Medical Microbiology

Section 4 Clinical manifestation and diagnosis of infections by body system

21 Sexually transmitted infections

Introduction

Sexually transmitted infections usually cause diseases

In some instances, sexually transmitted infections (STIs) may not result in overt disease symptoms, such as in the early stages of HIV infection and asymptomatic gonorrhoea in females. This is particularly concerning since people with asymptomatic or unreported STIs are unlikely to receive treatment, thus facilitating further cycles of infection and spread. While STIs are of major medical importance throughout the world, HIV infection has had the greatest global impact, estimated to affect nearly 35 million people by 2009. In addition to HIV, new cases of other STIs occur globally with alarming frequency (hundreds of millions of new cases) each year.

The incidence of most STIs is increasing

This is typified by the situation in the UK where, in 2009, 482 700 new STD cases were reported by genitourinary medicine clinics, an increase of over 12 000 from the previous year. A similar situation exists in other countries, including the USA. The reasons for this increase include:

• increasing density and mobility of human populations

• the difficulty of engineering changes in human sexual behaviour

• the absence of vaccines for almost all STIs.

The last two factors may change. There is evidence of changes in male homosexual behaviour, leading to decreased transmission of some STIs in this group, and vaccines for infections such as human papillomavirus have been developed.

HIV infection and AIDS have overshadowed other STIs with immense impact as a highly lethal infectious disease. Measuring plasma HIV-1 RNA load, CD4 counts and percentage, together with antiretroviral resistance and tropism testing by sequence analysis have become mainstays in the management of HIV infection with regard to monitoring disease progress and response to antiretroviral therapy in resource-rich countries.

The most common STIs are listed in Table 21.1Table 21.2 gives examples of the strategies used by the microorganisms to overcome host defences.

Table 21.1 The most common sexually transmitted infections (STIs)

Table 21.2 Strategies adopted by sexually transmitted microorganisms to combat host defences

Host defences

Microbial strategies

Examples

Integrity of mucosal surface

Specific attachment mechanism

Gonococcus or chlamydia to urethral epithelium

Urine flow (for urethral infection)

Specific attachment; induce own uptake and transport across urethral epithelial surface in phagocytic vacuole

Gonococcus

Infection of urethral epithelial or subepithelial cells

Herpes simplex virus (HSV), chlamydia

Phagocytes (especially polymorphs)

Induce negligible inflammation

Treponema pallidum, mechanism unclear, perhaps poorly activates alternative complement pathway due to sialic acid coating

Resist phagocytosis

Gonococcus (capsule) T. pallidum (absorbed fibronectin)

Complement

C3d receptor on microbe binds C3b/d and reduces C3b/d-mediated polymorph phagocytosis

Candida albicans

Inflammation

Induce strong inflammatory response, yet evade consequences

Gonococcus, C. albicans, HSV, chlamydia

Antibodies (especially IgA)

Produce IgA protease

Gonococcus

Cell-mediated immune response (T cells, lymphokines, natural killer cells, etc.)

Antigenic variation; allows re-infection of a given individual with an antigenic variant

Gonococcus, chlamydia

Poorly understood factors cause ineffective cell-mediated immune response

T. pallidum, HIV

STIs and sexual behaviour

The general principles of entry, exit and transmission of the microorganisms that cause STIs are set out in Chapter 13.

The spread of STIs is inextricably linked with sexual behaviour

There are therefore many more opportunities for controlling STIs than, for instance, respiratory infections. Infected but asymptomatic individuals play an important role, and important determinants are promiscuity and sexual practices involving contact between different orifices and mucosal surfaces (see Ch. 13). For example, transmission between heterosexuals or male homosexuals can take place following oral or anal intercourse. The gonococcus, for instance, causes pharyngitis and proctitis, although it infects stratified squamous epithelium less readily than columnar epithelium. As described more fully in Chapter 31, calculations regarding the number of infected secondary cases resulting from each primary STD case depends on a variety of behavioural factors since the number of sexual partners acquired by a given individual, i.e. the level of promiscuity, varies considerably. Those who have many sexual partners are both more likely to acquire and to transmit infection and play a key role in the persistence of such infections in the community of sexually active individuals. People with many sexual partners are therefore an obvious target for treatment and education about safer sex practices (e.g. condom use, etc.)

Various host factors influence the risk of acquiring an STI

It is not surprising that the type of sexual activity is important or that genital lesions or ulcers increase the risk of acquiring infections such as HIV. Other factors are less well understood, such as the numerous observations that uncircumcised men have a higher risk of infection.

STIs do not necessarily occur singly, and the possibility of multiple infections must always be borne in mind. For instance, syphilis can accompany gonorrhoea, and there is evidence that genital herpes may be reactivated during an attack of gonorrhoea.

Syphilis

Syphilis is caused by the spirochete Treponema pallidum

Treponema pallidum is closely related to the treponemes that cause the non-venereal infections of pinta and yaws (Table 21.3Fig. 21.1). T. pallidum has a worldwide distribution, and syphilis remains a serious problem not only in resource-rich countries but especially in resource-poor areas, due to the serious sequelae and the risk of congenital infection. Although syphilis rates in the USA fell to an all-time low in 2000, the incidence has since increased with a 70% greater risk in men during the past 5    years. A similar trend has also been seen in the UK.

Table 21.3 Spiral organisms of medical importance

Figure 21.1 (A) Typical penile chancre of primary syphilis. (Courtesy of R.D. Catterall.) (B) Yaws and (C) pinta are endemic in tropical and subtropical countries and are spread by direct contact.

(Courtesy of P.J. Cooper and G. Griffin.)

T. pallidum enters the body through minute abrasions on the skin or mucous membranes. Transmission of T. pallidum requires close personal contact because the organism does not survive well outside the body and is very sensitive to drying, heat and disinfectants. Horizontal spread (see Ch. 13) occurs through sexual contact, and vertical spread via transplacental infection of the fetus (see Ch. 23).

Local multiplication leads to plasma cell, polymorph and macrophage infiltration, with later endarteritis. The bacteria multiply very slowly, and the average incubation period is 3    weeks.

Classically, T. pallidum infection is divided into three stages

The three classical stages of syphilis are primary, secondary and tertiary syphilis (Table 21.4). However, not all patients go through all three stages; a substantial proportion remains permanently free of disease after suffering the primary or secondary stages of infection. The lesion of primary syphilis is illustrated in Figure 21.1. The secondary stage may be followed by a latent period of some 3–30    years, after which the disease may recur – the tertiary stage. Unlike most bacterial pathogens, T. pallidum can survive in the body for many years despite a vigorous immune response. It has been suggested that the healthy treponeme evades recognition and elimination by the host by maintaining a cell surface rich in lipid. This layer is antigenically unreactive and the antigens are only uncovered in dead and dying organisms when the host is then able to respond. Tissue damage is mostly due to the host response.

Table 21.4 The pathogenesis of syphilis

Stage of disease

Signs and symptoms

Pathogenesis

Initial contact

 

Multiplication of treponemas at site of infection; associated host response

2–10    weeks (depends on inoculum size)

Primary chancrea at site of infection

 

Primary syphilis

Enlarged inguinal nodes, spontaneous healing

Proliferation of treponemas in regional lymph nodes

1–3    months

   

Secondary syphilis
 
2–6    weeks

Flu-like illness; myalgia, headache, fever; mucocutaneous rasha; spontaneous resolution

Multiplication and production of lesion in lymph nodes, liver, joints, muscles, skin and mucous membranes

Latent syphilis

 

Treponemas dormant in liver or spleen

3–30    years

 

Re-awakening and multiplication of treponemas

Tertiary syphilis

Neurosyphilis; general paralysis of the insane, tabes dorsalis

Further dissemination and invasion and host response (cell-mediated hypersensitivity)

 

Cardiovascular syphilis; aortic lesions, heart failure

 
 

Progressive destructive disease

Gummas in skin, bones, testis

A feature of Treponema pallidum infection is its chronic nature, which seems to involve a delicately balanced relationship between pathogen and host.

a Chancre: Initially a papule; forms a painless ulcer; heals without treatment within 2    months. Live treponemas can be seen in dark-ground microscopy of fluid from lesions; patient highly infectious.

Despite many years of effort, T. pallidum still cannot be cultivated in the laboratory in artificial media. It has therefore been difficult to study possible virulence factors at a molecular level, although a variety of genes have been cloned, the entire chromosome has been sequenced, and major proteins have been characterized.

An infected woman can transmit T. pallidum to her baby in utero

Congenital syphilis is acquired after the first 3 months of pregnancy. The disease may manifest as:

• serious infection resulting in intrauterine death

• congenital abnormalities, which may be obvious at birth

• silent infection, which may not be apparent until about 2 years of age (facial and tooth deformities).

Laboratory diagnosis of syphilis

As T. pallidum cannot be grown in vitro, laboratory diagnosis hinges on microscopy and serology.

Microscopy

Exudate from the primary chancre should be examined by either:

• dark-field microscopy immediately after collection

• ultraviolet (UV) microscopy after staining with fluorescein-labelled antitreponemal antibodies.

The organisms have tightly wound, slender coils with pointed ends and are sluggishly motile in unstained preparations. T. pallidum is very thin (about 0.2    mm in diameter, compared with E. coli, which is about 1    mm) and cannot be seen in Gram-stained preparations. Silver impregnation stains can be used to demonstrate the organisms in biopsy material.

Serology

Serologic tests for syphilis are the mainstay of diagnosis. They are divided into non-specific and specific tests for the detection of antibodies in patients’ serum.

Non-specific tests (non-treponemal tests) for syphilis are the VDRL and RPR tests

The term non-specific is used because the antigens are not treponemal in origin, but are from extracts of normal mammalian tissues. Cardiolipin, from beef heart, allows the detection of anti-lipid IgG and IgM formed in the patient in response to lipoidal material released from cells damaged by the infection, as well as to lipids in the surface of T. pallidum. The two tests in common use today are:

• the Venereal Disease Research Laboratory (VDRL) test

• the rapid plasma reagin (RPR) test.

Both are available in kit form.

Non-specific tests show up as positive within 4–6    weeks of infection (or 1–2    weeks after the primary chancre appears) and decline in positivity in tertiary syphilis or after effective antibiotic treatment of primary or secondary disease. Therefore, these tests are useful for screening. However, they are non-specific and may give positive results in conditions other than syphilis (biologic false positives, Table 21.5). All positive results should therefore be confirmed by a specific test. However, treatment (e.g. especially during the primary and secondary stages) tends to result in seroreversion to these tests. Thus, with confirmed disease (see below), these tests can provide at least an indication of therapeutic efficacy.

Table 21.5 Serologic tests for syphilis and conditions associated with false-positive results

Test

Conditions associated with false-positive results

Non-specific (non-treponemal)
    VDRL
    RPR

Viral infection, collagen vascular disease, acute febrile disease, post-immunization, pregnancy. leprosy, malaria, drug misuse

Specific (non-treponemal)
    FTA-ABS
    TP-PA
    TPHA

Diseases associated with increased or abnormal globulins, lupus erythematosus, Lyme disease, autoimmune disease, diabetes mellitus, alcoholic cirrhosis, viral infections, drug misuse, and pregnancy

FTA-ABS, fluorescent treponemal antibody absorption test; MHA-TP, microhaemagglutination assay for T. pallidum; RPR, rapid plasma reagin test; TPHA, T. pallidum haemagglutination test; TP-PA, T. pallidum particle agglutination test; VDRL, Venereal Disease Research Laboratory test.

Commonly used specific tests for syphilis are the treponemal antibody test, FTA-ABS test and the MHA-TP

These tests use recombinant proteins or treponemal antigens extracted from T. pallidum. Tests in common use include:

• enzyme-linked immunosorbent assays which detect IgM and IgG

• the fluorescent treponemal antibody absorption (FTA-ABS, Fig. 21.2) test in which the patient’s serum is first absorbed with non-pathogenic treponemes to remove cross-reacting antibodies before reaction with T. pallidum antigens

• the microhaemagglutination assay for T. pallidum (MHA-TP).

These tests should be used to confirm that a positive result with a non-specific test is truly due to syphilis. Also, because they become positive earlier in the course of the disease, they can be used for confirmation when the clinical picture is strongly indicative of syphilis. They tend to remain positive for many years and may be the only positive test in patients with late syphilis. However, they remain positive after appropriate antibiotic treatment and cannot therefore be used as indicators of therapeutic response. They can also give false-positive reactions (see Table 21.5).

Figure 21.2 The fluorescent treponemal antibody absorption test for syphilis. Antibody in the patient’s serum binds to bacteria and is visualized by a fluorescent dye.

Confirmation of a diagnosis of syphilis depends upon several serologic tests

Positive serologic test results for babies born to infected mothers may represent passive transfer of maternal antibody or the baby’s own response to infection. These two possibilities can be distinguished by testing for IgM and retesting at 6    months of age, by which time maternal antibody levels have waned. Antibody titres remain elevated in babies with congenital syphilis.

At present, several serologic tests are needed to confirm a diagnosis of syphilis. None of these tests distinguishes syphilis from the non-sexually transmitted treponematoses, yaws and pinta. Western blot assays using whole T. pallidum cells as antigen are an important newer confirmatory test.

Treatment

Penicillin is the drug of choice for treating people with syphilis and their contacts

Penicillin is very active against T. pallidum (see Table 21.1). For patients who are allergic to penicillin, treatment with doxycycline should be given. Only penicillin therapy reliably treats the fetus when administered to a pregnant mother.

Prevention of secondary and tertiary disease depends upon early diagnosis and adequate treatment. Contact tracing with screening and treatment is also important. Several STIs may be present in one patient concurrently, and patients with other STIs should be screened for syphilis.

Congenital syphilis is completely preventable if women are screened serologically early in pregnancy (<    3    months) and those who are positive are treated with penicillin.

Gonorrhoea

Gonorrhoea is caused by the Gram-negative coccus Neisseria gonorrhoeae (the ‘gonococcus’)

This bacterium is a human pathogen and does not cause natural infection in other animals. Therefore its reservoir is human and transmission is direct, usually through sexual contact, from person to person. The organism is sensitive to drying and does not survive well outside the human host, so intimate contact is required for transmission. It is thought that a woman has a 50% chance of becoming infected after a single sexual intercourse with an infected man, while a man has a 20% chance of acquiring infection from an infected woman.

Asymptomatically infected individuals (almost always women, see below) form the major reservoir of infection. Infection may also be transmitted vertically from an infected mother to her baby during childbirth. Infection in babies is usually manifest as ophthalmia neonatorum (see Ch. 23).

The gonococcus has special mechanisms to attach itself to mucosal cells

The usual site of entry of gonococci into the body is via the vagina or the urethral mucosa of the penis, but other sexual practices may result in the deposition of organisms in the throat or on the rectal mucosa. Special adhesive mechanisms (Fig. 21.3) prevent the bacteria from being washed away by urine or vaginal discharges. Following attachment, the gonococci rapidly multiply and spread through the cervix in women, and up the urethra in men. Spread is facilitated by various virulence factors (Fig. 21.3), although the organisms do not possess flagella and are non-motile. Production of an IgA protease helps to protect them from the host’s secretory antibodies.

Figure 21.3 The spread of Neisseria gonorrhoeae is facilitated by various virulence factors. Changes in the surface structure of the gonococcus render the organism avirulent.

Host damage in gonorrhoea results from gonococcal-induced inflammatory responses

The gonococci invade non-ciliated epithelial cells, which internalize the bacteria and allow them to multiply within intracellular vacuoles, protected from phagocytes and antibodies. These vacuoles move down through the cell and fuse with the basement membrane, discharging their bacterial contents into the subepithelial connective tissues. Neisseria gonorrhoeae does not produce a recognized exotoxin. Damage to the host results from inflammatory responses elicited by the organism (e.g. lipopolysaccharide and other cell wall components; see Ch. 2). Persistent untreated infection can result in chronic inflammation and fibrosis.

Infection is usually localized, but in some cases bacteria isolates (e.g. resistant to the bactericidal action of serum, etc.) can invade the bloodstream and so spread to other parts of the body.

Gonorrhoea is initially asymptomatic in many women, but can later cause infertility

Symptoms develop within 2–7    days of infection and are characterized:

• in the male by urethral discharge (Fig. 21.4) and pain on passing urine (dysuria)

• in the female by vaginal discharge.

At least 50% of all infected women have only mild symptoms or are completely asymptomatic. They do not therefore seek treatment and will continue to infect others. Asymptomatic infection, however, is not the usual course of events in men. Women may not be alerted to their infection unless or until complications arise, such as:

• pelvic inflammatory disease (PID)

• chronic pelvic pain

• infertility resulting from damage to the fallopian tubes.

Ophthalmia neonatorum is characterized by a sticky discharge (see Fig. 23.5).

Figure 21.4 Gonococcal urethritis. Typical purulent meatal discharge with inflammation of the glans.

(Courtesy of J. Clay.)

Gonococcal infection of the throat may result in a sore throat (see Ch. 18), and infection of the rectum also results in a purulent discharge.

In men, local complications of urethral infection are rare (Fig. 21.5). Invasive gonococcal disease is much more common in infected women than in men, but prompt treatment is important in containing local infection. The common occurrence of asymptomatic infection in women is an important factor in the occurrence of complications (i.e. the infection is unrecognized and untreated). In 10–20% of untreated women, infection spreads up the genital tract to cause pelvic inflammatory disease (PID) and damage to the fallopian tubes.

 

Figure 21.5 Local and systemic complications of gonococcal infection. (A) Skin lesions start as erythematous papules, which often become pustular and haemorrhagic with necrotic centres. (Courtesy of J.S. Bingham.) (B) Septic arthritis of the ankle with marked erythema and swelling of the ankle and leg.

(Courtesy of T.F. Sellers, Jr.)

Disseminated infection occurs in 1–3% of women, but is less common in men (see above and Fig. 21.6). It is a function not only of the strain of gonococcus (see above), but also host factors (e.g. about 5% of people with disseminated infection have deficiencies in the late-acting components of complement (C5–C8)).

Figure 21.6 Local and systemic spread of gonococcal infection and complications.

A diagnosis of gonorrhoea is made from microscopy and culture of appropriate specimens

Urethral and vaginal discharges and other specimens where indicated are used for microscopy and culture. Although a purulent discharge is characteristic of local gonococcal infection, it is not possible to distinguish reliably between gonococcal discharge and that caused by other pathogens such as Chlamydia trachomatis on clinical examination.

With experience, the finding of Gram-negative intracellular diplococci in a smear of urethral discharge from a symptomatic male patient is a highly sensitive and specific test for the diagnosis of gonorrhoea.

Culture is essential in the investigation of infection in women and asymptomatic men, and for specimens taken from sites other than the urethra. Specimens from symptomatic men should also be cultured:

• to confirm the identity of the isolate; misinterpretation of microscopy or culture results can cause severe distress and may result in litigation

• to perform antibiotic susceptibility tests (see Ch. 32)

• to aid in the distinction between treatment failure and reinfection.

Because of the organism’s sensitivity to drying, cultures should be made on warmed selective (i.e. modified Thayer Martin) and non-selective (chocolate blood agar) medium to insure recovery. Inoculation into appropriate transport medium is required if transfer to the laboratory will be delayed (no more than 48    h). Blood cultures should be collected if disseminated disease is suspected, and joint aspirates may yield positive cultures.

Serologic tests are unsatisfactory. Commercial nucleic acid-based approaches (specific probes, amplification, etc.) are now available, providing reliable results within a few hours.

Antibacterials used to treat gonorrhoea are cefixime or ceftriaxone

The antibacterial agents of choice are shown in Table 21.1. Penicillinase-producing N. gonorrhoeae were first observed in 1976 with increasing resistance that has severely compromised the effective treatment of gonorrhoea in many parts of the world, especially SE Asia. Resistance to fluoroquinolones has also occurred. Since patients with gonorrhoea may also be infected with chlamydia (see below), treatment regimens often include a combination of agents targeting both organisms (e.g. ceftriaxone and doxycycline, respectively). Early treatment of a significant proportion of sexually promiscuous patients achieves a striking reduction in the duration of infectiousness and transmission rates. Prophylactic use of antibacterials has no effect in preventing sexually-acquired gonorrhoea, but the application of antibacterial eye drops to babies born to mothers with gonorrhoea or suspected gonorrhoea is effective. Infection can be prevented by the use of condoms.

Follow-up of patients and contact tracing are vital to control the spread of gonorrhoea. At present, effective vaccines are not available, but the possibility of using some of the pilus proteins or other outer membrane components of the gonococcal cell as antigens has been under investigation. However, immunization may prevent symptomatic disease without preventing infection, and the dangers of asymptomatic infection have been discussed above.

Repeated infections can occur with strains of bacteria with different pilin proteins (e.g. antigenic variation; see Ch. 16).

Chlamydial infection

C. trachomatis serotypes D–K cause sexually transmitted genital infections

The chlamydiae are very small bacteria that are obligate intracellular parasites. They have a more complicated life cycle than free-living bacteria because they can exist in different forms:

• The elementary body (EB) is adapted for extracellular survival and for initiation of infection.

• The reticulate body (RB) is adapted for intracellular multiplication (Fig. 21.7).

Traditionally, three species of Chlamydia were recognized: C. trachomatisC. psittaci and C. pneumoniae. However, the latter two have been moved to the genus, Chlamydophila (Table 21.6). Chlamydophila psittaci and Chlamydophila pneumoniae infect the respiratory tract and have been discussed in Chapter 19. The species Chlamydia trachomatis can be subdivided into different serotypes (also known as serovars) and these have been shown to be linked characteristically with different infections:

• Serotypes A, B and C are the causes of the serious eye infection trachoma (see Ch. 25).

• Serotypes D–K are the cause of genital infection and associated ocular and respiratory infections (Table 21.7).

• Serotypes L1, L2 and L3 cause the systemic disease lymphogranuloma venereum (LGV) (see below).

C. trachomatis serotypes D–K have a worldwide distribution, whereas the distribution of LGV serotypes is more restricted.

Figure 21.7 The life cycle of Chlamydia. EB, elementary body; RB, reticulate body.

Table 21.6 Medically important species of Chlamydiaceae

Table 21.7 Clinical syndromes and complications caused by C. trachomatis, serotypes D–K

Infection in

Clinical syndromes

Complications

Men

Urethritis, epididymitis, proctitis, conjunctivitis

Systemic spread, Reiter’s syndromea

Women

Urethritis, cervicitis, bartholinitis, salpingitis, conjunctivitis

Ectopic pregnancy, infertility, systemic spread: perihepatitis arthritis dermatitis

Neonates

Conjunctivitis

Interstitial pneumonitis

a Urethritis, conjunctivitis, polyarthritis, mucocutaneous lesions.

The majority of infections are genital and are acquired during sexual intercourse. Asymptomatic infection is common, especially in women. Ocular infections in adults are probably acquired by autoinoculation from infected genitalia or by ocular–genital contact. Ocular infections in neonates are acquired during passage through an infected maternal birth canal, and the infant is also at risk of developing C. trachomatis pneumonia (see Ch. 19).

Chlamydiae enter the host through minute abrasions in the mucosal surface

They bind to specific receptors on the host cells and enter the cells by ‘parasite-induced’ endocytosis (see Ch. 13). Once inside the cell, fusion of the chlamydia-containing vesicle with lysosomes is inhibited by an incompletely understood mechanism and the EB begins its developmental cycle (Fig. 21.7). Within 9–10    h of cell invasion, the EBs differentiate into metabolically active RBs, which divide by binary fission and produce fresh EB progeny. These are then released into the extracellular environment within a further 20    h.

The clinical effects of C. trachomatis infection appear to result from cell destruction and the host’s inflammatory response

The released EBs invade adjacent cells or cells distant from the site of infection if carried in lymph or blood.

Growth of C. trachomatis serotypes D–K seems to be restricted to columnar and transitional epithelial cells, but serotypes L1, L2 and L3 cause systemic disease (LGV). The site of infection determines the nature of clinical disease (see Table 21.7). Genital tract infection with serotypes D–K is locally asymptomatic in most women, but usually symptomatic in men.

Laboratory tests are essential to diagnose chlamydial urethritis and cervicitis

Chlamydial urethritis and cervicitis cannot be reliably distinguished from other causes of these conditions on clinical grounds alone. The methods traditionally available include cell culture and direct antigen detection.

Most infected patients develop antibodies, but serology is unreliable for diagnostic purposes. As chlamydiae are obligate intracellular parasites, traditional identification by isolation must be performed by growth in cell cultures. When this approach is available, the specimen is suspended in fluid and centrifuged on to a monolayer of tissue culture (McCoy) cells pretreated with cycloheximide, which enhances the uptake of chlamydiae. After 48–72    h, C. trachomatis forms characteristic cytoplasmic inclusions, which stain with iodine because they contain glycogen (Fig. 21.8) or can be visualized with immunofluorescent stains.

Figure 21.8 Chlamydial inclusion bodies stained dark brown with iodine.

C. trachomatis can be detected directly on microscopy using the direct fluorescent antibody test

C. trachomatis can be detected directly in smears of clinical specimens made on microscope slides stained with fluorescein conjugated monoclonal antibodies and viewed by UV microscopy – the direct fluorescent antibody (DFA) test. The EBs stain as bright yellow-green dots (Fig. 21.9). Results can be obtained within a few hours. Compared with culture, this method is extremely specific, but often not sensitive enough for asymptomatic infections. Chlamydial antigens can also be detected in specimens using an enzyme-linked immunosorbent assay (ELISA), but this test also suffers from reduced sensitivity in asymptomatic patients.

Figure 21.9 Direct fluorescent antibody test for Chlamydia trachomatis. Elementary bodies can be seen as bright yellow-green dots under the ultraviolet microscope.

(Courtesy of J.D. Treharne.)

A variety of nucleic acid-based tests are commercially available for chlamydial detection

Nucleic acid probe and amplification-based tests are capable of directly detecting C. trachomatis in specimens from infected individuals (e.g. cervix, urethra, urine, etc.). As mentioned previously, these commercially available kits can provide rapid (2–4    h) and specific detection of both N. gonorrhoeae and Chlamydia DNA, which is important since patients are often co-infected with both organisms. These quick and accurate molecular approaches are increasingly used as the preferred test for the detection of these organisms.

Chlamydial infection is treated or prevented with doxycycline or azithromycin

It is important to remember that chlamydiae are not susceptible to the beta-lactam antibiotics, which are important for the treatment of gonorrhoea and syphilis. It is recommended that patients receiving treatment for gonorrhoea also be treated with doxycycline or azithromycin for possible concurrent chlamydial infection (see Table 21.1). In addition, patients with clinically diagnosed chlamydial genital infections, their sexual contacts and babies born to infected mothers should be treated. Erythromycin should be used for babies.

Prevention depends upon recognizing the importance of asymptomatic infections. Early diagnosis and treatment of cases and of their sexual partners is important in order to avoid complications and reduce opportunities for transmission. Remember that STIs are not mutually exclusive, and patients may have concurrent infections with quite different pathogens.

Other causes of inguinal lymphadenopathy

Genital infections are common causes of inguinal lymphadenopathy (swelling of lymph nodes in the groin) among sexually active people. Syphilis and gonorrhoea have been discussed above. Lymphogranuloma venereum (LGV), chancroid and donovanosis are more common in tropical and subtropical countries than in Europe and the USA but may be imported by travellers who have acquired the disease through sexual contact in these areas.

Lymphogranuloma venereum (LGV)

Lymphogranuloma venereum is caused by C. trachomatis serotypes L1, L2 and L3

Lymphogranuloma venereum (LGV) is a serious disease especially common in Africa, Asia and South America. It occurs sporadically in Europe, Australia and North America, particularly among homosexual males. The prevalence appears to be higher among males than females, probably because symptomatic infection is more common in men.

Lymphogranuloma venereum is a systemic infection involving lymphoid tissue and is treated with doxycycline or erythromycin

The clinical picture can be contrasted with the more restricted infection seen with C. trachomatis serotypes D–K (see above). The primary lesion is an ulcerating papule at the site of inoculation (after an incubation period of 1–4    weeks) and may be accompanied by fever, headache and myalgia. The lesion heals rapidly, but the chlamydiae proceed to infect the draining lymph nodes, causing characteristic inguinal buboes (Fig. 21.10), which gradually enlarge. Chlamydiae may disseminate from the lymph nodes via the lymphatics to the tissues of the rectum to cause proctitis. Other systemic complications include fever, hepatitis, pneumonitis and meningoencephalitis. The infection may resolve untreated, but:

• Abscesses may form in lymph nodes, which suppurate and discharge through the skin.

• Chronic granulomatous reactions in lymphatics and neighbouring tissues can eventually give rise to fistula in ano or genital elephantiasis.

Figure 21.10 Lymphogranuloma venereum. Bilateral enlargement of inguinal glands.

(Courtesy of J.S. Bingham.)

Cell culture methods are available (see above), but the chlamydial isolation rate is reported to be low (24–30%). Historically, the ‘Frei’ skin test was used in diagnosis involving intradermal injection of the LGV antigen. However, the test is no longer used due to a lack of reliability and sensitivity in early disease and it lacks specificity because the Frei antigen is only genus specific. As discussed above, nucleic acid-based tests are also available. Treatment with doxycycline or erythromycin (see Table 21.1) is recommended. Pregnant women and children under 9    years of age should be treated with erythromycin.

Chancroid (soft chancre)

Chancroid is caused by Haemophilus ducreyi and is characterized by painful genital ulcers

Infection by the Gram-negative bacterium Haemophilus ducreyi is manifest as painful non-indurated genital ulcers and local lymphadenitis (Fig. 21.11). Note the difference between this and the chancre of primary syphilis, which is painless, but the ulcers may be confused with those of genital herpes, though they are usually larger and have a more ragged appearance. While the disease is endemic in some areas of the USA, cases generally tend to occur in distinct outbreaks. However, in Africa and Asia chancroid is the commonest cause of genital ulcers. Epidemiologic information is important because the diagnosis is usually clinical as the organism is difficult to grow in the laboratory. Chancroid may also be confused with donovanosis (see below).

Figure 21.11 Chancroid. Several irregular ulcers on the prepuce.

(Courtesy of L. Parish.)

Chancroid is diagnosed by microscopy and culture and treated with azithromycin, ceftriaxone, erythromycin or ciprofloxacin

Gram-stained smears of aspirates from the ulcer margin or enlarged lymph node characteristically show large numbers of short Gram-negative rods and chains, often described as having a ‘school of fish’ appearance, within or outside polymorphs. Aspirates should be cultured on a rich medium (GC agar with 1–2% haemoglobin, 5% fetal bovine serum, 10% CVA and vancomycin, 3    μg/mL) at 33°C in 5–10% carbon dioxide. H. ducreyi will not tolerate higher temperatures. Growth is slow, and it may take 2–9    days for colonies to appear. Treatment with a macrolide (e.g. erythromycin or azithromycin) or ceftriaxone (see Table 21.1) is generally recommended.

Donovanosis

Donovanosis is caused by Calymmatobacterium granulomatis and is characterized by genital nodules and ulcers

Donovanosis (granuloma inguinale or granuloma venereum) is rare in temperate climates, but common in tropical and subtropical regions such as the Caribbean, New Guinea, India and central Australia. The infection is characterized by nodules, almost always on the genitalia, which erode to form granulomatous ulcers that bleed readily on contact. The infection may extend and the ulcers may become secondarily infected. The pathogen is a Gram-negative rod, traditionally called Calymmatobacterium granulomatis. However, genomic analysis has placed this organism into the genus Klebsiella (i.e. Klebsiella granulomatis); however, some literature continues to use the C. granulomatis designation. The bacteria invade and multiply within mononuclear cells and are liberated when the cells rupture.

Donovanosis is diagnosed by microscopy and treated with doxycycline

The diagnosis of donovanosis is made by examining a smear from the lesion stained with Wright’s or Giemsa stain. ‘Donovan bodies’ appear as clusters of blue- or black-stained organisms in the cytoplasm of mononuclear cells. Treatment with doxycycline, azithromycin or co-trimoxazole is recommended.

Mycoplasmas and non-gonococcal urethritis

Mycoplasma hominis, M. genitalium, and Ureaplasma urealyticum may be causes of genital tract infection

Although Mycoplasma pneumoniae has a proven role in the causation of pneumonia (see Ch. 19), the role of M. hominisM. genitalium and Ureaplasma urealyticum (which metabolizes urea; also called ‘T strains’) in STIs is less certain. These organisms frequently colonize the genital tracts of healthy sexually active men and women. They are less common in sexually inactive populations, which supports the view that they may be sexually transmitted. It is difficult to prove that they cause infection of the genital tract, but M. genitalium may cause non-gonococcal urethritis, M. hominis may cause PID, postabortal and postpartum fevers, and pyelonephritis. U. urealyticum has also been associated with non-gonococcal urethritis and prostatitis.

M. hominisM. genitalium, and U. urealyticum are treated with either doxycycline or azithromycin (some Ureaplasmas are tetracycline resistant) which is also the treatment for chlamydial infections.

Other causes of vaginitis and urethritis

Candida infection

Candida albicans causes a range of genital tract diseases, which are treated with oral or topical antifungals

These vary from mild superficial, localized infections in an otherwise healthy individual to disseminated, often fatal infections in the immunocompromised. This yeast is a normal inhabitant of the female vagina, but in some women and in circumstances which are not clearly understood, the candidal load increases and causes an intensely irritant vaginitis with a cheesy vaginal discharge. This may be accompanied by urethritis and dysuria and may present as a urinary tract infection (see Ch. 20). The diagnosis can be confirmed by microscopy and culture of the discharge (Fig. 21.12).

 

Figure 21.12 Candida albicans. (A) Light microscopic appearance and (B) culture of vaginal discharge.

Treatment with an oral antifungal such as fluconazole or a topical preparation such as nystatin is recommended, but recurrence is frequent in a small proportion of women. Balanitis (inflammation of the glans penis) is seen in approximately 10% of male partners of females with vulvovaginal candidiasis, but urethritis is uncommon in men and is rarely symptomatic.

Trichomonas infection

Trichomonas vaginalis is a protozoan parasite and causes vaginitis with copious discharge

Trichomonas vaginalis inhabits:

• the vagina in women

• the urethra (and sometimes the prostate) in men.

It is transmitted during sexual intercourse. In women, heavy infections cause vaginitis with a characteristic copious foul-smelling discharge, though the infection may be asymptomatic in some females. There is an associated increase in the vaginal pH. The infection should be distinguished from bacterial vaginosis (see below) by microscopic examination of the discharge, which shows actively motile trophozoites (Fig. 21.13). Trichomonas may be cultured from a vaginal (not cervical) swab and nucleic acid detection can be used for diagnosis though it is not yet generally available.

Figure 21.13 Motile trophozoites in vaginal discharge in T. vaginalis infection. (Giemsa stain.)

(Courtesy of R. Muller.)

Metronidazole is recommended for symptomatic T. vaginalis infections. Tinidazole may also be used but belongs to the same class of compounds as metronidazole, so there is a need for orally active alternative agents.

In men, Trichomonas vaginalis is rarely symptomatic, but sometimes causes a mild urethritis. Regular sexual partners of symptomatic women should be treated at the same time to prevent reinfection.

Bacterial vaginosis

Bacterial vaginosis is associated with Gardnerella vaginalis plus anaerobic infection and a fishy-smelling vaginal discharge

This non-specific vaginitis is a syndrome in women characterized by at least three of the following signs and symptoms:

• excessive malodorous vaginal discharge

• vaginal pH    >       4.5

• presence of clue cells (vaginal epithelial cells coated with bacteria, Fig. 21.14)

• a fishy amine-like odour.

There is a significant increase in the numbers of G. vaginalis in the vaginal flora and a concomitant increase in the numbers of obligate anaerobes such as Bacteroides.

Figure 21.14 Clue cells in bacterial vaginosis.

G. vaginalis is consistently found in association with vaginosis, but is also found in 20–40% of healthy women. It is generally present in the urethra of male partners of women with vaginosis, indicating that it can be sexually transmitted. G. vaginalis has also been isolated from blood cultures from women with postpartum fever.

G. vaginalis has had a chequered taxonomic history, being first classified as a haemophilus, then as a corynebacterium, reflecting the fact that it tends to be Gram-variable (sometimes appearing Gram-negative, sometimes Gram-positive). It grows in the laboratory on human blood agar in a moist atmosphere enriched with carbon dioxide. The organism is treated with oral metronidazole. Species of the genus Mobiluncus appear to be related to G. vaginalis and have also been implicated in vaginosis.

The pathogenesis of bacterial vaginosis is still unclear, but appears to be related to factors that disrupt the normal acidity of the vagina and the equilibrium between the different constituents of the normal vaginal flora. Whether any of these or other unknown factors are sexually transmissible is unclear.

Genital herpes

Herpes simplex virus (HSV)-2 is the most common cause of genital herpes, but HSV-1 is being detected more frequently

Herpes simplex virus (HSV) is a ubiquitous infection of humans worldwide. HSV-1 is generally transmitted via saliva, causing primary oropharyngeal infection in children, and cold sores occur after virus reactivation. However, HSV-2 emerged as a result of independent transmission by the venereal route. HSV-2 shows biologic and antigenic differences from HSV-1 and can be distinguished by molecular typing methods as well as older techniques such as immunofluorescence. There is little cross-immunity. Although originally recovered from separate sites, orogenital sexual practices have obscured the topographic difference between the strains, so that HSV-1 and HSV-2 can be recovered from oral and genital sites. HSV-2 is one of the most common STIs and it has been estimated that there are over 500 million individuals with HSV-2 globally. In the USA, a Centers for Disease Control (CDC) survey from 2005–2008 reported that HSV-2 antibody was detected in 16% of the study population, greater among females. One of the worrying aspects surrounding HSV-2 is that most people do not know that they have an HSV-2 infection as up to 75% may not have symptoms and therefore will not realize that they may transmit this infection. Finally, HSV-2 infection can result in a twofold increased risk of developing HIV infection. This is likely to be due to breaches in the mucosal barrier as a result of the HSV ulcers.

Genital herpes is characterized by ulcerating vesicles that can take up to 2 weeks to heal

The primary genital lesion on the penis or vulva is seen 3–7    days after infection. It consists of vesicles that soon break down to form painful shallow ulcers (Fig. 21.15). Local lymph nodes are swollen, and there may be constitutional symptoms including fever, headache and malaise. Occasionally the lesions are on the urethra, causing dysuria or pain on micturition. Healing takes up to 2    weeks, but the virus in the lesion travels up sensory nerve endings to establish latent infection in dorsal root ganglion neurones (see Ch. 24). From this site it can reactivate, travel down nerves to the same area, and cause recurrent lesions (‘genital cold sores’).

Figure 21.15 Genital herpes. Vesicles (A) on the penis and (B) in the perianal area and vulva. Those on the labia minora and fourchette have ruptured to reveal characteristic herpetic erosions.

(Courtesy of J.S. Bingham.)

Aseptic meningitis or encephalitis occurs in adults as a rare complication, and spread of infection from mother to infant at the time of delivery can give rise to neonatal disseminated herpes or encephalitis.

Genital herpes is generally diagnosed from the clinical appearance, and acyclovir can be used for treatment

Herpes simplex virus DNA can be detected in vesicle fluid or ulcer swabs. More classic techniques involved virus isolation and subsequently typing the isolate by immunofluorescence using type-specific monoclonal antibodies. Recurrent genital infection is more frequent with HSV-2; therefore typing is of help in determining the prognosis. The cytopathic effect is characteristic and is generally seen within 1–2    days post-inoculation, with ballooning degenerating cells and multinucleate giant cells. HSV DNA detection methods which include type differentiation may be used, and have a much greater sensitivity than virus isolation. A number of antivirals, including oral acyclovir, valaciclovir and famciclovir can be used for treatment of severe or early lesions, and acyclovir may need to be given intravenously if there are systemic complications. Recurrent attacks are troublesome, and treatment options include starting an antiviral when prodromal symptoms occur or alternatively taking 6–12    months low-dose acyclovir or one of the alternative agents to stop or at least reduce the frequency of recurrences.

Human papillomavirus infection

There are over 120 distinct types of human papillomaviruses, all infecting skin or mucosal surfaces, and the DNA of each showing less than 50% cross-hybridization with that of others. These are evidently ancient viral associates of humans that have evolved extensively, and many of the different types are adapted to specific regions of the body.

Many papillomavirus types are transmitted sexually and cause genital warts

Warts (condylomata acuminata) appear on the penis, vulva and perianal regions (Fig. 21.16) after an incubation period of 1–6    months (see Ch. 26). They may not regress for many months and can be treated with podophyllin. The lesion on the cervix is a flat area of dysplasia visible by colposcopy as a white plaque (Fig. 21.17) after the local application of 5% acetic acid. Because of their association with cervical cancer, especially types 16 and 18, cervical lesions are best removed by laser or loop excision.

Figure 21.16 Genital warts. (A) Warts on the penis are usually multiple, and on the shaft are often flat and keratinized. (B) Warts in the perianal area often extend into the anal canal. (C) Warts in the vulvoperineal area can enlarge dramatically and extend into the vagina.

(Courtesy of J.S. Bingham.)

Figure 21.17 Cervical dysplasia caused by papillomavirus should be removed by laser.

(Courtesy of A. Goodman.)

Human immunodeficiency virus

Human immunodeficiency virus (HIV) is a retrovirus (Table 21.8), so-called because this single-stranded RNA virus contains a pol gene that codes for a reverse transcriptase (Latin: retro, backwards).

Table 21.8 Human retroviruses

Virus

Comment

HTLV1

Endemic in West Indies and SW Japan; transmission via blood, human milk; can cause adult T-cell leukaemia, and HTLV1-associated myelopathy, also known as tropical spastic paraparesis

HTLV2

Uncommon, sporadic occurrence; transmission via blood; can cause hairy T-cell leukaemia and neurological disease

HIV-1, HIV-2

Transmission via blood, sexual intercourse; responsible for AIDS. HIV-2 West African in origin, closely related to HIV-1 but antigenically distinct

Human foamy virus

Causes foamy vacuolation in infected cells; little is known of its occurrence or pathogenic potential

Human placental virus(es)

Detected in placental tissue by electron microscopy and by presence of reverse transcriptase

Human genome viruses

Nucleic acid sequences representing endogenous retroviruses are common in the vertebrate genome, often in well-defined genetic loci; acquired during evolutionary history; not expressed as infectious virus; function unknown; perhaps should be regarded as mere parasitic DNA

Human T-cell lymphotropic virus (HTLV)1, HTLV2, HIV-1 and HIV-2 have been cultivated in human T cells in vitro. The human placental and genome viruses are not known as infectious agents. Retroviruses are also common in cats (FAIDS), monkeys (MAIDS), mice (mouse leukaemia) and other vertebrates. ARC, AIDS-related complex.

Acquired immune deficiency syndrome (AIDS) was first recognized in 1981 in the USA

In 1981, the Communicable Disease Center, Atlanta, USA, noted an increase in requests to use pentamidine for Pneumocystis carinii (now classified as P. jirovecii) infection in previously well individuals who also suffered severe infections by other normally harmless microorganisms. These included C. albicans oesophagitis, mucocutaneous HSV, toxoplasma CNS infection or pneumonia, and cryptosporidial enteritis; Kaposi’s sarcoma was also often present. Patients had evidence of impaired immune function, as shown by skin test anergies, and depletion of CD4-positive T-helper (Th) lymphocytes. This immunodeficiency syndrome appearing in an individual without a known cause such as treatment with immunosuppressive drugs was referred to as ‘acquired immune deficiency syndrome’ (AIDS). An internationally agreed definition of AIDS soon followed. Epidemics subsequently occurred in San Francisco, New York and other cities in the USA, and in the UK and Europe a few years later.

Human immunodeficiency virus, the causative virus of AIDS, was isolated from blood lymphocytes in 1983

It was recognized as belonging to the lentivirus (slow virus) group of retroviruses and related to similar agents in monkeys and to visnavirus in sheep and goats. The structure of the viral particle and its genome are illustrated in Figure 21.18 and its replication mechanism in Figures 21.19 and21.20.

Figure 21.18 The structure and genetic map of HIV. The rev and tat genes are divided into non-contiguous pieces and the gene segments spliced together in the RNA transcript. Occasional host proteins such as major histocompatibility complex (MHC) molecules are present in the envelope. (p) is protein and (gp) is glycoprotein. About 109 HIV-1 particles are produced each day at the peak of infection, and this, together with the low fidelity of reverse transcriptase, means that new virus variants are always appearing. Mutations are seen especially in env and nef genes. Any one patient contains many variants, and drug resistant and immune-resistant mutants emerge. There are also macrophage-tropic and T-cell-tropic populations of virus and syncytium-inducing and non-syncytium-inducing populations, with effects on disease progression. By genetic analysis, HIV-1 strains are subdivided into group M (most HIV-1 isolates), which contains at least 10 subtypes (A–J) differing in geographic distribution, and groups N and O (African). The degree of cross-immunity between these strains is not clear.

Figure 21.19 The HIV replication cycle. The virus enters the cell either by fusion with the cell membrane at the cell surface or via uptake into a vacuole and release within the cell.

Figure 21.20 Electron micrograph showing HIV budding from the cell surface before release.

(Courtesy of D. Hockley.)

Virus replication is regulated by at least six genes. The replication cycle is often halted after integration of the provirus so that the infection remains latent in the cell. The tat and rev genes function as transactivating factors, and can increase production of viral RNAs and proteins when latently infected cells are:

• stimulated to differentiate (e.g. Th cells by antigen)

• stimulated by infection with certain other viruses such as HSV or cytomegalovirus (CMV).

Human immunodeficiency virus infection probably started in Africa in the 1950s, and 35 million people worldwide were infected by 2009

The molecular biologic evidence (in terms of nucleic acid sequence) indicates that both HIV-1 and the closely related HIV-2 seen in West Africa probably arose from closely related primate viruses. HIV-1 is separated into three groups, namely M (main), N (new) and O (outlier). The M group comprises the HIV-1 subtypes A to J, with the N and O groups focused in western central Africa. The geographical prevalence of the subtypes differs, with subtype B being most common in North America and Europe, and the non-B strains such as A and C being found more frequently in Africa. However, with increasing travel the subtype distribution is changing and, together with the potential for mixed or superinfections, i.e. an HIV-infected individual becoming infected with another strain, and viral recombination events, other subtypes are being seen such as the circulating recombinant forms (CRF). These may be important in having different rates of disease progression.

HIV-1 may have been present in humans in central Africa for many years, but in the late 1970s it began to spread rapidly (Fig. 21.21), possibly with changed biologic properties, as a result of increased transmission following major socioeconomic upheavals and migrations of people from central to east Africa. Female prostitutes and male soldiers and workers travelling around the country played a major part. The disease soon appeared in Haiti and the USA, followed by Europe and Australasia.

Figure 21.21 Early spread of HIV infection (now worldwide). HIV-1 may have been present in central Africa for many years before increased migration and socioeconomic upheaval caused it to begin spreading in the late 1970s. Outside Africa, most infections occurred in men.

In the late 1980s, HIV began to appear in Asian countries, beginning with Thailand, and by 1995 explosive spread was based on heterosexual transmission, with high infection rates in female sex workers and transmission among users of injected drugs in Asia.

Worldwide by the end of 2009, about 35 million adults and children were infected with HIV including:

• 22.5 million in sub-Saharan Africa

• 5 million in South, South-East and East Asia

• 1.4 million in Eastern Europe and Central Asia

• 1.5 million in North America

• 0.8 million in Western and Central Europe.

Nearly 3 million people were newly infected in 2009, and 1.8 million died as a result of HIV infection that year.

In 2010, the UNAIDS global report stated that there had been a stabilization in the growth of the global AIDS epidemic. HIV incidence, that is, the number of new infections, had been falling since the late 1990s as had the AIDS-related mortality figures. It was estimated that when comparing the data from 1999 to 2009, the incidence had fallen in 2009 by 19% to 2.6 million newly infected individuals, having peaked in 1997. Over the same time period, there was a 25% fall in new HIV infections in 33 countries, 22 of which were in sub-Saharan Africa, but a 25% increase was seen in 7 other countries, including parts of North and Central Asia (source: http://www.unaids.org/globalreport/Global_report.htm).

Human immunodeficiency virus mainly infects cells bearing the CD4 cell surface antigen and also requires chemokine co-receptors

The HIV transmission route for more than 80% of adults involves mucosal surfaces, in particular cervicovaginal, penile and rectal. The remainder may be infected by intravenous or percutaneous routes. The window period for detecting the virus is 7–21    days, as HIV multiplies in the mucosa and draining lymphoreticular tissues. The first targets are CD4 receptor-bearing cells that include Th cells, Langerhans cells and other dendritic cells and microglia (Figs 21.2221.23). Monocyte-derived macrophages are not as good targets compared with the others. The CD4 molecule acts as a high-affinity binding site for the viral gp120 envelope glycoprotein. Productive replication and cell destruction does not occur until the Th cell is activated. Th cell activation is greatly enhanced not only in attempts to respond to HIV antigens, but also as a result of the secondary microbial infections seen in patients. Monocytes and macrophages, Langerhans cells and follicular dendritic cells also express the CD4 molecule and are infected, but are not generally destroyed, potentially acting as a reservoir for infection. Langerhans cells, for example dendritic cells in the skin and genital mucosa, may be the first cells infected. Later in the disease, there is a remarkable disruption of histologic pattern in lymphoid follicles as a result of the breakdown of follicular dendritic cells.

Figure 21.22 Scanning electron micrograph of an HIV-infected Th cell. (×    20 000).

(Courtesy of D. Hockley.)

Figure 21.23 Phases of infection following exposure to human immunodeficiency virus (HIV). Infection begins with transmission across a mucosal barrier, either by a cell-free virus, infected cell, or virion attached to dendritic cells (DCs) or Langerhans cells (LCs). Early low-level propagation probably occurs in partially activated CD4  + T cells, followed by massive propagation in activated CD4  + T cells of the gut-associated lymphoid tissue lamina propria. Dissemination of HIV to other secondary lymphoid tissues and establishment of stable tissue viral reservoirs ensue. Immune response lags behind the burst of viraemia and provides only partial control of viral replication. Abbreviations: CTL, cytotoxic T lymphocyte; PD-1, programmed death 1.

(Redrawn from Moir, S. et al: Pathogenic mechanisms of HIV disease, Annu Rev Pathol Mech Dis 6:223–248, 2011.)

HIV-1 enters host cells by binding the viral gp120 to the CD4 receptor and a chemokine co-receptor on the host cell surface. The CCR5 beta-chemokine receptor is important in establishing the infection. Those people with CCR5 gene deletions are resistant to infection. On the other hand, disease progression has been associated with HIV variants using the CXCR4 alpha-chemokine receptor. Cell susceptibility to infection is therefore affected by the levels of these chemokine co-receptors; for example, their expression may be up-regulated by opportunistic infections.

Productive infection of resting CD4 T cells in the lymphoreticular system of the gastrointestinal tract occurs. These cells express integrin receptors, viral attachment molecules, as well as Th cell surface markers, and HIV-1 infection rapidly expands with a rise in HIV-1 RNA levels at the same time as the irreversible depletion of reservoirs of Th cells. A latency state is soon established with the formation of persistent lymphoid tissue viral reservoirs.

At first the immune system fights back against HIV infection, but then begins to fail

During the first few months virus-specific CD8-positive T cells are formed and reduce the viraemia which is referred to as the HIV load. This is followed by the appearance of neutralizing antibodies. Even so, up to 1010 infectious virus particles and up to 109 infected lymphocytes are produced daily. Then the immune system begins to suffer gradual damage, and the number of circulating CD4-positive T cells steadily falls and the HIV load rises. Nearly all infected CD4-positive T cells are in lymph nodes. The cell-mediated immune responses to viral antigens, as judged by lymphoproliferation, weaken, whereas responses to other antigens are normal. Perhaps the virus initially engineers a specific suppression of protective responses to itself. Eventually, the patient loses the battle to replace lost T cells, and the number falls more rapidly. Skin test delayed-type hypersensitivity (DTH) responses are absent, natural killer (NK) cell and cytotoxic T-cell (Tc) activity is reduced, and there are various other immunologic abnormalities, including polyclonal activation of B cells. Functional changes in T lymphocytes – reduced responses to mitogens, reduced interleukin 2 (IL-2) and interferon-gamma (IFNγ) production – are also seen. As AIDS develops, responses to HIV and unrelated antigens are further depressed. The immune system has lost control. Plasma HIV-1 RNA load measurements have been shown to predict clinical outcome and are used in clinical management to help determine disease stage and progression as well as antiretroviral therapy response.

The exact mechanism of the immunosuppression in HIV infection is still unclear

The following factors need to be considered:

• Th cells directly killed by virus

• Th cells induced to commit suicide (apoptosis, programmed cell death) by virus

• Th cells made vulnerable to immune attack by Tc cells

• T-cell replenishment impaired by damage to the thymus and lymph nodes and by infection of stem cells

• defects in antigen presentation associated with infection of dendritic cells

• immunosuppressive virus-coded molecules (gp120, gp41).

The host response is further handicapped by the high rate of viral evolution assisted by the lack of a reverse transcriptase proofreading function. The virus exists as a quasispecies, in other words the infection comprises a number of heterogeneous strains. Some of the variants show resistance to currently circulating Tcs (i.e. are immune escape variants). Others show increased pathogenicity.

Before the advent of highly active antiretroviral therapy the immunosuppression was permanent, the patient remained infectious, the virus persisted in the body and death was due to opportunist infections and tumours.

HIV-2 appears to be transmitted less easily than HIV-1, probably because the viral load is lower, and the progression to AIDS is slower. HIV-2 is endemic in West Africa, and has spread to Portugal and parts of India.

Routes of transmission

In resource-rich countries, homosexual men have so far been the group most vulnerable to HIV infection and AIDS, especially the passive partner in anal intercourse. Infection is transmitted primarily from male to male and from male to female (Fig. 21.24), although not very efficiently compared with other STIs. Transmission from female to male, however, is a common and well-established feature of HIV in Africa and Asia. In randomly selected rural communities in parts of central and east Africa, up to 40% of the population is infected with HIV, mostly young adults.

Figure 21.24 Major routes of transmission of HIV. Although the heterosexual route of transmission has so far been well established only in resource-poor countries, there is evidence that this route is becoming more important in the resource-rich countries. IVDU, intravenous drug user.

Heterosexual transmission has not so far been as important in resource-rich as in resource-poor countries

One explanation for the greater heterosexual spread in resource-poor countries is that other STIs are more common, causing ulcers and discharges, which are sources of infected lymphocytes and monocytes. Genital ulcers are associated with a fourfold increase in the risk of infection. Also, viral strains from Asia and sub-Saharan Africa have been shown to infect Langerhans cells in genital mucosa more easily than do other strains. It is not clear whether HIV can infect males by the urethra or whether pre-existing genital skin breaks are necessary. As with other STIs, uncircumcised males are more likely to be infected.

HIV can also be transmitted vertically from infected mothers to their babies, but the infant is not infected in 55–85% of pregnancies, the upper limit being associated with avoiding breastfeeding. Overall, the infant is infected in about 20% of pregnancies in utero and intrapartum. The transmission rate peri- and postnatally is around 11–16%, the higher end of the range depending on whether the child has been breastfed for up to 24    months. In resource-rich countries, antenatal HIV screening, offering antiretroviral drugs during pregnancy and caesarean section delivery, avoiding breastfeeding, and giving antiretroviral drugs to the newborn infant have reduced the risk of HIV transmission to the child. In resource-poor countries it has been shown that giving one dose of one antiretroviral drug to both mother and child reduced HIV transmission by 47%.

By the end of 2009, there were around 370 000 children newly infected with HIV, a fall of 24% from 2004.

Nineteen per cent fewer children died from AIDS-related illnesses than the estimated 320 000 who died in 2004.

Haemophiliacs who have received contaminated blood products have also been infected, though less commonly, as well as have injecting drug users. As with other blood-borne virus infections, using contaminated needles can lead to infection, i.e. in tattooing, body piercing and acupuncture.

Finally, healthcare workers are at risk of HIV infection after sustaining needlestick or mucous membrane splash injuries involving an HIV-infected source. The risk of infection is approximately 1 in 400 and is dependent on a number of factors, including depth of the injury and amount of blood to which the recipient has been exposed. Wearing protective clothing such as gloves and goggles is part of universal precautions to avoid exposure.

Development of disease

Primary HIV infection may be accompanied by a mild mononucleosis-type illness

Signs and symptoms of the mild mononucleosis-type illness associated with HIV infection include fever, malaise and lymphadenopathy. A maculopapular rash may also occur. Antibody responses can be detected in a few weeks, and Tc cells are formed. The acute infection and rapid, widespread viral dissemination is followed by a chronic asymptomatic stage. Viral replication is reduced in line with the immune response, and the individual usually remains well. The duration of this stage is dependent on a number of factors including the viral phenotype, host immune response and use of antiretroviral therapy (Fig. 21.25). Infected cells are, however, still present, and at a later stage the infected individual may develop weight loss, fever, persistent lymphadenopathy, oral candidiasis and diarrhea. Further viral replication takes place until finally, some years after initial infection, full-blown AIDS develops (Fig. 21.26).

Figure 21.25 Kinetics of immunologic and virologic events associated with human immunodeficiency (HIV) infection during acute and early chronic phases. The schematic represents the sequence of events, including the appearance of viral antigens, HIV-specific antibodies, and HIV-specific CD8  + T cells during the acute and early chronic phases of infection. HIV reservoirs are established during the acute phase of infection soon after emergence of plasma viraemia. Throughout the acute phase of infection, characterized by massive virus replication and high levels of plasma viraemia, an acute HIV syndrome develops in the majority of infected individuals, and the virus rapidly spreads to various lymphoid organs, causing extensive depletion of CD4  + T cells. Although anti-HIV immunity, including virus-specific CD8  + T cells and antibodies, develops during the acute phase of infection, escape viral mutants rapidly emerge. Abbreviations: ELISA, enzyme-linked immunosorbent assay; PCR, polymerase chain reaction.

(Redrawn from Moir, S. et al: Pathogenic mechanisms of HIV disease, Annu Rev Pathol Mech Dis 6:223–248, 2011.)

Figure 21.26 The clinical features and progression of untreated HIV infection. CMV, cytomegalovirus; CNS, central nervous system; HSV, herpes simplex virus; PML, progressive multifocal leukoencephalopathy.

Progression to AIDS

Viral invasion of the CNS, with self-limiting aseptic meningoencephalitis as the most common neurological picture, occurs in early infection.

A progressive HIV-associated encephalopathy is seen in individuals with AIDS and is characterized by multiple small nodules of inflammatory cells; most of the infected cells appear to be microglia or infiltrating macrophages. These cells express the CD4 antigen, and it has been suggested that infected monocytes carry the virus into the brain, but the picture is complicated by the various persistent infections that are activated and give rise to their own CNS pathology. These include infections by HSV, varicella-zoster virus (VZV), Toxoplasma gondii, JC virus (progressive multifocal leukoencephalopathy, PML) and Cryptococcus neoformans.

HIV exercises complex control over its own replication (see Fig. 21.19). Replication is also affected by responses to other infections, which act as antigenic stimuli, and some of them directly as transactivating agents.

Some patients, especially in Africa, develop a wasting disease (‘slim’ disease), possibly due to unknown intestinal infections or infestations, and perhaps also to the direct effects of the virus infecting cells of the intestinal wall.

AIDS, symptomatic disease, consists of a large spectrum of microbial diseases acquired or reactivated as a result of the underlying immunosuppression due to HIV (Fig. 21.27Table 21.9). The disease picture of AIDS is therefore an indirect result of infection with HIV.

Figure 21.27 Opportunist infections and tumours associated with HIV infection. (A) Hairy leukoplakia – raised white lesions of oral mucosa, predominantly along the lateral aspect of the tongue, due to Epstein–Barr virus infection. (Courtesy of H.P. Holley.) (B) Extensive oral candidiasis. (Courtesy of W.E. Farrar.) (C) Kaposi’s sarcoma – brown pigmented lesions on the upper extremities. (Courtesy of E. Sahn.) (D) Pneumocystis pneumonia, with extensive infiltrates in both lungs. (Courtesy of J.A. Innes.) (E) Cytomegalovirus retinitis showing scattered exudates and haemorrhages, with sheathing of vessels. (Courtesy of C.J. Ellis.) (F) Cryptosporidiosis – electron micrograph showing mature schizont with several merozoites attached to intestinal epithelium.

(Courtesy of W.E. Farrar.)

Table 21.9 Opportunist infections and tumours in AIDS

Viruses

Disseminated CMV (including retina, brain, peripheral nervous system, gastrointestinal tract)
HSV (lungs, gastrointestinal tract, CNS, skin)
JC virus (brain – PML)
EBV (hairy leukoplakia, primary cerebral lymphoma)

Bacteriaa

Mycobacteria (e.g. Mycoplasma aviumM. tuberculosis – disseminated, extrapulmonary)
Salmonella (recurrent, disseminated) septicaemia

Protozoa

Toxoplasma gondii (disseminated, including CNS)
Cryptococcus neoformans (CNS)
Histoplasmosis (disseminated, extrapulmonary)
Coccidioides (disseminated, extrapulmonary)

Tumours

Kaposi’s sarcomab
B cell lymphoma (e.g. in brain, some are EBV induced)

Other

Wasting disease (cause unknown)
HIV encephalopathy

a Also pyogenic bacteria (e.g. Haemophilus, Streptococcus, Pneumococcus) causing septicaemia, pneumonia, meningitis, osteomyelitis, arthritis, abscesses, etc.; multiple or recurrent infections, especially in children.

b Associated with HHV-8, an independently-transmitted agent; 300 times as frequent in AIDS as in other immunodeficiencies. AIDS is defined as the presence of antibodies to HIV plus one of the conditions in this table. CMV, cytomegalovirus; CNS, central nervous system; EBV, Epstein–Barr virus; HSV, herpes simplex virus; PML, progressive multifocal leukoencephalopathy.

Before the advent of antiretroviral therapy, one study in New York reported a mortality rate of 80%, 5    years after the onset of the disease, and the average survival time after hospital admission was 242    days.

Treatment

Antiretroviral therapy results in a dramatic improvement in disease prognosis

In the 1990s, a range of antiretroviral therapies was introduced which included the nucleoside reverse transcriptase inhibitors (NRTIs), non-nucleoside reverse transcriptase inhibitors (NNRTIs) and protease inhibitors (PIs). These were developed further over the next two decades in terms of new drugs in all classes and combinations. In 2003, a fusion inhibitor was added to the list and by 2009 two other classes were available, an integrase inhibitor and chemokine receptor antagonist (see Ch. 33 for more detail). In combination with two NRTIs, the NNRTI or PI drugs have had a dramatic effect on progression to AIDS and led to the term highly active antiretroviral therapy (HAART). One drawback is the number of important side effects of the drugs, including mitochondrial toxicity and altered fat distribution known as lipodystrophy. Treatment compliance with certain drugs is a problem because of the side effects and the number and frequency of pills taken each day. This is important, as missing any doses can lead to the development of drug resistance, thus limiting treatment options. Improved monitoring using plasma HIV load measurements and CD4 counts and percentages has shown the success of HAART, with rapid falls in plasma HIV load and rises in CD4 cells seen after initiating therapy. HIV is found in various compartments of the body including the CSF and genital tract. Antiretroviral drugs may not penetrate these sites, resulting in a high viral load detectable in semen despite suppression of the plasma HIV load. As a result of improved diagnosis, surveillance, prevention and use of HAART, the number of AIDS-related deaths among children and adults worldwide was stable at 1.8 million in 2001 and 2009.

Development of antiretroviral resistance and cross-resistance is a feature

Plasma HIV-1 RNA load is a good indicator of viral replication, and failure of antiretroviral therapy is seen by a rise in viral load. Antiretroviral resistance testing and therapeutic drug monitoring are part of clinical management. Drug resistance testing may be carried out when the plasma HIV-1 load is not suppressed whilst on antiretroviral therapy. Specific mutations in the reverse transcriptase and protease regions of plasma virus, associated with reduced susceptibility to one or more antiretroviral drugs, have been identified by nucleic acid sequencing, known as genotypic analysis. Some drug resistance mutations confer resistance to more than one drug of the same class, whereas others appear unique to specific drugs.

Transmission of drug-resistant HIV is an important issue. Drug resistance mutations were detected in around 16% of samples tested between 2001 and 2006 from antiretroviral therapy naive adults infected with HIV in the UK. The prevalence of drug-resistant viruses in newly infected individuals will depend on factors such as whether the individual was infected by someone failing on antiretroviral therapy, this being less likely in those infected in resource-poor regions. Baseline antiretroviral drug resistance testing is part of the management guidelines in many countries before starting treatment, as infection with a drug-resistant virus may affect the efficacy of subsequent therapy.

Treatment of AIDS involves prophylaxis and treatment of opportunist infections as well as using antiretrovirals

Depending on the CD4 count, prophylaxis is given for specific opportunistic infections such as Pneumocystis jirovecii and Cryptococcus neoformans. When opportunist infections are diagnosed, they are treated appropriately, for example co-trimoxazole or pentamidine with or without steroids for P. jirovecii, ganciclovir for CMV, and fluconazole or amphotericin for C. neoformans infection.

Laboratory tests

Laboratory tests for HIV infection involve both serological and molecular analysis

Acquired immune deficiency syndrome (AIDS) is a clinical definition; in the presence of antibodies to HIV, any of the conditions listed in Table 21.9, regardless of the presence of other causes of immunodeficiency, indicate AIDS. The range and complexity of tests used for HIV-1 and -2 screening, diagnosis of infection, and monitoring disease progression and response to therapy have increased dramatically.

Viral replication occurs during the incubation period, during which time the viral genome and, briefly, viral p24 antigen but not the host’s antibody response may be detected. HIV-1 and -2 diagnostic tests can be divided into antibody detection, combined antibody and antigen detection, antigen detection, and genome detection. The last can be divided into qualitative HIV-1 proviral DNA and quantitative HIV-1 RNA detection. In addition, antiretroviral drug resistance assays are becoming part of standard management.

Initially, an HIV-1 and -2 antibody/antigen combination assay which includes antibody and p24 antigen is carried out (see Ch. 32). These assays have been developed to reduce the diagnostic window period. Assay reactivity is confirmed using an alternative HIV test format on the original unseparated sample stored in the laboratory. This is to ensure that a specimen separation error has not occurred. HIV type differentiation may be carried out using an immunoblot, where the antigens are coated on nitrocellulose strips. A positive result is confirmed on a further blood sample, to ensure that the original sample had not been mislabelled at collection.

HIV-1 RNA or proviral DNA tests may be carried out on plasma and whole blood samples, respectively, if it is difficult to make a diagnosis because low level reactivity is detected when the serum sample is tested and the result is indeterminate, or the patient may have a seroconversion illness and the screening tests are negative.

Part of monitoring HIV-1-infected individuals on or off antiretroviral therapy involves measuring the plasma HIV-1 RNA load, which can be quantified using several commercial or in-house assays using different methods. The main assay formats are based on reverse transcription polymerase chain reaction (RT-PCR), branched DNA signal amplification, and RNA transcription isothermal amplification.

In addition, part of the laboratory portfolio involves antiretroviral resistance genotypic analysis by automated DNA sequencing. This is a costly, specialized test, and the interpretation of the results may be complicated.

Diagnosis of HIV infection in newborn infants can be difficult because passively acquired maternal IgG will be detected in the first 12    months after birth. Reference laboratories may have virus-specific IgM and IgA in-house tests, which would signify in utero infection (see Ch. 23), as part of their test portfolio. Samples from infants are tested at various time intervals up to 12–24    months for p24 antigen, HIV-1 RNA and/or HIV-1 proviral DNA, and HIV antibody to assess their HIV status.

Measures to control spread

Many resource-rich countries have taken measures to reduce the spread of HIV

Up to 2010, the number of new HIV infections fell by nearly 20%. HIV prevalence fell by more than 25% among young people in 15 of the most severely affected countries after this group adopted safer sexual practices. In addition, 53% of HIV-positive pregnant women received treatment to prevent transmission of the virus to their child in 2009 compared with 35% in 2007. In resource-rich countries such as the UK compared with Africa and Asia, most new infections in 2009 involved men who have sex with men (MSM) (Figure 21.28).

Figure 21.28 Number of new HIV diagnoses1 by prevention group, UK: 2000–2009. Data are adjusted for missing route of infection. 2Includes mother to child transmission and blood product recipient.

(Data from Health Protection Agency Health Protection Report Volume 4 Number 47 Published on: 26 November 2010 HIV in the United Kingdom : 2010 Report.)

The risk of transmitting HIV via blood and blood products is reduced considerably by donor screening programmes, in addition to other blood-borne viruses, and heat treatment, respectively. Those at risk of infection are advised not to donate. Heat treatment of factor VIII is a further precaution before this product is used to treat haemophiliac patients. HIV has a delicate outer envelope and is highly susceptible to heat and chemical agents. HIV is inactivated under pasteurization conditions and also by hypochlorites, even at concentrations as low as 1 in 10 000    ppm; 2.5% glutaraldehyde and ethyl alcohol are also effective against the virus.

The main effort in the prevention of HIV infection concerns mass public education programmes. These involve advice to change sexual behaviour and the use of barrier contraceptives such as condoms. The problem of transmission between injecting drug users is being tackled in some areas by measures that were originally controversial, such as the free distribution of clean needles and syringes.

The importance of reducing transmission rates is discussed in Chapter 31. The biggest risk for the future in resource-rich countries is that heterosexual transmission becomes more common. Unfortunately, the determinants of heterosexual transmission are not understood, but the means for prevention are nevertheless clear – condoms and change in sexual behaviour. Public health educational programmes have been presented by various types of media in order to reduce the incidence of all STIs.

Vaccination

There are a number of challenges in developing a successful vaccine against HIV infection

More than 50 vaccine regimens have undergone clinical trials since 1999. However, only four transferred to test-of-concept or efficacy trials. The prospects are limited for a number of reasons including viral antigenic variation and sequence diversity, slow neutralizing antibody response to HIV infection, viral evasion of immune responses and establishment of latent viral reservoirs. Various subunit envelope glycoproteins, whole killed virus vaccines, plasmid DNA vaccines and virus vectors to carry HIV antigens have been investigated and tested. Trials have been carried out in animal (monkey) models and also humans.

The aim is to prevent infection or reduce the HIV load and clinical progression post infection. The immune correlates of protection have yet to be well defined and are critical in order to protect against infection. Two vaccine candidates involved in efficacy studies were an envelope gp120 protein vaccine that resulted in type-specific but not broadly reactive neutralizing antibody responses and a replication-incompetent adenovirus vector expressing HIV-1 gag, pol and nef gene products. The latter resulted in cellular immune responses in most recipients. However, the vaccine was neither protective nor reduced HIV loads post infection. The fact that there is a successful killed virus vaccine for a feline retrovirus (feline leukaemia), and that a similar vaccine protects monkeys from simian AIDS does, however, give some hope for the development of an HIV vaccine.

To prevent sexual transmission, mucosal immunity is needed, and this is likely to come from a mucosally administered vaccine. The major route of HIV-1 transmission is via mucosal surfaces. Worldwide, the cervical and vaginal mucosa are the major portals but the rectal mucosa is the more common route in North America and Europe. Penile foreskin increases the risk of HIV transmission due to the high density of HIV target Langerhans cells in addition to the inner mucosal surface not being keratinized. Circumcision has been shown to reduce the risk of transmission. Macaque SIV infection models have shown additional mucosal routes of entry, suggesting that HIV infection might also be transmitted through oropharyngeal and upper gastrointestinal mucosa. A T-cell vaccine would need to induce a long-lasting mucosal immune response that includes mucosal neutralizing IgA and IgG antibody and T-cell responses. Mucosal CD8  + CTLs would limit infection and subsequent HIV viraemia as well as clearance of viral reservoirs in the gut mucosa.

Opportunist STIs

Opportunist STIs include salmonellae, shigellae, hepatitis A, Giardia intestinalis and Entamoeba histolytica infections

Although STIs are classically transmitted during heterosexual intercourse, they can also be transmitted whenever two mucosal surfaces are brought together. Anal intercourse allows the transfer of microorganisms from penis to rectal mucosa or to anal and perianal regions. Gonococcal or papillomavirus lesions, for instance, may occur in any of these sites. A few microorganisms (hepatitis B, HIV) are transmitted more often across rectal mucosa. If there is oral–anal contact, a variety of intestinal pathogens are given the opportunity to spread as STIs and can then be regarded as ‘opportunistic STIs’. These include salmonellae, shigellae, hepatitis A virus, Giardia intestinalis and Entamoeba histolytica (see Ch. 22). Together with chronic infections such as CMV and cryptosporidiosis, they contribute to intestinal symptoms and diarrhea in AIDS patients.

Hepatitis B virus is often transmitted sexually

Hepatitis B virus is detectable in semen, saliva and vaginal secretions. HBV transmission, like HIV, is more likely when genital areas are ulcerated or contaminated with blood (e.g. menstrual blood). Hepatitis B transmission among MSMs parallels the transmission of HIV, with passive anal intercourse as a high-risk factor. Hepatitis D transmission can only follow hepatitis B as it is a defective virus that needs HBsAg to replicate. Hepatitis C is less commonly transmitted sexually; <    5% of long-term sexual partners are infected.

Arthropod infestations

Infection with the pubic or crab louse causes itching and is treated with permethrin shampoo

The ‘crab louse’, Phthirus pubis, is distinct from the other human lice, Pediculus humanus humanus and Pediculus humanus capitis. The crab louse is well adapted for life in the genital region, clinging tightly to the pubic hairs (see Ch. 6). Occasionally, hairs on the eyebrows or in the axilla are colonized. It takes up to 10 blood feeds a day and this causes itching at the site of the bites. Eggs called ‘nits’ are seen attached to the pubic hairs, and the characteristic lice, up to 2    mm long, are visible (often at the base of a hair) under a hand lens or by microscopy. Infestation is common, e.g. there are more than 10 000 cases/year in the UK.

Treatment is by the application of permethrin cream or aqueous phenothrin.

Genital scabies is also treated with permethrin cream

Sarcoptes scabiei (see Ch. 26) may cause local lesions on the genitalia, and can thus be sexually transmitted. Patients may have evidence of scabies elsewhere on the body, with burrows between the fingers or toes. Genital scabies is also treated with permethrin cream.

 Key Facts

• Microorganisms transmitted by the sexual route in humans include representatives from all groups apart from the rickettsiae and helminths.

• STIs are found in the general community rather than being confined only to high-risk groups.

• Genital herpes, warts, chlamydial urethritis, and gonorrhoea are by far the most common of all the STIs, but HIV infection has had a major impact, eclipsing all the other well-known STIs because it is usually eventually lethal.

• Except for hepatitis and human papillomavirus there are no vaccines for these infections, but antimicrobial chemotherapy is often available.

• At present, the best method of control is prevention.

• Transmission depends upon human behaviour, which is notoriously difficult to influence.

• Long intervals between the onset of infectiousness and disease increase the chances of transmission.