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Editorial Comment

Sexually transmitted diseases

Pozniak, Anton L.

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Current Opinion in Infectious Diseases: February 2000 - Volume 13 - Issue 1 - p 27-28
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Since the invention of the microscope, nothing has revolutionized the field of microbiology as much as genomic sequencing. It is just five years since the first bacterium had its chromosome mapped, and soon the ultimate gene sequence, the human genome, will be unravelled.

Syphilis, which is especially common in parts of Africa and Asia, remains a global health problem. There has recently been a major rise in its incidence in the former Soviet Union and, because of foreign travel and immigration, an increase in cases has been seen in other European countries. This resurgence of syphilis has stimulated a heightened interest in developing a vaccine. Syphilis has one of the smallest bacterial genomes with just over a million base pairs, and sequencing its genome was completed about 18 months ago. Treponema pallidum, the syphilis organism, has the greatest similarity overall with the organism responsible for Lyme disease, Borrelia burgdorferi, another spirochete. Even more interesting is how similar yet different the genes of the various treponema species are. Sequencing a bacterium's genome doesn't only satisfy phyllogenetic and other scientific curiosity, and Norris and Weinstock (pp. 29-36) review the potential clinical applications of knowing the gene sequence of T. pallidum, including the identification of new antigen targets for vaccines. Such antigens may also be the basis of immunodiagnostic tests. Unfortunately, before genomic sequencing there had been problems in identifying T. pallidum outer membrane proteins, which may be important in developing a vaccine. Now their identification may be simplified by locating genes which code for them. The future remains intriguing for what must be one of history's most notorious organisms.

It comes as a surprise to many physicians who do not specialize in sexually transmitted diseases that trichomonosis is the most common non-viral sexually transmitted disease in the world. It is estimated that 167 million people become infected annually and up to half of these will be asymptomatic. The old belief that trichomonosis was mainly a disease of women is now being challenged with the use of polymerase chain reaction techniques. These suggest that men are probably as equally infected as women. Lehker and Alderete (pp. 37-45) review the biology of this little appreciated organism and point out that further treatment options for this organism are now needed as relapses are often due to metronidazole resistance. Several new drugs based on nitroimidazole compounds and some old drugs such as disulfiram have shown some effect in vitro. Understanding the biology of this organism, however, may lead to the design of new drugs, especially those that might inhibit Trichomonas vaginalis thymidine kinase. There seems to be much heated debate regarding the evolutionary relationship of this organism in comparison with other eukaryotes. This debate has highlighted the unique biochemistry of trichomonosis and may allow targets for therapy to be developed. Even with new therapies it will be difficult to eradicate trichomonosis without huge specific worldwide control programmes. If a vaccine were developed this would require integration of immunization into existing health strategies.

A reduction in teenage pregnancies has become a priority for the UK government. This is not just an issue for this small island alone as rates in the USA are even higher. Unprotected intercourse not only exposes teenagers to the risk of pregnancy, but also to sexually transmitted infection. (James, pp. 47-51). Sex education in schools is often not standardized in its quantity and quality and an integrated approach for teenagers with school based or other community health clinics might improve access to care. However, there is a perception that many of the barriers to developing such novel approaches come from school governing bodies and parents rather than the pupils themselves.

The revolution in diagnostic testing for sexually transmitted infections can be used to great advantage for teenagers, especially for gonorrhoea and chlamydia when molecular techniques can be used. Other molecular tests are being developed which may reduce the need for pelvic examinations or urethra swabs for diagnoses. Such a testing strategy may perhaps increase the likelihood that teenagers would access clinic services. As the highest prevalence of chlamydia is in those under 20 years-of-age there has been a suggestion that twice yearly screening of sexually active adolescents should be undertaken. If this is the case it appears sensible that screening for all sexually transmitted infections rather than specific organisms should be undertaken. A one shot treatment for chlamydia and gonorrhoea which can be directly observed may also help make services more attractive to adolescents as they don't need to hide pills from their parents or peers. This approach also gives healthcare workers an opportunity to reinforce positive health messages for this hard to reach group.

Postexposure prophylaxis (PEP) (Hawkins, pp. 53-57) has moved on considerably since the first case controlled study in the mid-1990s. Although azidothymidine (AZT) has been used since soon after its appearance in 1987, for serious occupational exposure by needlestick most of the current guidelines recommend triple therapy. An interesting comparison suggests that the efficacy and tolerance of AZT to prevent maternal fetal transmission is similar to its use in preventing PEP. Whether single doses of drugs such as those seen in the maternal transmission HIV NET 012 trial would be sensible for PEP requires further study.

The major problems with most needlestick protocols used today include the assessment of the severity of the needlestick, starting the antiretrovirals in time and remaining on them once they have been started. We now have the spectre of HIV drug resistance virus, which has been transmitted from patient to patient and any potential PEP regimen may need to be based on local resistance prevalence data. The thorny issue of PEP after sexual exposure or injecting drug use exposure is bravely discussed by Hawkins (pp. 53-57). He makes the important point that any perceived benefit of PEP in such circumstances should not reduce safer sex behaviour. Whether effective guidelines can be introduced around post sexual exposure will be a great challenge for physicians and government alike.

© 2000 Lippincott Williams & Wilkins, Inc.