Sexually transmitted infections and HIV – a complex interaction

A fall in syphilis rates is not always good news, if the findings of South African researchers, presented last week at the 13th ICASA meeting in Nairobi, Kenya are to be believed.

In much of Africa, where syphilis has been widespread compared to other continents, rates of the disease have fallen in recent years.

In South African antenatal clinics, where syphilis blood tests are routine, the fall was particularly dramatic – from 11.0% in 1997 to 2.8% in 2001, at the same time as HIV rates shot up.

There could obviously be several explanations for this – such as behavioral interventions changing behaviour, improved management of STIs with the introduction of syndromic management, or even changing criteria for diagnosing syphilis. However, when detailed mapping was carried out in South Africa, this showed that the falls were concentrated in the populations worst affected by AIDS, both geographically and in terms of age. After rejecting those other explanations, the conclusion was that AIDS is selectively disabling and killing those people most likely to be infected with and/or passing on syphilis to their partners – a so-called “core group” for the disease (Prerna).

This raises a question about an association seen earlier in Uganda, between a decline in syphilis and a reduction in HIV prevalence seen in antenatal clinics. Could it be that both declines had as much to do with death rates among HIV positive women, as with behaviour change towards safer sex?

An additional possibility is that because HIV positive women are less likely to become pregnant, population-based testing carried out in antenatal clinics becomes less representative of women as the HIV rate increases and women with HIV suffer more illness The fertility reduction factor is real and estimated at around 15% when pregnancy rates among HIV positive and negative women are compared. This means that rates of HIV in antenatal clinics can sometimes underestimate the rate of HIV among women, even though women in antenatal clinics may for obvious reasons be more likely to have had unprotected sex than non-pregnant women (Fabiani).

Syphilis and other sexually transmitted infections are often linked to HIV transmission, and it is now recognised that this is more than coincidence or linkage through shared behavioural risk factors. Inflammation due to STIs increases the extent to which HIV is shed into semen or vaginal fluids, and it has also been thought that this inflammation could make an HIV negative person with an STI more vulnerable to becoming infected when exposed to the virus.

However, a double-blind placebo-controlled clinical clinical trial in which HIV negative female sex workers in Nairobi, Kenya, were given monthly antibiotic treatment to prevent STIs did not alter their risk of getting HIV, even though treatment did reduce the number of cases of gonorrhoea, chlamydia and trichomoniasis (Moses).

This suggests that the most important effect of STIs on transmission of the virus is when the STI increases the infectivity of a person with HIV, and that interventions to improve the diagnosis and treatment of STIs among HIV positive people would be of the greatest value in reducing HIV infection.

When HIV treatment is introduced, the effect could be to increase the risk of syphilis and other STIs being transmitted in the community since the people who would otherwise have died or have been to unwell to transmit those disease may be at increased risk of passing them on. Maintaining, or better, strengthening STI control may therefore be particularly important in preventing the transmission of drug-resistant viruses.

A practical way to implement such a strategy is by adding STI screening and treatment to the service provided by HIV VCT programmes. A Ugandan study looked at the cost effectiveness of doing this for syphilis, and found it could be a very worthwhile investment (Kalule).

Sexual transmission of HIV is not the only risk from sexually transmitted infections. Mother-to-baby transmission may also be increased if an HIV positive mother has an untreated STI, and so the long-established case for screening and treatment of STIs in antenatal clinics is further reinforced when women are HIV positive (Imade).

Finally, it is no good treating women for STIs if they are only going to be reinfected by untreated regular male partners, so the need to include men in screening and treatment programmes is clear.