HIV Not Very Like Syphilis; Surprised?

If 80 or 90% of HIV transmission is through heterosexual sex in African countries, as UNAIDS says, patterns of infection might be expected to resemble those found for syphilis, at least to some extent. But data collected several years ago in Kenya suggests that syphilis prevalence is relatively low and that there are many differences in patterns of infection.

HIV is more difficult to transmit through penile-vaginal sex, yet prevlance rates are much higher than those  found for syphilis. But also, syphilis prevalence is similar among men and women. HIV prevalence is far higher among women than men in Kenya, overall, and more than five times higher in one large tribal group. Syphilis prevalence does not differ by urban/rural residence whereas HIV prevalence tends to cluster in urban areas. HIV prevalence in some rural areas is very low indeed.

While syphilis prevalence is highest among HIV positive men, HIV wasn't even found to be a significant risk factor for women. Syphilis prevalence tends to increase with age in men and women, whereas HIV prevalence tends to increase at a later age in men than in women, rising to higher prevalence rates among women than men, before dropping rapidly in older age groups among both men and women.

Syphilis prevalence is higher in Yaounde, Cameroon than it is in Kisumu, Kenya, although HIV prevalence in Yaounde is relatively low and in Kisumu it is very high. In South Africa, syphilis prevalence has declined far more rapidly than HIV in a similar period among antenatal clinic attendees.

Syphilis prevalence of over 10% has been recorded among Kenyan sex workers, among whom high STI rates could be expected. However, figures of 70-80% claimed for HIV prevalence among sex workers in the 1980s in Kenya and in the 1990s in Tanzania have never been recorded for syphilis, anywhere. In fact, in some countries HIV prevalence is not particularly high among sex workers unless they face additional risks, such as intravenous drug use.

Poorer men, and poorer women to a lesser extent, are more likely to have syphilis, as are men with lower levels of education. HIV prevalence tends to be higher among wealthier quintiles and among those who have higher levels of education, in Kenya, Tanzania and other countries.

It is worth bearing in mind that high prevalence of STIs does not mean that people necessarily engage in unusually high levels of unprotected sex. It does mean that health services, particularly sexual and reproductive health services, are inadequate.

I am concentrating on the differences between syphilis and HIV, but there are few remarkable similarities. I don't wish to deny that HIV is sometimes transmitted through heterosexual sex, just to question the extent of such transmission. Because, if transmission patterns are not very like those for syphilis, it would be a mistake to characterize HIV as an STI and design HIV prevention interventions accordingly.