Wednesday, February 16, 2011

Researchers Seek Answers But Reject the Bits they Don't Like

The fact that very high HIV prevalence is found in some countries and not in others is usually explained in terms of differences in rates of sexual behavior, especially 'unsafe' sexual behavior. However, it has never been demonstrated that rates of sexual behavior, unsafe or otherwise, really are higher in areas where HIV prevalence is high.

A recent article on herpes simplex virus (HSV-2) is a case in point (HSV-2 Among Bar and Hotel Workers in Northern Tanzania, Kapiga, Sam, Shao et al). A group of bar and hotel workers in Moshi, Tanzania, were studied and HSV-2 prevalence was found to be 43.5%. In the US, 22% of people aged 12 years or over are infected with HSV-2. Rates of unsafe sex probably explain high HSV-2 prevalence found in both countries, but that doesn't explain low HIV rates in US, nor high rates in Tanzania.

These two HSV-2 figures may look very different until you consider that unsafe sexual behavior is thought to be more common among bar and hotel workers in African countries than it is among the general population. HSV-2 prevalence increases with age, also, so prevalence among a comparable group in the US could be expected to be a lot closer to prevalence found in the Tanzanian group.

HSV-2 is almost always transmitted sexually. HIV is sometimes transmitted sexually, though heterosexual sex is an inefficient route of transmission. HIV is far more efficiently transmitted through various non-sexual routes, such as unsafe health care, especially through contaminated injecting equipment.

The relative contribution of sexual and non-sexual transmission of HIV to high prevalence HIV epidemics, such as those found in some African countries, is not known. The assumption that HIV is almost always transmitted sexually in African countries is, therefore, without foundation. And the figures for HSV-2 prevalence in the US and Tanzania suggest that rates of unsafe sexual behavior are similar, so difference in HIV prevalence is probably due to non-sexual factors.

The problem is that high levels of unsafe sexual behavior in (some) African countries are inferred from the fact that HIV prevalence is high and low levels of unsafe sexual behavior in Western countries are inferred from the fact that HIV prevalence is low. Yet, rates for some types of unsafe sexual behavior, age at sexual debut, number of sexual partners, overlapping sexual partners, etc, are probably very similar in both African and non-African countries. Not a lot of research has focused on comparing the two.

The paper studying HSV-2 shows that condom use is low and inconsistent, so it's not surprising that rates of sexually transmitted infections are high. But, while consistent condom use may protect against sexual HIV transmission, it does not protect against non-sexual HIV transmission. Therefore HSV-2 patterns in Moshi were, in many respects, quite different from HIV patterns.

For example, women were 4 times more likely to be infected with HSV-2 than men, prevalence standing at 53.1% and 29.2%, respectively. But women were almost 10 times more likely to be HIV positive than men. HSV-2 increases the probability of transmitting HIV and of being infected with HIV, but because one is almost always sexually transmitted and the other is only sometimes sexually transmitted, infection patterns remain distinct.

In Tanzania, HIV prevalence is higher among Christians than among Muslims. The opposite is true of HSV-2, which has a 50% lower risk among Catholics (the finding was not significant among non-Catholics). Rates of unsafe sexual behavior are also found to be higher among Muslims (multiple partnerships and concurrent relationships), which could explain higher rates of HSV-2 but only explains lower rates of HIV if you accept that HIV must also be transmitted non-sexually.

The complex relationships between levels of education and HSV-2 and HIV prevalence, respectively, are also quite different. The authors of this paper interpret both diseases purely in terms of sexual transmission, which muddies the waters. But while HSV-2 rates go down with increasing education among women, HIV rates go up.

The authors speculate about possible correlations between relative levels of income/wealth and HSV-2 transmission but don't produce any relevant figures. This is a pity because unsafe sexual behavior may well be higher among females with lower incomes but HIV rates in Tanzania are often higher among wealthier women.

The age patterns for HSV-2 and HIV prevalence are also quite different, with HSV-2 rates rising with increasing age. HIV rates in women are much more closely connected with pregnancy and childbearing, which carry increased levels of non-sexual risk. HIV rates among men start to rise later in life and peak somewhat later than among women.

Also HSV-2 is rare in non-sexually active people, especially the very young. But rates of HIV can sometimes be alarmingly high among these groups. Prevalence among male and female children are quite similar, suggesting similar levels of risk. HIV rates in adult females are almost always higher than in males in high prevalence countries (and almost always far lower than in males in low prevalence countries).

The researchers were surprised to find that women who had uncircumcised sex partners had a 50% reduction in HSV-2 risk. The Tanzanian government is currently considering circumcising 2.8 million men because they have been convinced that it will reduce HIV transmission from women to men. So it might be worth doing some unbiased research before carrying out such a risky intervention.

So the article may tell us a lot about HSV-2 and sexual risk, and might even shed some light on sexually transmitted HIV. But it assumes that HIV is, like HSV-2, a sexually transmitted infection, which is certainly not accurate and may even be dangerously inaccurate.

The article gives little insight into why HIV prevalence is so high in some parts of some African countries. Yet the authors purport to find implications for HIV prevention. But such implications are unlikely to be significant or helpful unless the relative contributions of sexually and non-sexually transmitted HIV are also quantified.


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