that the risk may be as low as 0.1 percent per episode of intercourse. Female-to-male transmission by intercourse is less efficient than male-to-female spread (Mastro et al., 1994; Haverkos and Quinn, 1995). Reasons for the greater susceptibility of women may include greater trauma to the genitalia and the vaginal epithelium in women during intercourse and longer exposure to HIV when infected ejaculate is retained in the vagina. Similar mechanisms may account for greater female susceptibility to non-ulcerative STDs, such as gonorrhea, chlamydia, and trichomoniasis (Wasserheit, 1992). Studies of HIV transmission among heterosexual couples have demonstrated a seroprevalence of 20 to 50 percent among originally discordant couples; in two African studies, annual seroconversion rates among discordant couples were on the order of 4 to 9 percent per year (Allen et al., 1992; Serwadda et al., 1995). From European data, factors that appear to increase heterosexual transmission include and-receptive intercourse, more advanced illness in the male partner, and history or presence of STDs (de Vincenzi, for the European Study Group on Heterosexual Transmission of HIV, 1994).

It is generally accepted that homosexual contact between men is a minor route of HIV transmission in sub-Saharan Africa, given that such behavior is reported only rarely in the region (see Chapter 4). Although homosexual transmission may be rare, underreporting of such behavior may be common because homosexuality is highly stigmatized in most African societies. However, when homosexual sex does occur, male-to-male transmission may play a part in sustaining the epidemic; there is little reason to think that the risk factors would not be similar to those in Western countries, where anal-receptive intercourse appears to be a primary risk factor for homosexual transmission of HIV. Similarly, other studies among male homosexual populations have demonstrated an association between HIV-1 and other STDs, such as syphilis and herpes.

HIV and STDs

A growing body of data suggests that HIV transmission cannot be considered in isolation from the classical STDs, principally syphilis, gonorrhea, chlamydia, chancroid, trichomoniasis, and herpes simplex virus type 2 (HSV-2). HIV shares modes of transmission and behavioral risk factors with these other STDs. More important, there is evidence that classical STDs may increase susceptibility to and transmission of HIV; their control may thus serve as an important element in curbing the HIV epidemic. Thus, the following sections on sexual transmission, as well as some subsequent sections on behavior and care seeking, refer to both HIV and the classical STDs.

Prevalence of STDs in Africa Although accurate determination of the prevalence of STDs in Africa is hindered by a lack of population-based data and adequate surveillance, existing information suggests that infection rates are very

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