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Although unprotected receptive anal sex has long been shown to be the highest-risk sexual activity for men who have sex with other men, a study in the July 1 edition of The Journal of Infectious Diseases suggests that the risk to the insertive partner'or 'top''may be much higher than originally thought. Researchers gauged levels of HIV RNA in the blood, semen, and rectal mucosa secretions of 64 HIV-positive gay and bisexual men visiting sexually transmitted disease clinics in Seattle and Lima, Peru. Surprisingly, HIV was detected at much higher levels in the rectal secretions than in either blood or semen. HIV was even present in the rectal fluids of men who were suppressing blood-based viral loads to undetectable levels through highly active antiretroviral therapy, according to the study. About half of the men studied were on HAART regimens; the remainder were treatment-naive. The median viral load detected in all the study subjects' blood was 17,400 copies; the median semen-based viral load was much lower, only 3,550 copies. But the median viral levels detected in rectal mucosa secretions'at 91,200 copies'was more than five times higher than that seen in blood and more than 25 times higher than detected in semen. The viral load pattern held true for study subjects taking anti-HIV medications and for those who were not on antiretroviral regimens. The data suggest that HIV-positive receptive anal sex partners'or 'bottoms''may be putting their partners at a higher risk for infection than originally thought, especially when engaging in unprotected sex. The risk might be particularly high for insertive partners who are uncircumcised, since previous studies have shown that the Langerhans cells, located on the foreskin, are especially susceptible to HIV infection and that semen, rectal secretions, and vaginal fluids can be easily trapped under the foreskin and placed into prolonged contact with the cells. They also conclude that HIV viral levels in rectal mucosa may not be directly affected by the use of anti-HIV drugs, as are blood- and semen-based levels. This 'may be due to differential levels of antiretroviral drugs or to anatomic and immunologic differences in the male genital tract versus rectal mucosa,' the researchers say.