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New findings may redefine high-risk sex.

Researchers who set out to study mother-to-infant AIDS transmission in monkeys have, much to their own surprise, dragged a nettlesome AIDS question into public view and challenged definitions of "high-risk" sexual activities that have stood since the start of the AIDS era 15 years ago.

The work began with an attempt to determine whether infant monkeys become infected at birth by ingesting blood from infected mothers. That led to a follow-up study suggesting that oral intercourse in humans may be riskier than the anal variety-contrary to a mountain of reports that identify unprotected anal intercourse as the likeliest route to an AIDS-abbreviated life. The investigators are quick to point out that their findings come from a study of simian immunodeficiency virus (SIV) in macaques, not of HIV, the AIDS-causing virus, in people. Moreover, unlike sex, exposure in the lab does not cause the tiny abrasions that can convey the virus to the bloodstream. The researchers report in the June 7 Science that the minimum SIV dose required to infect a monkey by mouth was 6,000 times lower than the dose needed to infect via the rectum. Six of seven full-grown macaques have become infected in this way, say the researchers, who used SIV because monkeys are essentially unaffected by HIV. Two of the six have died of the simian version of AIDS.

Ruth M. Ruprecht of the Dana-Farber Cancer Institute in Boston and her colleagues conclude that "unprotected receptive oral intercourse should be added to the list of risk factors for HIV-1 transmission." Although researchers at the Centers for Disease Control and Prevention (CDC) in Atlanta acknowledge that "such sexual activity does carry a risk of HIV-1 transmission," they argue in a 1996 supplement to the journal AIDS that the risk "is substantially less than the risk of transmission during unprotected penile vaginal and penile anal sex."

That conclusion arises from studies of the self-reported sexual practices of people have who have become infected. Though imperfect, such studies provide the best information available in humans.

Case reports suggest that 17 people worldwide-three to five of them in the United States-have acquired HIV through oral sex. "I think our animal study should serve as a warning in light of the human case reports that oral sex is not safe sex," Ruprecht says.

"We can agree with that-low-risk doesn't mean no-risk," says Scott D. Holmberg, a CDC epidemiologist who studied one such case. "But we need to be careful about drawing conclusions based on macaque experiments." Ruprecht and her team became intrigued with the question of oral transmission of SIV when they found that infant monkeys become infected by swallowing blood during birth. The team theorized that newborns lack sufficient stomach acid to disarm the virus.

To test that idea, they dropped SIV on the tongues of three adult macaques, two of which had been given antacids. When, contrary to expectation, all three became infected, the team tested seven monkeys to find the lowest dose of SIV needed to infect the monkeys orally and rectally. The higher efficiency of oral infection was a "surprise because, in humans, anal receptive intercourse has been recognized as the most dangerous activity," says Ruprecht. It is difficult to establish any link between oral sex and AIDS in humans because it is hard to find people who have only oral sex. Researchers have also found a protein in human saliva called secretory leukocyte protease inhibitor-a natural cousin of the new AIDS drugs on the market-that guards against HIV infection (SN: 2/18/95, p. 108).

Ruprecht and her colleagues have not studied this protein in macaques, nor have they tried to determine whether SIV-infected cells, which shed virus, are as infectious as the cellfree virus they used. Other questions concern the cells that SIV infects and the route by which it enters the monkey's system. "We need to do a lot more work," Ruprecht says. "We have raised more questions than this study can answer."
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Author:Sternberg, Steve
Publication:Science News
Date:Jun 8, 1996
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