HOW DANGEROUS IS ORAL SEX?

By Christine Gorman

Gay men tend to worry a lot more about the health hazards of anal sex than oral. Everybody knows, after all, that it's much easier for the virus that causes AIDS to cross the lining of the rectum than to infect someone through the mouth. Or is it? The surprising results of a study on rhesus monkeys published last week in Science not only suggest otherwise but also underscore how little scientists know about how, at the microscopic level, HIV spreads from one person to the next.

In the study, scientists from Boston's Dana-Farber Cancer Institute and Tulane University tried to infect 15 sedated monkeys with SIV, the simian cousin of the AIDS virus. To simulate oral sex, researchers dribbled an SIV solution onto the tongues of seven animals. Then, for comparison, they carefully placed SIV in the rectums of eight other monkeys. Much to their surprise, they found that it took less of the viral solution to infect a monkey orally than rectally--6,000 times less.

So what do these findings have to say about people? Exact correlations are impossible to make. SIV and HIV, although similar in many respects, are different viruses. And the scientists did not try to create the tiny tears in the lining of the rectum like those that are produced during anal sex and that increase the chance of HIV infection in humans. But generally speaking, the results support the idea that the number of HIV particles found in an infected man's semen--though not in the saliva--is sufficient to be passed on through the mouth or throat. One likely route: the tonsils, which contain large numbers of the kinds of lymph cells favored by HIV and SIV. "We're not saying that oral exposure is more dangerous than anal exposure," notes Dr. Ruth Ruprecht of Dana-Farber. "What we're saying is that oral sex is not safe."

Public-health officials had already come to the same conclusion. A number of epidemiological studies have shown that anal sex was the principal means by which HIV spread through the homosexual community in the 1970s and '80s. More recently, however, as more and more gay men adopted "safer" sex techniques, reports began surfacing of people who had become infected with HIV even though they had engaged in oral sex only. Erring on the side of caution, the federal Centers for Disease Control and Prevention in Atlanta added unprotected oral sex to the list of unsafe sex practices several years ago. "From our standpoint, the study reinforces the evidence we've collected over the years," says Dr. Helene Gayle, an HIV-prevention expert at CDC. "Just as in any other means of sexual contact, people have to understand what the risk is and take the appropriate precautions." In other words, it's not enough to switch to oral sex; it's best to use a condom too.

--By Christine Gorman. Reported by Alice Park/New York

With reporting by Alice Park/New York