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6 August 2001
How Safe Is Oral Sex?
by George Atkinson

Some time ago, researchers at the University of Texas Medical Branch at Galveston (UTMB) showed that, thanks to its low saltiness compared to other bodily fluids, saliva kills blood cells that find their way into the mouth, including those cells harboring the human immunodeficiency virus (HIV). That happens through osmosis as the blood cells suck in water from saliva and burst. Those experiments, reported in the February 8, 1999, Archives of Internal Medicine, explained why AIDS isn't spread by casual contact such as kissing or sneezing. Now, the same researchers have solved a related and paradoxical conundrum: If saliva is so protective, how is it that HIV can be spread through breast-feeding and oral sex?

For some time, epidemiologists (scientists who study the behavior of large groups of people) have observed that oral consumption of HIV-infected breast milk and semen promotes transmission of the AIDS virus. In new experiments, the UTMB researchers tackle the question, "Why doesn't saliva disable the HIV-infected blood cells in those two substances?"

The short answer, which appears in the February 2000 issue of the Journal of Infectious Diseases, is that milk and semen, being much saltier than saliva and so much greater in volume, reduce or eliminate saliva's beneficial effects. (Mother's milk and semen, like other bodily fluids that support cells, are seven times saltier than saliva.)

The researchers came to their conclusion via test-tube experiments in which they added HIV-infected white blood cells to mother's milk and semen obtained from non-infected donors. Next the scientists immersed the infected substances in a solution that was 90 percent saliva and 10 per cent the saltier milk or semen. This was followed by other experiments in which they incrementally reduced the percentage of saliva and boosted the percentage of the saltier substances to see what happened to the infected cells.

"When saliva was reduced to only one-third of the mixture, with the remaining two-thirds being the saltier substances, then saliva was no longer protective against HIV," says Samuel Baron, professor of microbiology and medicine at UTMB and principal author of the study. "Under normal circumstances there is just about one-fifth of a teaspoon of saliva in the mouth. Deposited semen typically would equal more than four times that volume and mother's milk much, much more. So it's easy to see that saliva can't kill the infected cells because there's just too much of the saltier substances."

Embedded in the study are novel assumptions that amount to what Baron calls "a paradigm shift for some people" in understanding how the AIDS virus causes infection. Baron and his fellow authors Joyce Poast, C. Joan Richardson, Derrick Nguyen and Miles Cloyd, all at UTMB, think that virus inside infected blood cells is what causes infection.

"There is very good evidence that infected white blood cells are the main transmitting agent of HIV in the vagina and the rectum," Baron continues. "But most physicians still believe that HIV is transmitted by free virus, that is, virus found outside cells that appears, for instance, in mucosal secretions such as milk and semen. In fact, however, in people infected with HIV, most cell-free virus is not infectious, probably because HIV carriers make sufficient antibodies, which bind to the virus and hamper its ability to infect other cells."

"In contrast," Baron continues, "HIV-positive milk and semen are highly infectious, probably because these fluids contain white blood cells that are infected by HIV and the antibodies can't get inside these cells to attack the virus."

As an outgrowth of the new study, Baron and his colleagues are now working to develop a gel that might stop some of the estimated six million transmissions of HIV that occur every year, most sexually. The idea is to mimic the protective effect of saliva in two environments that are normally saltier than the mouth: the vagina and rectum. The only related substance now available is a spermaticide called nonoxinol-9. But although that product works in tissue cultures and laboratory animals, Baron says, it has not proved protective in human beings, probably because it irritates human tissues. That irritation, he says, may create a portal for the AIDS virus to enter the body. "Perhaps this new understanding of how saliva naturally protects against casual transmission of HIV, and how infected milk and seminal fluid overcome that protection," Baron says, "may lead us to develop products to protect the susceptible vagina and rectum."

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