Kiss and Tell
Is low salt key to saliva's safety?

Is a kiss safe when it comes to HIV? Based on years of research, the answer has been yes, with some caveats. Past studies show that proteins in saliva appear to neutralize or disable the HIV virus by two- to five-fold, limiting oral (mouth-to-mouth) transmission of HIV. That doesn't completely rule out any risk: Deep kisses are trickier if either partner has bleeding gums or cuts in the mouth, where a tear in the mucosal wall might facilitate transmission. So prevention activists still urge people to be careful when kissing if either partner has bleeding gums or sores on their lips or inside their mouths. HIV-positive people are also urged to fill any dental cavities they may have, since HIV may hide out in these pockets.

Now, according to a small study involving six HIV-negative people conducted by Dr. Samuel Baron and colleagues at the University of Texas Medical Branch at Galveston, there's new evidence to back a provocative theory regarding a more powerful protective effect from saliva that has nothing to do with enzymes. Baron's test-tube studies found that saliva, due to its low level of salt, inactivates more than 90 percent of the HIV-infected leukocytes (blood cells) that may be the main transmitter of HIV on mucosal surfaces, thereby blocking HIV transmission by more than 10,000-fold.

That's because salt is needed to keep cells alive, explains Baron, who says that if you put red blood cells (that contain salt) in a glass of water, for example, they quickly pull in the liquid, swell up, and burst. The absence of salt in saliva causes HIV-infected cells to do the same thing. And that's why kissing appears so safe in many instances. Saliva, concludes Baron, "is breaking down the infected cells." His group presented its newest data, which will be published early next year in the Archives of Internal Medicine, at the recent Institute of Human Viro-logy meeting in Baltimore.

What about oral sex or breast-feeding? Both are epidemiologically linked to cases of transmission, based on case reports. Breast milk is considered enough of a risk factor that the World Health Organization, amid controversy, recently recommended that women in the developing world stop breast-feeding to lower the rate of HIV transmission to their newborns. Oral sex sparks equal debate, and this newest data may fire things up.

According to the Texas researchers, test-tube studies show that breast milk, colostrum (pre-milk breast fluid in nursing women), seminal fluid, and balanced salt solution (a solution of salts present in the body) may-by replacing the salts missing in saliva and allowing the infected cells to survive-"overcome the protection by saliva in vitro." So semen and breast milk, which have a higher level of salt than saliva, effectively reverse saliva's ability to kill HIV-infected cells. By itself, saliva may stop the virus-infected cells, but if semen or breast milk is also present, it won't, says Baron.

"If you add the infected cells in the presence of a large amount of blood, semen, or breast milk and then add saliva, those three fluids protect those infected cells from being killed by saliva," he states, choosing his words carefully. "That coincides with the epidemiological evidence that those three fluids can transmit HIV. But in the absence of those three fluids, there appears to be only rare transmission of HIV."

Baron's theory is based on previous reports suggesting that HIV-infected cells, not free virus, may be the main culprits responsible for transmitting HIV. A lot of researchers disagree with that theory. In its work, Baron's group found that saliva targets HIV-infected cells but weakly inhibits the free virus that can also float outside cells in such body fluids as semen (though in smaller numbers than in virus found in infected cells). Baron is the first to admit his study is based on very small numbers and needs to be verified by other people through careful, controlled studies. "It challenges some preconceptions," he admits. His critics say it's too simple to believe.

What about vaginal fluid and the risk of mouth-to-vagina sex? Un-fortunately, Baron's group hasn't looked at that, but he says it will. Like seminal fluid, vaginal fluids contain salts, which suggests that they might also block saliva's ability to attack HIV-infected cells. "That is a prediction that needs to be tested," states Baron. "We plan to do so." He also thinks the volume of fluids containing salt might play a role in HIV transmission.

Looking ahead, a number of researchers are examining various compounds that might work like saliva to kill infected leukocytes and provide protection against HIV. Baron is testing bile salts, a natural, cheap agent that he says is "100 times more effective than saliva" in attacking the membrane of a cell. What about plain water itself? Could a vaginal or rectal douche after sex reduce one's risk of HIV infection? "We simply don't know," admits Baron, who is nevertheless dubious about the idea. "Large volumes of water would have to reach all the crevices where you'd find HIV-infected cells, and that seems unlikely."

For now, he urges caution in drawing premature conclusions about the degree of HIV risk associated with oral sex. Practicing safer sex, he says, is still your best bet.

That, and lots of safe and tender kisses.

-Anne-christine d'Adesky