Can oral sex be hazardous to one's health? Specifically, mightputting the AIDS virus where the mouth is cause infection anddisease?

These far-out, offbeat questions get a crisp answer in today's Science.The apposite research report bears the title: "Infection and AIDS inadult macaques after nontraumatic oral exposure to cell-free SIV."

Retrovirologist and clinician Ruth Ruprecht, the paper's principalauthor, directs the Laboratory of Viral Pathogenesis at the DanaFarber Cancer Institute in Boston, an affiliate of Harvard MedicalSchool.

"The key message from our report in Science," she told BioWorldToday, "is that the simian immunodeficiency virus [SIV], after beingplaced far back on the tongue of anesthetized monkeys that had novisible sores, cuts or gum disease in the mouth, was able to reach thebloodstream, and result in systemic infection."

But Ruprecht hastened to add a reassuring rider: Her researchfindings that the oral cavity is a route of exposure for SIV, andtherefore HIV, "do not imply that casual oral contact _ such askissing, sharing spoons or toothbrushes, for instance _ is at all a riskfor people to get infected with the AIDS virus.

"Nowadays," she observed, "the warning that unprotected anal sex isthe number-one risk behavior is getting through to people, and thatbehavior is dropping. But maybe there is now a compensatoryincrease in unprotected oral sex."

She pointed out that "there are patients in the medical literature whobecame infected with HIV-1, and listed oral-genital contact as theironly exposure to the virus."

At Mucosal Level, Monkeys Act Like People

Ruprecht's laboratory, jointly with Tulane University's primateresearch center in Covington, La., designed a set of experiments withadult macaques, to measure transmucosal infectivity of SIV in oraland rectal tissues, as compared with direct intravenous inoculation.

"We demonstrated for the first time," Ruprecht said, "that in adults,virus can pass into the bloodstream after being placed withoutcausing trauma into the mouth. Six of the seven monkeys we exposedin this way," she continued, "became infected, and two of those sixalready have died of end-stage AIDS. Others have signs of thedisease also."

When a rhesus monkey (Macaca mulatta) comes down with an SIVinfection, experimental evidence generated to date indicates that SIVinfection and disease in these primates closely resembles HIV-1infection and AIDS in humans. HIV and SIV share genomicstructure, modes of transmission and target cells.

"There have been very close parallels," Ruprecht observed, "betweenthe SIV monkey model and what's known in the human situation."

She and her associates went on to measure the number of viralparticles required to penetrate each of the intact target mucosaltissues.

"When we compared the lowest infective dose orally and rectally inour monkeys," she said, "we saw an unexpected result: It was actuallyeasier, it took less virus, to infect systemically after oral exposurethan after putting the virus into the rectum."

The primate researchers performed the rectal inoculations by placingthe virus very carefully through an endoscope, into anesthetizedanimals. "This didn't involve any sexual activity," Ruprechtobserved, "so it wasn't exactly what is happening in the humansituation."

The result: "To achieve systemic infection took 6,000 times less virusfor the oral route than for the rectal. However," she added, "we donot interpret this seemingly discrepant finding to say that oral sex ismore risky than anal intercourse."

Ruprecht explained: "We think that human unprotected rectal sexcauses tears in the mucosal lining, which makes it easier for HIV toreach underlying lymphoid tissue, or get access directly to thebloodstream."

On this score, clinical epidemiologist Kenneth Mayer at BrownUniversity observes: "Oral exposure is so ubiquitous in the course ofanal sex that you can't disentangle them."

Ruprecht's team also compared "the lowest dose of virus needed toresult in systemic infection after oral exposure to that resulting inwidespread infection by intravenous inoculation."

The difference was 830-fold. That is, "It took 830 times as muchvirus to result in generalized infection after oral exposure. But viraldoses of such magnitude don't occur in casual oral contact,"Ruprecht pointed out.

She and her group now are taking up several items of unfinishedbusiness:

* So far, they have dosed their monkey models with cell-freesupernatant viral suspensions, rather than actual infected cells."Human semen of an HIV-1-positive man," she observed, "containscell-free virus and infected cells. It's difficult to know which one, orboth, is responsible for transmitting the infection.

"I don't think we can get the answer from human studies. This iswhere work with animal models becomes important."

* Equally knotty is mapping the devious route a virus particle travelsfrom lesion-free mucosal exposure to the nearest blood vessel. "Inour set of experiments _ placing cell-free virus onto the back of thetongue _ we don't know where the virus crosses over into thebloodstream. We don't know the site, whether mouth, throat, tonsils,esophagus, stomach or what."

Her group now is organizing this investigation. "In neonatalmonkeys," Ruprecht said, "we already are in the middle of doing it.We'll have to look in tissue sections at various time points after acuteinfection."

While reiterating her reassurance that casual oral contact gives nocause for concern, Ruprecht does warn that "Oral sex is not safe sex,even in the absence of disease in the mouth." Hence, "unprotectedreceptive oral intercourse should be added to the list of risk behaviorsfor HIV-1 transmission."

As for what then is protected oral sex, she cites the Centers forDisease Control and Prevention's recommendation: "Use a condom."n

-- David N. Leff Science Editor

(c) 1997 American Health Consultants. All rights reserved.