The AIDS virus (search) quickly spreads from the mouth to the rest of the body, a monkey study shows.

The finding means a new hurdle for AIDS vaccines (search). It suggests they'll have to work quickly and efficiently to prevent HIV infection.

The finding also raises new questions about the safety of oral sex, suggests study leader Donald L. Sodora, PhD, assistant professor of internal medicine, infectious diseases, and microbiology at the University of Texas Southwest Medical Center in Dallas.

"We have highlighted unappreciated entry points for the AIDS virus," Sodora tells WebMD. "The study shows that oral exposure to HIV is a way a person can hypothetically be infected. We know babies get infected from HIV in breast milk. So it is not a big stretch to think semen could infect you orally. ... I don't think this is a safer way to have sex."

Humans vs. Monkeys

Kimberly Page-Shafer, PhD, MPH, notes that Sodora's study looked at monkeys inoculated with high doses of the monkey version of the AIDS virus. That may be very different from what happens in humans. Oral sex only rarely spreads HIV from person to person, she says.

Page-Shafer is a researcher at the Center for AIDS Prevention at the University of California, San Francisco. At the 2004 World AIDS Conference, she presented data from a study of 400 men whose only form of sexual behavior was receiving oral sex. Despite little condom use with multiple partners — including partners known to be infected with HIV — none of the men came down with HIV infection.

"We had zero infections over 1,493 person-years of exposure to oral receptive sex," Page-Shafer tells WebMD. "This doesn't mean there aren't factors that contribute to easier HIV transmission by oral sex. It does happen. But data confirm it is a pretty rare occurrence."

Sodora says that while his study used relatively high doses of a deadly monkey AIDS virus, it's a good model for human infection. The mouth and throat are just too full of HIV entry points for comfort. His safe-sex advice: Always use condoms.

AIDS Vaccine Implications

The real reason for Sodora's study was not to evaluate the relative danger of receiving unprotected oral sex. The idea was to look at how HIV enters the body when it encounters mucous membranes.

One theory is that HIV has to multiply on mucous membranes before it can cross into the bloodstream. It would be great if this were the case. That would give vaccine-stimulated immune responses time to find and neutralize the virus.

But the Sodora study shows this isn't the case. Just one day after placing AIDS virus in monkey's mouths, the animals had detectable virus in their lymph nodes. Three days after that, the virus spread throughout the body. The findings appear in the December issue of the journal AIDS.

"If you really want to prevent HIV infection, you need to block the virus before it ever infects that first target cell," Sodora says. "This requires antibodies at the mucosal surface. Interestingly, a lot of vaccines studied now are designed to elicit [killer-cell] responses. That is supposed to kill infected cells and suppress viral replication. But I think that is too late to prevent infection."

Many of the AIDS vaccines now under study do, in fact, stimulate anti-HIV killer cells. Even if they don't prevent infection, these vaccines are likely to delay or even prevent AIDS in people who become infected.

"We are working very diligently to develop effective AIDS vaccines," Sodora says. "The first generation of AIDS vaccines are now being tested. There is hope they will help people. But we should think about the second generation of vaccines. ... We should be striving to prevent infection. It is a long shot — a home run — but our sights should be set on that. We should be swinging for the fences."

Despite this new hurdle, Chinglai Yang, PhD, thinks AIDS vaccines have a good chance of preventing HIV infection. Yang is an AIDS vaccine researcher at Atlanta's Emory University.

"I work in this field and I know this is a very tough job to accomplish. But I do think you can actually have a vaccine with a preventive effect," Yang tells WebMD. "If you have a vaccine that can reduce the incoming virus load by 90 percent, you will effectively reduce the transmission rate by 60 or 70 percent. Yes, some vaccinated individuals will get infected. But now the estimate is one HIV transmission per 1,000 acts of intercourse. If you can change that to one in 5,000, you have a much smaller epidemic — assuming people's behavior stays the same."

By Daniel J. DeNoon, reviewed by Michael W. Smith, MD

SOURCES: Milush, J.M. AIDS, December 2004; vol 18: pp 1-10. Donald L. Sodora, PhD, assistant professor of internal medicine, infectious diseases, and microbiology, University of Texas Southwest Medical Center, Dallas. Kimberly Page-Shafer, PhD, MPH, associate adjunct professor, department of medicine, and Center for AIDS Prevention Studies, University of California, San Francisco. Chinglai Yang, PhD, assistant professor, Emory University, Atlanta.