WEDNESDAY, April 6 (HealthDay News) -- In a development that could enhance HIV-prevention research, a new study of heterosexual couples confirms that the risk of transmitting HIV rises with the level of the virus in semen and cervical fluid.

The finding -- that more virus translates to higher likelihood of transmission -- hasn't been proven to this extent before, said study lead author Dr. Jared M. Baeten of the University of Washington in Seattle.

"This confirms what we had thought about the biology of HIV," he said, "and it gives us new information about genital levels of HIV being particularly important, even independent of blood levels."

For the study, researchers obtained samples of genital fluid from 2,521 heterosexual couples living in seven African countries. Most were married and living together. At the start of the two-year study, one partner in each couple was infected with HIV, the AIDS-causing virus, and none was taking anti-HIV drugs.

Over the course of the study, published April 6 in the journal Science Translational Medicine, 78 partners became infected within the relationship.

The researchers compared cervical and semen fluid samples from partners who transmitted the virus with samples from men and women who didn't transmit the virus and found that the risk of HIV transmission approximately doubled with each specified HIV increase in genital fluids. (In a few cases, HIV transmission occurred without any sign of the virus in genital fluids, although it was in the blood.)

The results are "really useful for figuring out new research studies looking at new strategies," said Baeten, an assistant professor of global health and medicine. "You can develop strategies that reduce HIV levels only in the genital tract, not in the blood, like microbicides."

The study is useful for a couple of reasons, said Dr. Peter A. Anton, director of the Center for HIV Prevention Research at University of California Los Angeles, who co-wrote a commentary accompanying the study. Not only does it suggest a way of determining who is most likely to infect a partner, it also enables researchers to study those who didn't infect the people they had sex with.

This can help researchers better understand "the natural protections that the penis, the vagina and the rectum have that we want to make sure we preserve," he said. The study "is highlighting what we need to look at going forward," he added.

Still, the study, which Anton said was "really well done," has some limitations. It only looked at heterosexual couples and not at people at higher risk of becoming infected with HIV, such as sex-trade workers and gay men.

And the study doesn't examine how often HIV-positive people with no detectable virus in their blood transmit the disease to their partners. Anti-HIV drugs can often reduce the level of HIV in blood to zero, while the virus hides in other parts of the body.

Worldwide, more than 7,000 new HIV infections are diagnosed daily, according to background information in Anton's commentary. In the big picture, these new findings can only do so much to curb the rate of HIV infection, he said.

Noting that many HIV-positive people are unaware they have the disease, Anton said, "The biggest issue in transmission is that many people don't know their status."

More information

For more about HIV/AIDS, visit the U.S. National Library of Medicine.