A new study finds that when high concentrations of HIV, the AIDS virus, turn up in a person's genital secretions, there is a higher risk the virus will be transmitted to that person's heterosexual partner. The discovery sheds new light on the biology of HIV infection.
For the past 20 years, AIDS researchers have looked at concentrations of HIV in genital secretions as a potential indicator of infectiousness. But none of the studies has been large enough to reach any firm conclusions.
The latest study, headed by University of Washington in Seattle Assistant Professor of Global Health and Medicine Jared Baeten, involved more than 2,500 heterosexual couples in seven African countries. In each couple, one partner was infected and the other was not.
Baeten says researchers followed the couples, more than 5,000 people in Botswana, Kenya, Rwanda, South Africa, Tanzania, Uganda and Zambia, for as long as two years, and counseled them on safe-sex practices.
Nevertheless, 46 women transmitted HIV to their uninfected male partners and 32 men eventually gave the virus to their HIV-negative female partners.
Baeten says researchers found that the higher the concentration of HIV in samples of vaginal secretions or male semen, the greater the risk of HIV transmission between the partners.
"The relationship was linear," noted Baeten. "As the amount of HIV in the genital samples went up, the risk of transmission went up. And this was true for transmission from women to men and men to women."
Baeten says researchers have known for the past decade there is a relationship between the amount of HIV in the blood of infected individuals and the risk of transmission.
Baeten notes, however, that throughout most of the world, the disease is not spread through contact with blood, but through sexual intercourse and contact with genital fluids.
Baeten says blood concentrations of HIV can vary from day to day, and some infected individuals naturally have lower blood levels of the virus. He says this led to another interesting finding.
"Levels of HIV in the genital tract predicted HIV risk even accounting for levels of HIV in blood, arguing that the levels in the genital tract, because those are the most close to where HIV occurs, are potentially the best marker for transmission risk," Baeten added.
Baeten says he expects the discovery's impact will be not in the clinic, but in the laboratory, where it could be a boon to researchers looking for ways to reduce HIV transmission.
"Researchers can test interventions that reduce the genital-HIV levels and understand that those would have a substantial effect in preventing HIV-transmission risk," Baeten explained.
An article on gauging HIV infection risk through levels of the virus in genital secretions is published in the journal Science Translational Medicine.