A new study conducted in Africa has found that treating HIV-positive individuals with anti-retroviral drugs reduces the risk of them transmitting the disease to their heterosexual partners by more than 90 percent.  

The data emerged from a six-year study of 3,400 heterosexual couples in Botswana, Kenya, Uganda, Rwanda, South Africa, Tanzania and Zambia.  Taking place in the study were HIV-infected individuals and their HIV-negative partners.

In the course of the study, nearly 350 infected individuals went on anti-retroviral therapy, or ART, because the level of their CD4 cells, immune system cells that are under attack by the AIDS virus and a measure of disease activity, dropped dangerously low.

Only one of 103 transmissions occurred among couples who went on anti-retroviral therapy, according to study lead author Deborah Donnell with the Vaccine and Infectious Disease Institute in Seattle, Washington.

Donnell says the study showed a 92 percent reduced risk of HIV transmission among couples with one partner on anti-retroviral therapy compared to couples whose infected partners did not begin ART.

"We only saw one infection amongst people after they started ART compared to the 102 [infections] we saw when people didn't go on anti-retroviral therapy.  So, the strength of effect is pretty high," she said.

It's well known that the risk of mothers transmitting the AIDS virus to their infants is dramatically reduced in women who take anti-retroviral drugs.  Donnell says hers is the first study to show a similar effect in heterosexual couples.

Researchers used genetics tests to confirm that any HIV transmissions that occurred were between married couples and not from some source.

In the study, anti-retroviral therapy was reserved for partners with the lowest CD4 cell count.  But Donnell says researchers found that 70 percent of transmissions occurred among married couples in which the infected partner had a higher immune cell count but had not received ART.

"It makes you wonder whether as we roll out anti-retroviral therapy we could focus some of the initial roll out of ART to people with higher viral loads, whether that might also have an effect in preventing the transmission of HIV," she said.

Since the AIDS pandemic began in early 1980, more than 60 million people have been infected with the deadly virus.  Researchers believe that anti-retroviral therapy may have a significant public health benefit as well as helping the people who are being treated for HIV infection.

The study on the reduction in HIV transmission among married couples receiving anti-retroviral therapy is published this week in the journal The Lancet.