August 5 (HealthDay News) -- The drug nevirapine -- widely used in developing countries to prevent transmission of HIV from mothers to babies -- persists in the breast milk and blood of mothers, a new Stanford University study finds.

That, in turn, could increase the risk that they and their children will develop drug-resistant strains of HIV, the researchers added.

The scientists looked at 32 HIV-positive pregnant women in Zimbabwe who received a single dose of nevirapine when they went into labor. The women had received no other treatment for their infection with HIV, the virus that causes AIDS.

Two weeks after delivery, more than half of the women still had detectable levels of the drug in their blood, and two-thirds had measurable levels of nevirapine in their breast milk. The longer the drug stays in the body, the more likely it is to develop drug-resistant mutations, the Stanford researchers said.

At the start of the study, none of the women had drug-resistant HIV strains. But two months after they gave birth, one-third of the women had drug-resistant strains in their blood, and 65 percent had drug-resistant strains in their breast milk as well, and could pass those strains to their babies during breast-feeding. Women with more advanced HIV were most likely to develop drug-resistant strains.

The study was expected to be presented Tuesday at the International AIDS Conference in Mexico City.

"In the short term, nevirapine is better than nothing. But in the long term, I'm concerned about conferring resistance. If you're talking about resistance on a broad scale, it could jeopardize future treatment for mothers and infants," principal investigator Dr. David Katzenstein, a professor of infectious diseases, said in a Stanford news release.

Nevirapine and another drug called zidovudine (AZT) play a major role in public health programs to prevent mother-to-child transmission of HIV in developing nations. Worldwide, the drugs have been used as preventive tools in nearly 900,000 women and infants.

Access to better antitretroviral treatment would reduce the risk of the development of drug-resistant HIV, said study first author Dr. Seble Kassaye, an instructor in infectious diseases.

"[The study] reinforces the need to treat these women with combination therapy, thereby providing better prevention for the infant, while providing better treatment for the mother. Public health efforts should continue to expand combination therapy so that mothers and babies aren't left vulnerable to drug resistance," Kassaye said in the news release.

SOURCE: Stanford University, news release, Aug. 5, 2008