Previous studies have shown that maternal and neonatal treatment with nevirapine can reduce an HIV-positive mother's chance of infecting her newborn to about 13 percent, roughly halving the risk, though the treatment also carries the risk of promoting drug-resistant virus. New research presented Tuesday at the 17th International AIDS Conference in Mexico City suggests that nevirapine persists in the mother's blood and breast milk for at least two weeks, which could lead to the development of resistant viral strains.

"In the short term, nevirapine is better than nothing," said Dr. David Katzenstein, a member of the Stanford University School of Medicine team that authored the study. "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."

The findings are based on a study of 32 HIV-positive pregnant women administered nevirapine in Zimbabwe. The researchers noted detectable blood levels of nevirapine in more than half the women within two weeks of delivery. At two weeks, the drug was found in the breast milk of two-thirds of the women. Though none of the women had drug-resistant HIV at the outset, RNA testing at two months revealed resistant virus in the blood of one-third of the mothers and the breast milk of two-thirds. The authors found advanced disease, as evidenced by lower CD4+ cell counts, to be predictive of developing drug resistance.

In a statement, study co-author Dr. Seble Kassaye said the results point to "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."

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