By Will Dunham

WASHINGTON, March 29 (Reuters) - African women infected with the AIDS virus cut the risk of transmitting it to their babies when they fed them exclusively breast milk and not also formula, animal milk or solid food, a study found on Thursday.

Researchers in South Africa, writing in the Lancet medical journal, tracked 1,372 HIV-infected women and found a 4 percent risk of postnatal transmission of the human immunodeficiency virus to babies fed only breast milk for six months after birth.

The infants who were breast-fed but also given baby formula or animal milk were almost twice as likely to get the virus from the mother as those consuming breast milk alone, the study found. Babies fed solid foods in addition to breast milk were nearly 11 times more likely to become infected, it found.

The researchers cited a biological reason that might explain the findings. They said the mucous membrane within the intestines may serve as a barrier to HIV infection, and that breast milk could reinforce and protect that lining.

The study also found that the death rate by 3 months of age for babies who were exclusively breast-fed was less than half that of infants who received infant formula alone.

Fifteen percent of babies whose HIV-infected mothers did not breast-feed them died by age 3 months, compared with 6 percent of the babies whose mothers fed them exclusively through breast-feeding, the study found.

The study indicated that for women in impoverished areas where AIDS is most prevalent, the health benefits of breast milk appeared to outweigh the risk of passing on HIV through breast-feeding.

PROS AND CONS

Experts say breast milk provides nutrients an infant needs for the first months of life as well as antibodies that can protect against bacterial and viral infections.

But the breast milk of HIV-infected women may contain the virus and risk infecting the child. Thus, under ideal conditions, experts believe HIV-infected women should not breast-feed babies. But in sub-Saharan Africa, the epicenter of the AIDS epidemic, conditions often are not ideal.

Since infant formula is mixed with water before being given to the baby, woman living in communities with impure water and poor sanitary conditions risk exposing babies to waterborne illnesses that can cause life-threatening diarrhea or other ailments.

The study was led by Dr. Hoosen Coovadia and Dr. Nigel Rollins of the University of KwaZulu-Natal in South Africa.

Rollins said an estimated 150,000 to 350,000 babies were infected with HIV by their mothers through breast milk annually. The study's findings suggest that if infected women living in impoverished areas exclusively breast-fed their babies, somewhere around 50,000 to 100,000 lives could be saved annually, Rollins said.

"For the health and well-being of her child, exclusive breast-feeding is more than likely going to protect the child both from transmission and the other risks to her child's survival," Rollins said in a telephone interview.

More than 25 million people have died of AIDS since the incurable disease was first recognized in 1981. About 40 million people now live with HIV, most in sub-Saharan Africa.

The researchers were not sure why the addition of solid food particularly heightened the mother-to-child transmission risk, but noted the larger, more complex proteins in such foods may help enable the virus to slip through the gut wall or otherwise facilitate viral entry.