For the first time, health officials report that the AIDS virus can be spread by a mother pre-chewing her infant's food, a practice mainly seen in poor, developing countries.

Three such cases were reported in the United States from 1993-2004, government scientists said Wednesday in a presentation in Boston at a scientific conference.

It's blood, not saliva, that carried the virus because in at least two of the cases the infected mothers had bleeding gums or mouth sores, according to investigators at the U.S. Centers for Disease Control and Prevention.

CDC officials say more study is needed. But they are asking parents and caregivers with HIV not to pre-chew infants' food, and are trying to educate doctors about this kind of transmission.

Health officials believe chewed-food transmission is rare in the United States, where such behavior is considered unusual. In some countries, mothers do it because they have no access to baby food or a means of pulverizing food for toothless infants.

"But even one case is too many," said the CDC's Dr. Ken Dominguez, who helped investigate the U.S. cases.

The first involved a 15-month-old African-American boy in Miami, diagnosed in 1993. His great-aunt was infected with HIV and pre-chewed food for the boy when he was between the ages of 9 months and 14 months.

Then a 3-year-old Caribbean-American boy was diagnosed in 1995, also in Miami. His HIV-infected mother pre-chewed food for her son.

Still uncertain they had definitively connected the practice to the spread of HIV, the doctors wanted more evidence. It was years later before they could confirm a third case, which occurred in 2004. A 9-month-old African-American girl was diagnosed with HIV in Memphis. The mother began pre-chewing the girl's food when she was about four months old.

All three children were infected with HIV at a time they would have been teething and had inflamed gums. It may be that both a caregiver and a child must have wounds in their mouths for the virus to have a good chance of passing from one bloodstream to another, the investigators said.

Previous studies have linked pre-chewing to the spread of other infections including Helicobacter pylori, a bacteria that causes stomach ailments, and streptococcal pharyngitis, which triggers sore throat. That research, too, is preliminary and needs to be confirmed, CDC officials said.

In developing nations without other feeding options, any campaign against pre-chewing could be nutritionally harmful, said Kimberly Hagen at the Emory Center for AIDS Research in Atlanta.

"This would really take a lot of thinking before you could say, 'We've had three cases in 11 years, so you have to stop pre-chewing your child's food,'" Hagen said.