After four weeks of age, infants who breast feed from mothers infected with HIV continue to be at risk for infection with HIV for as long as they breastfeed, according to an analysis conducted and funded by the National Institute of Child Health and Human Development of the National Institutes of Health. Previously, researchers thought the risk for being infected with the virus from breast milk diminished as an infant grew older.
The analysis determined that a significant proportion of infants - 42 percent - were infected by breast feeding after they were 4 weeks old.
The study also found that infants were at greater risk for contracting the virus through breastfeeding if their mothers had low levels of CD4+ cells, an immune cell targeted by the AIDS virus. Moreover, male infants were more likely to contract the virus through breastfeeding than were female infants.
The analysis was conducted by NICHD and the Ghent Working Group on HIV in Women and Children, appears in the June 15, 2004 issue of The Journal of Infectious Diseases.
"In many poor countries, mothers who are infected with HIV don't have the option of bottle feeding their infants to prevent spreading the virus to them," said Duane Alexander, MD, director of the NICHD. "This information will help us to devise new ways to help prevent infants from becoming infected with the AIDS virus."
In developed countries like the United States, mothers infected with HIV are generally advised not to breast feed their infants.
The analysis pooled information from a number of studies that took place in Africa, said the NICHD author of the study, Jennifer Read, MD, MPH, of NICHD's Pediatric Adolescent and Maternal AIDS Branch. Read explained that one of the greatest strengths of the study was the large number of children included in the analysis. Taken together, the number of children in the study was much larger than in any of the studies that attempted to address the issue previously. All of the studies included in the larger analysis regularly assessed the infants' feeding patterns as well as their HIV infection status, beginning shortly after birth
For the analysis, researchers examined information on 4,085 children in nine studies. A total of 3,025 children in the study had negative HIV test results at 4 weeks of age and were breastfed through at least 28 days of age. Of these 3,025 children, 223 had late postnatal transmission - testing negative for HIV at four weeks of age, but testing positive after that time. The remainder of the 3,025 children who were uninfected at 4 weeks of age did not become infected.
Late postnatal infections occurred throughout the duration of breastfeeding, with children becoming infected at any time, from when they were four weeks old until they were 18 months old. In all, late postnatal transmissions occurred among 42 percent of the 993 children for whom timing of HIV infection was known.
The analysis also revealed that children of mothers who had low levels of CD4+ cells were more likely to become infected with HIV than were children whose mothers had higher CD4+ levels.
"The association of lower maternal CD4+ counts and a higher risk of transmission was not unexpected," Read said. "Individuals with lower CD4+ counts may have higher concentrations of HIV in the bloodstream and in breastmilk."
The researchers do not know why male infants were more likely to develop late postnatal infections than female infants were. A possible explanation is that, due to gender differences in immune functioning, girls are less susceptible to the infection after four weeks of age than are boys. The researchers wrote that other studies have found that the immune systems of infected male infants vary somewhat from the immune systems of infected female infants.
According to Read, the study findings might be useful in devising new strategies to prevent infected mothers from passing HIV to their infants through breast milk. Such strategies are essential, she said, as many women in poor countries don't have the option of formula feeding their infants. In some areas, formula may be too expensive for women to afford. Other areas may lack clean water, and mixing formula with local water may place infants at greater risk of contracting often times fatal diarrheal diseases. In other areas, it may be socially unacceptable for women not to breast feed their infants. In such areas, she explained, choosing not to breast feed - or any other sign that a woman is infected with HIV - may result in adverse consequences for the woman, such as being the victim of domestic violence.
The researchers discussed possible strategies for preventing the virus from spreading through breast milk, such as giving anti-HIV drugs to mothers who are breast feeding. Similarly, such drugs might be given to infants while they are breast feeding.
"An important implication of our analyses is that, since children of HIV-1-infected mothers have a consistent and substantial risk of acquisition of HIV-1 throughout the period of breast-feeding, to be most effective, interventions to prevent transmission through breast-feeding should be continued until the cessation of breast-feeding," the researchers wrote.