HIV viral load in breast milk is highest during the first 14 days of lactation, and mother-to-baby transmission of HIV is most likely to occur during this period, according to a study published in 1st March 2003 edition of the Journal of Infectious Diseases. These findings are being hailed as “unique” by the study authors.
Investigators obtained up to seven breast milk-samples from 275 HIV-positive mothers over a two year period in a longitudinal study which recruited in Nairobi, Kenya from 1992-98.
HIV-positive pregnant women were enrolled to the study at 32 weeks of gestation when blood samples to check CD4 count and HIV viral load were obtained. Swabs were also taken to determine genital shedding of HIV.
Up to seven breast milk samples were collected from women in the two years after they gave birth. Infants were tested for HIV within a week of delivery and then at regular intervals over the next two years.
A total of 648 breast milk samples were collected from the 275 women in the study. The average duration of breast-feeding was 21.3 months. Of the 275 women in the study, 70 transmitted HIV to their baby whilst breast-feeding.
At week 32 of pregnancy, the women in the study had an average CD4 count of 436 cells/mm3, with the majority of women having a count between 200 –499 cells/mm3. Average HIV viral load was a little under 38,000 copies//ml (range 112 – 2,500,000 copies/ml).
HIV was detected in at least one breast milk sample of 89% of women in the study. Average viral load in breast milk was 398 copies/ml but there was a considerable range (
The average HIV viral load in the breast milk of women who transmitted HIV to their babies whilst lactating was higher than in non–transmitting women (3.14 log versus 2.81 log). Amongst women transmitting HIV to their infants, 60% were found by the researchers to shed HIV in breast milk at each sample, with 40% intermittently shedding whilst lactating. Women in the two groups provided a similar number of samples - an average of 3.5 vs 3.9 samples.
In addition, the amount of HIV in breast milk was found to fluctuate over time, and was highest in breast milk produced within two weeks of giving birth (2.59 log versus 2.04 log in mature breast milk; p=0.004). However, at all sampling times before 15 months, transmitting mothers were found to have significantly higher HIV viral load in their milk than non-transmitting mothers.
Investigators found that women who had high viral loads and low CD4 counts at 32 weeks of gestation were likely to have more HIV in their breast milk, with every 1.0 log increase in plasma viral load translating to a +0.58 log increase in HIV viral load in breast milk. In addition, detection of HIV in sexual fluids at 32 weeks of pregnancy was also predictive of higher viral load in breast milk.
The investigators note that their two year study “provides a unique and comprehensive perspective on the fluctuation in breast-milk HIV-1 levels during long-term lactation.” They add that breast-milk virus levels were associated with transmission of HIV and that women who consistently shed virus in their milk were more likely to transmit HIV to their infants.
”For each 10-fold reduction in breast-milk virus load prior to infection, there was a 2-fold reduction in the overall rate of mother-to-baby transmission.” Reducing viral load in breast milk would be a way of reducing mother-to-baby transmission of HIV in situations where HIV-positive women breast-feed.
Seeking to explain the reduction in HIV in breast milk after 14 days, the investigators suggest that this could be due to declining cell concentrations, or because of better immune response.
Further information on this website
Mother-to-baby transmission - Factsheet
Rousseau CM et al. Longitudinal analysis of HIV type-1 RNA in breast milk and its relationship to infant infection and maternal disease. Journal of Infectious Diseases 187:741-46, 2003.