Approximately 30-50 percent of all mother-to-child HIV-1 transmission occurs through breastfeeding. This problem is most common in sub-Saharan Africa, where there is limited access to antiretroviral drugs, infant formula and safe drinking water. Although breast milk is an important mode of transmission, little is known about the dynamics and behavior of HIV in the breast. Viral levels in milk often correlate with levels in the bloodstream, but are typically 100-fold lower. Mastitis, or inflammation of breast, is a common, often asymptomatic problem in breastfeeding women, and is linked with dramatically increased viral loads in milk, in turn leading to increased likelihood of transmission. It is unclear to what extent HIV in breast milk is related to the virus population in the bloodstream, and whether mastitis causes expansion of the HIV in breast milk or infiltration of blood borne virus into the breast as part of the inflammation response.
To learn more about viral interplay between breast and blood, VIDD affiliate investigators Dr. Soren Gantt and Dr. Lisa Frenkel and colleagues collected milk and blood samples from nine recently post-partum HIV-positive Zimbabwean women. They assayed the milk for chemical evidence of mastitis by measuring the sodium concentration and sequenced individual viral genomes from the blood and milk samples, from which they created phylogenetic trees to model the evolutionary dynamics and relationships of the viral populations within each subject. Identical viral genomes were more frequent in milk than in blood, indicating some degree of viral isolation in breast tissue. However, the researchers found a great deal overlap in the overall phylogenetic trees for each subject between the blood populations and the populations in milk from each breast, so the viral populations must have intermixed to a great extent. Six of the women had mastitis in one breast; in those milk samples the median viral loads were approximately 17-fold higher than the median in non-mastitic milk samples. They found a positive correlation between the milk sodium concentration and increasing genetic distance between milk and blood viral sequences, possibly indicating local replication of HIV in the breast that is driven by inflammation. However, even in mastitic samples there was a great degree of viral sequence diversity and overlap with blood samples.
Gantt S, Carlsson J, Heath L, Bull ME, Shetty AK, Mutsvangwa J, Musingwini G, Woelk G, Zijenah LS, Katzenstein DA, Mullins JI, Frenkel LM. Genetic analyses of HIV-1 env sequences demonstrate limited compartmentalization in breast milk and suggest viral replication within the breast that increases with mastitis. J Virol. 2010 Oct;84(20):10812-9.