Introduction: In September 2002, possible transmission of West Nile virus via human milk was reported for the first time.
Methods: Since 2003, the Centers for Disease Control and Prevention collected reports of maternal or infant West Nile virus illness during the breastfeeding period. All of the reported instances were reviewed. In addition, milk samples from women infected during pregnancy were tested for West Nile virus RNA and West Nile virus-specific antibodies.
Results: Six infants were reported to have breastfed from mothers with West Nile virus fever. Five of the 6 infants had no illness or detectable antibodies to West Nile virus in serum after onset of maternal illness. One infant who was not tested and developed a rash was otherwise well 1 week after onset of maternal illness. In addition, 2 infants were reported to have developed West Nile virus illness while breastfeeding; preceding maternal illness was not documented. Two breastfed infants whose mothers acquired West Nile virus fever in the last week of pregnancy developed West Nile virus-specific antibodies; both infant infections could have been congenitally acquired. Of 45 milk samples from women infected with West Nile virus during pregnancy, 2 had West Nile virus RNA, and 14 had immunoglobin M antibodies to West Nile virus.
Conclusions: Of 10 reported instances since 2003 of maternal or infant West Nile virus illness while breastfeeding, transmission of West Nile virus through human milk could neither be ruled out nor confirmed for 5 cases; in 5 others, serologic tests indicated no vertical transmission. Transmission of West Nile virus through breastfeeding seems to be rare, but more information is needed.