Zika Virus in Breast Milk Inactivated by Pasteurization and Prolonged Storage

With all of the Zika virus-associated complications for pregnant women and their babies, a little bit of good news regarding possible inactivation of one route of transmission for the mosquito-borne infection goes a long way.

For these reasons, the findings of a study scheduled to be published in the January 15, 2017 issue of the journal Virus Research, and available online as of November 25, have provided at least a certain degree of optimism, as the authors from Switzerland and Germany were able to show that prolonged storage and pasteurization effectively inactivated Zika in the breast milk taken from women infected with the virus.

According to study co-author Volker Thiel, PhD, from the Department of Infectious Diseases and Pathobiology, University of Bern and the Federal Department of Home Affairs, Institute of Virology and Immunology in Bern and Mittelhäusern, Switzerland, the project was initiated after the research team read a study on the presence of Zika in breastmilk published earlier this year in The Lancet, and after Stephanie Pfaender (lead author of the Virus Research paper) came on board as a postdoctoral fellow. She had been part of a team that showed that human breastmilk effectively acts as an antiviral against hepatitis C.

For their study, Dr. Thiel and his colleagues collected milk samples from three healthy human donors and stored it at −80°C. They then incubated the African lineage strain of Zika virus (MP-7501) and the circulating strain from Puerto Rico (PRVABC59) at a ratio of 1:10 with human breast milk, or for use as a control. They then infected 1 × 104 Vero B4 cells with the virus/milk mixture in a limiting dilution assay or stored the mixed samples at 4°C before virus titration.

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The research team found a reduction of viral titers within 1 day of storage. In addition, after 1 to 3 days of storage, Zika virus in the cells became inactivated and viral RNA was undetectable. Interestingly, in the control cells, the researchers noted that Zika remained stable for up to 5 days, which they believe suggests that human breast milk inactivates the virus in a time-dependent manner. Furthermore, they noted that pasteurization of the milk (via Holder pasteurization—at 62.5°C for 30 minutes) rendered Zika undetectable, regardless of the milk donor or virus strain used in the experiments.

“Although transmission of Zika virus via breast milk has not been demonstrated so far, infectious virus has been detected in the breast milk of infected mothers,” Dr. Thiel told Contagion. “We don’t actually consider this route of transmission; however, for individual cases we should not exclude or neglect this possibility.”

Dr. Thiel said, “Our results provide a scientific rationale for an easy and cost-effective approach to inactivate potential infectious viral particles in breastmilk to prevent possible transmission via breastfeeding in cases of doubt, and thus may have direct impact on public health.”

Brian P. Dunleavy is a medical writer and editor based in New York. His work has appeared in numerous healthcare-related publications. He is the former editor of Infectious Disease Special Edition.