U.S. has found H5N1 flu virus in milk — here’s why the risk to humans is likely low

By Lawrence Goodridge, J Scott Weese, University of Guelph

Reports that the H5N1 virus has been found in raw and pasteurized milk in the United States have raised questions regarding the safety of H5N1-contaminated milk for human consumption. Although H5N1 influenza usually affects birds, it was recently found in U.S. dairy cows. According to the Public Health Agency of Canada’s April 26 update, no cases have been reported in Canada yet.

To answer questions about the safety of drinking milk contaminated with the H5N1 virus, it is important to understand some basic biology of H5N1 and other related viruses.

Influenza virus structure

There are four types (A, B, C and D) of influenza viruses. Seasonal flu in humans is caused by types A and B.

Subtypes of influenza A virus are named based on two proteins found on the surface of the virus: hemagglutinin (H) and neuraminidase (N). There are 18 different hemagglutinin subtypes and 11 different neuraminidase subtypes. That means H5N1 is identified as having H subtype 5 and N subtype 1.

Influenza is an enveloped virus, meaning that it takes its outer layer from the host cell that it’s infecting, enveloping itself. Then the H and N proteins are inserted into that outer layer.

Many studies have shown that enveloped viruses are not stable outside of the animal host, in contrast to non-enveloped viruses (such as foot and mouth disease virus, which infects cattle), which are much more stable.

The lack of stability of the H5N1 virus is an important piece of information in trying to understand if milk (and other foods) that may contain the virus are safe to consume. For example, influenza viruses are unstable at high temperatures, and sensitive to low pH (acidity). Based on this information, and knowing how pasteurization is performed, we can come up with reasonable conclusions about the safety of pasteurized milk.

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