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Concerns Rise as H5N1 Bird Flu Found in Cow’s Milk

WASHINGTON — The discovery of H5N1 bird flu in cow’s milk last year has heightened concerns about the virus potentially evolving to become more transmissible among humans. However, as virologist Richard Webby detailed at the World Vaccine Congress in Washington on April 23, that scenario has not yet materialized.

It was unexpected for cows to harbor the H5N1 virus. Influenza viruses typically attach to sialic acid chains linked to sugar molecules on the surface of cells. Remarkably, the configuration of sialic acid on the mammary gland cells in cows closely resembles those found in birds. This specific structure allows the H5N1 virus to effectively infect both avian and bovine hosts, according to Webby from St. Jude Children’s Research Hospital in Memphis, Tennessee.

Furthermore, bovine mammary glands provide a fertile environment for the virus to thrive. “The levels of the virus in these cows align closely with what we can produce in laboratory conditions. If you’re a virus, it’s like being in a candy store in the udders of these cows,” Webby noted. He also emphasized that such ample growth may mitigate evolutionary pressure for the virus to adapt to human receptors. This means the virus has no pressing need to change its structure to facilitate human transmission—though this could change rapidly.

However, the presence of H5N1 in cattle carries significant risks, as Webby pointed out. An increasing number of human cases linked to contact with infected livestock or milk raises the likelihood of the virus mutating to spread more efficiently between people. To date, there have been 70 reported human infections in the United States, with 41 cases directly tied to exposure to dairy herds. Additionally, one individual died after contracting a variant associated with cattle, though that person had been in contact with backyard chickens and wild birds.

In the United States, two forms of H5N1 avian influenza have affected dairy cattle, and instances of related variants have also been reported in various other mammals including dolphins and porpoises—species not previously thought susceptible to bird flu, according to Webby.

Nonetheless, he cautions that this rise in infections among different mammal species may not necessarily indicate a shift in the virus’s capacities. Instead, it reflects an increase in overall viral presence due to its high replication rates in infected birds and animals.

Source
www.sciencenews.org