An outbreak of highly pathogenic avian influenza has impacted over 184 million domestic poultry since 2022 and, as of spring 2024, has spread to more than 1,000 milking cow herds. Researchers from Iowa State University have found that the mammary glands of several production animals—including pigs, sheep, goats, beef cattle, and alpacas—may also be biologically capable of harboring avian influenza. This is due to high levels of sialic acids present in their mammary tissue.
Sialic acid is a sugar molecule on animal cell surfaces that acts as a docking station for influenza viruses. Previous research by many of the same scientists showed that dairy cattle udders contain high amounts of sialic acid, which helped explain the rapid transmission of H5N1 avian influenza among dairy herds.
The study, published November 27 in the Journal of Dairy Science and conducted by researchers at Iowa State University's College of Veterinary Medicine and the U.S. Department of Agriculture's National Animal Disease Center in Ames, also found similar receptors in human mammary glands.
Dr. Todd Bell, professor of veterinary pathology and co-author of the study, said only sporadic cases of H5N1 infection have been reported in these other animal species so far but noted limited testing: "If we don't look, we don't know," Bell said.
Infected cows are producing milk contaminated with the virus. As a result, the USDA has implemented nationwide surveillance testing on raw cow milk samples. Pasteurization effectively kills influenza viruses in milk intended for sale; however, concerns remain regarding raw milk from other livestock species. "Some people do consume the raw milk of these other animals," Nelli said.
Nelli further explained that virus presence in infected cow’s milk likely contributes to H5N1 transmission and increases risks for humans: "If a virus in livestock is being spread by respiratory infections, few humans will be in close enough contact to catch it. But milk is an entirely different situation because it's transported into communities," he said.
All examined mammary gland tissues contained sialic acid receptors favored by both avian and seasonal human influenza viruses. The potential for these viruses to mix between species raises concern about more dangerous strains emerging. H5N1 has previously shown a fatality rate around 50% among humans; however, out of 71 confirmed human cases during this outbreak there have been two deaths.
"We need to try to stay ahead of this so it doesn't have a chance to continue to replicate and potentially evolve into something even more troublesome," Bell said.
