In early March, Dr. Barb Petersen, a veterinarian specializing in large animals in Texas, started getting calls from her dairy farm clients in the Panhandle. The workers on these farms noticed something unusual: cows with mastitis, an infection of the udder, that didn’t fit the typical causes like bacterial infection or tissue damage. The milk from these cows was thickened and discolored, and the problem seemed to be spreading.
Farmers were baffled. One even mentioned that many of his farm pets were also falling ill, suggesting the problem wasn’t limited to cows. As the calls kept coming, Petersen knew she had to find out what was causing the outbreak. She collected samples from the sick cows and sent them to the Texas A&M state veterinary lab and colleagues at Iowa State University for analysis. The results came back shocking: high levels of the H5N1 influenza virus, a form of bird flu.
The dairy industry, along with public health officials worldwide, were alarmed by the finding. The presence of H5N1 in cows was unexpected, and it posed a significant risk. How did cows contract the virus, and what could it mean for humans? Researchers in the US and Denmark set out to find answers.
A preprint study from these researchers revealed that cows have the same receptors for flu viruses as humans and birds. This raised concerns that cows could act as a “mixing bowl,” allowing the virus to potentially adapt and spread more easily between people. Though such an event is rare, it could trigger another pandemic.
H5N1 had primarily been a problem in birds, decimating poultry flocks in the US. It had even infected some mammals, but cows hadn’t been on anyone’s radar as potential hosts. Now, with 42 infected herds in nine states, the US Department of Agriculture was on high alert, though it noted that only one person had contracted the virus from infected cows. The Centers for Disease Control and Prevention (CDC) said the risk to public health was low but was monitoring people with animal exposures.
The reason for the concern lies in the unique way flu viruses invade cells. For Covid-19, it’s the ACE2 receptor; for flu viruses, it’s a sugar molecule called sialic acid. Different animals have different shapes of sialic acid receptors. Birds have one type, and humans another. Until this study, it was assumed cows didn’t have receptors for A-strain flu viruses like H5N1.
Dr. Lars Larsen, a professor of veterinary clinical microbiology at the University of Copenhagen, was surprised by what they found. Cows had both bird and human-like sialic acid receptors, especially in the mammary glands. This raised the possibility of flu viruses swapping genetic material, creating new strains that could pose a greater risk to humans.
Dr. Richard Webby, a WHO influenza expert, explained that flu viruses could mutate in two ways: a significant shift through reassortment or gradual drift through genetic changes over time. The findings in cows suggested a potential risk for both, but reassortment, though rare, could lead to a virus that spreads more easily between humans.
Given the potential risk, experts like Dr. Sam Scarpino of Northeastern University believe this new information demands urgent action. He suggests increased safety measures for workers in dairy farms, stricter controls on milk production, and more research funding to understand influenza in cows better. Although the findings are preliminary and require further study, the consensus is that the dairy industry and public health officials should take this development seriously and act quickly to prevent a potential pandemic.
