A newly published study from the University of Missouri has revealed that both beef and dairy cattle in the United States have been widely exposed to multiple strains of influenza A virus—specifically, those commonly found in humans and pigs. The findings highlight a growing concern about the potential for cattle to play a role in the evolution of new flu viruses that could impact animal and public health.
The research, appearing in mSphere, marks a key step in understanding how influenza A viruses circulate in cattle. Until the detection of H5N1 in U.S. dairy herds in 2024, cattle were not considered natural hosts for flu viruses.
Researchers tested blood samples from 1,724 cattle collected across 15 states since January 2023. Most of the samples came from Missouri and included over 30 different cattle breeds. Using an ELISA-based test, they found that nearly 34% of samples showed antibodies to influenza A, signaling prior exposure. This included 508 samples from 2023 and 78 collected so far in 2024.
Further analysis using a hemagglutination inhibition assay confirmed that many of the positive samples had antibodies to multiple strains, including the 2009 H1N1 pandemic virus, H3N2 seasonal flu, and swine-origin H1N2 and H3N2. Infections were recorded in all seasons, though more commonly during the colder months. Both male and female cattle were affected at similar rates.
Notably, none of the samples tested positive for the H5N1 virus currently affecting dairy herds. Researchers were surprised by the absence of H5N1 given its ongoing presence in livestock and wildlife. Still, the study reinforces the need to closely monitor flu activity in cattle, as they may serve as a mixing vessel for viruses that can jump between species.
Meanwhile, H5N1 continues to spread. The U.S. Department of Agriculture (USDA) Animal and Plant Health Inspection Service (APHIS) reported three new detections in Idaho dairy herds, bringing the total number of confirmed cattle outbreaks to 993 across 17 states. Idaho alone has reported 44 outbreaks, with nine occurring within the past month.
In addition to livestock cases, APHIS reported 24 new H5N1 infections in mammals. These included 11 domestic cats across six states: Colorado, Texas, Kansas, Oklahoma, California, and New York. A wild bobcat in Connecticut also tested positive.
Other infected wildlife included skunks in Colorado and Kansas, a red fox in Pennsylvania, three black bears in New Jersey, a house mouse in Indiana, and a bottlenose dolphin in Florida.
The growing number of H5N1 cases across species underscores the importance of ongoing surveillance and research to track the virus’s spread and potential risks to animal and human populations.
