One in four cats living near active H5N1 dairy cattle outbreaks in California tested positive for antibodies against the virus, according to new research. The finding raises fresh questions about how broadly the virus is circulating in communities adjacent to affected farms, and about the role companion animals may play in bridging avian influenza toward human populations.
The study, published July 28, 2026 in CDC’s Emerging Infectious Diseases journal, was led by researchers at the University of Maryland School of Public Health and the University of Texas Medical Branch. The team collected serum samples from 73 cats in Tulare County, California, during December 2024 through March 2026, during the height of the H5N1 dairy cattle crisis in the state’s Central Valley. Three cats tested positive for both influenza A nucleoprotein antibodies and neutralizing antibodies against H5N1 clade 2.3.4.4b, yielding an overall seropositivity rate of 4.1%. But when the researchers narrowed the analysis to cats living within 2 kilometers of a dairy farm, the seropositivity rate climbed to 25%.
That proximity signal is the study’s most epidemiologically significant finding. Seropositive cats lived, on average, 1.34 kilometers from the nearest dairy farm, compared with 4.83 kilometers for seronegative animals. The pattern held across very different cat types: one outdoor feral cat, one indoor-only cat, and one additional indoor-only cat all tested positive. Both indoor cats resided in the household of a dairy industry worker, a detail that echoes an earlier Michigan case in which two house cats belonging to a dairy worker died of H5N1 in 2024 and raises, though cannot confirm, the possibility of human-to-cat transmission.
Why companion animal surveillance cannot wait
Cats occupy a biologically concerning position in the H5N1 risk landscape. Like swine, they carry both the alpha-2,6- and alpha-2,3-linked sialic acid receptors that bind human and avian influenza strains respectively, making them a candidate mixing vessel for viral reassortment. Co-infection with both avian and human influenza strains in a single feline host could, in theory, generate novel variants with enhanced human transmissibility. That is not a hypothetical concern: documented cat-to-human influenza transmission has occurred three times in the United States, including a veterinary professional in Los Angeles exposed to an H5N1-infected cat in 2024.
The estimated case-fatality rate in cats with confirmed H5N1 infection is approximately 90%, which means the seroprevalence figures reported here almost certainly underestimate true infection rates. Animals that survive a mild or subclinical infection contribute to the seropositive count; those that die of acute encephalitis, paralysis, or blindness do not.
The study’s limitations are important to acknowledge. With only three seropositive animals, the findings are suggestive rather than conclusive. The sample size constrains statistical power, and because samples came from routine veterinary blood draws at a single animal welfare organization, the population studied may not be representative of all cats in the region. The transmission source for any of the three seropositive cats cannot be confirmed.
Still, companion animal surveillance has lagged far behind livestock and wildlife monitoring throughout the H5N1 dairy cattle crisis, and this study makes clear that domestic cats near outbreak epicenters are being exposed at rates that should prompt attention. The authors argue that cat surveillance could itself serve as a sentinel system, flagging nearby H5N1 activity in cattle or poultry that has not yet been detected or contained through conventional channels.
The authors point to dairy cattle vaccination as a structural solution that could reduce spillover risk to cats and, by extension, to humans living and working in close proximity to both. Until that protection is in place, guidance for farm workers, veterinarians, and pet owners living near affected farms remains an urgent and underdeveloped piece of the public health response.
Sources and further reading
Bemis IG, Shittu I, Gomez JF, Fadaka EA et al. Seroprevalence of Influenza A(H5N1) Virus in Domestic Cats at Epicenter of Dairy Cattle Outbreaks, California, USA, 2024–2026. Emerging Infectious Diseases. July 28, 2026.
This article was researched and sourced by Global Biodefense editors and reported with Claude AI assistance for drafting and editing.