Davis is a veterinarian, epidemiologist, and One Health expert. Carlin is a veterinarian and biodefense policy expert. Sorrell is a virologist and biosafety expert. Stiefel is a biodefense and national security policy expert.

As veterinarians, flu scientists, and biosecurity experts, we have watched with growing concern the rise in H5N1 avian influenza in cats. Since 2022, more than 100 domesticated cats in the United States have had confirmed infections -- some house pets, some barn cats, some feral -- with high mortality, although mortality data remain undocumented on federal sites. Contaminated raw milk and raw meat pet foods have been implicated in numerous cases, while others could be linked to exposure to wild birds. The virus has also killed great cats, at least 20 at a single sanctuary, cougars and bobcats among them.

Now, a new report from the Centers for Disease Control and Prevention and Michigan health authorities sheds light on yet another possible source of the virus for cats: infected people. Investigators found the virus in two indoor cats living in the home of dairy workers. These cats had no known direct exposure to other sick animals. This suggests that human-to-cat spread has occurred, a development that is as unsurprising as it is concerning. We often think of zoonotic spillover as the movement of pathogens from animal to human, but it works both ways.

To manage this rapidly developing problem, we recommend federal, state and industry action and investment to address gaps in regulatory oversight, expand surveillance efforts, and accelerate point-of-care testing that can be deployed both for people and animals.

Cats infected with H5N1 can develop severe and fatal illness. We don't know much more. There is minimal surveillance at state or national levels in the U.S. for companion animal infectious diseases, including those of pandemic concern, and few tests available to conduct such surveillance. This reality challenges prevention, detection, and control efforts for the very kind of pre-pandemic, panzoonotic crisis we are currently experiencing.

The death of barn cats on dairy farms can be a sentinel for producers. It was a major driver for Texas veterinarians to test their sick cows for influenza and led to the first reports of H5N1 in dairy cattle in March 2024. For this reason, some states added H5N1 to their standing rabies testing programs, and the American Veterinary Medical Association now recommends testing for both viruses in animals with neurological disease, a common symptom of avian influenza in cats.

While useful, the ad hoc addition of H5N1 testing to rabies programs is a passive structure. Given the pandemic risk from H5N1, active surveillance of companion animals is needed to recognize the full scope of the problem and identify critical control points for intervention. It is most needed in instances where exposure could be highest, including pets of dairy and poultry workers and those fed raw milk or meat.

Such surveillance is necessary given our rapidly evolving understanding of the multiple transmission routes for cats. One cat in the CDC study reportedly rolled on the work clothes of the owner, who hauled unpasteurized milk and had contact with H5N1-infected farms. We need a clearer understanding of whether indirect routes of exposure like this can lead to infection in cats.

One way to achieve this is for federal and state health authorities to include zoonosis risk questions in their contact tracing efforts. These efforts could capture voluntarily provided information related to the individual's interface with susceptible animals. Case tracing efforts could even be expanded to include not just in-contact people but also their cats and other pets.

This would of course require resources, both human and technological. One of the major barriers to implementing such an effort is fragmented governance. U.S. federal agency oversight for companion animals is limited and siloed. The United States Department of Agriculture has extremely constrained surveillance authority with respect to any animal that is not livestock or poultry or that does not directly affect them; its major focus is to protect and promote the U.S. food supply. Without clear mandates, resources, or authorities to do so, USDA has directed almost no personnel or financial resources to companion animals.

The Food and Drug Administration lacks these authorities, too. What it does have is broad authority over pasteurized milk, regulation of commercial pet food, and investigation of reports of adverse feed-associated events. While oversight for raw milk is left to the states, the FDA could restrict interstate sales of raw pet milk, a current loophole, and could do more to address the food safety issues related to contaminated raw meat pet foods.

With USDA focused on poultry and livestock, and FDA focused narrowly on food safety, most of the authority for outbreak response in companion animals falls to the states. State-level efforts are a critical source of outbreak awareness and management, but at a national level, there remains a need to ensure an application of surveillance and response that is as even and robust as possible. This would provide the White House's national security and pandemic preparedness teams with critical data they need to ensure their own situational awareness, and to coordinate effective strategies across federal agencies.

Data that reveal the prevalence of H5N1 in different populations of cats, coupled with interpolation of human H5N1 data, could help to unravel the ways in which cats may be part of the transmission chain of this promiscuous virus. Studies have identified that more than 30% of U.S. cats developed antibodies to SARS-CoV-2 when owners were infected. We now have a strong index of suspicion that similar dynamics are at play with H5N1. To capture these data, the Biomedical Advanced Research and Development Authority should accelerate development of rapid point-of-care or at-home tests for H5. These should be put to use in people in high-risk occupations and their close contacts. With USDA support, these or similar tests could also be developed for and validated in cats. Such rapid tests could be used in veterinary clinics to screen cats with respiratory or neurologic signs, getting answers to clinicians quickly. Confirmatory testing in state and federal laboratories will remain essential.

Testing can also be harnessed as a prevention strategy. High-risk workers who have access to free, rapid Flu A -- or better, H5-specific tests in development -- as well as protective gear, other countermeasures, and biocontainment education may be able to mitigate take-home exposures in real time. Reaching the barn and feral cat populations will be harder -- skilled trapping efforts are necessary to reach these "peridomestic" animals -- but investment in partnerships with researchers, animal shelters, and control organizations in rural areas could help.

Over and over again -- from prairie dogs with mpox to mink with Covid-19 -- our federal regulatory system has demonstrated that it does not have the adequate oversight authorities for companion animals, or frankly any species without clear agricultural purposes. This threatens our health and our economy and has become ever more evident with H5N1. Our current approach is a leaky, ad hoc effort that has repeatedly allowed contaminated products, equipment, and clothing as well as infected personnel and animals to escape containment. With solving the authority gap as the long-term goal, we must first urgently work on expanding surveillance -- particularly for barn cats and pets of agricultural workers -- and improving information flow in ways that help to protect workers, producers and the public. This will be central to any strategy designed to prevent further spread of this relentless virus.