A Backyard Poultry Outbreak Just Showed Us H5N1 Doesn't Stop at the Coop

A new case report out of Illinois found bird flu deep in the tissues of both a cat and two rats, and vet teams should take note.

When 105 birds on a small Illinois farm nearly wiped out overnight this past spring, the local veterinarian knew something was very wrong. What she could not have predicted is that the virus responsible would turn up not just in the flock, but in a 13 year old house cat and two rats living on the same property, according to a new case report published this month in Emerging Infectious Diseases.

The farm sat about two miles from a massive reservoir lake along the Mississippi Flyway, exactly the kind of overlap between wild waterfowl and backyard poultry that keeps veterinary epidemiologists up at night. Within days, 100 of 105 birds were dead. Testing confirmed highly pathogenic avian influenza A(H5N1), clade 2.3.4.4b, the same lineage that has been working its way through wild birds, poultry, dairy cattle, and a growing list of mammals since it crossed into North America in 2021.

Here is where it gets interesting for small animal and mixed practice alike. A domestic shorthair cat and a feral rat found dead on the property both tested positive, and researchers did not stop at a quick screen. When they ran full tissue panels, they found virus almost everywhere they looked in both animals, including lungs, brain, kidney, lymph nodes, and nasal and tracheal swabs. The cat's brain carried the heaviest viral load of any tissue tested, and the rat was not far behind. Even more notable, the rat's urine and kidney tissue came back positive too, hinting that rodent urine could be its own route of spread on a farm, not just something wild birds and cats have to worry about.

Genetic sequencing tied the cat, the rat, and the original chicken sample together as the same outbreak strain, and all three carried a mutation known to help influenza adapt to mammalian hosts. On necropsy, the cat showed lung and brain inflammation consistent with viral pneumonia and encephalitis, while the rat's brain and heart also showed signs of viral involvement, on top of a nasty bacterial pneumonia layered over the underlying infection.

The authors are careful not to overstate what one small outbreak can prove. They were not able to isolate live virus from tissue or urine due to biosafety limits, so this is strong PCR and pathology evidence, not definitive proof that rats are passing the virus onward. Still, the takeaway for practice teams is hard to ignore. Rats already show up on plenty of farms nobody wants them on, and this study adds real weight to the idea that rodents could be a quiet middle step between infected wild birds and the livestock, barn cats, and farm dogs living alongside them.

What this means for your exam room. For anyone working mixed animal or rural small animal practice, this is a good moment to build H5N1 into the differential for any farm cat presenting with sudden neurologic or respiratory signs, especially with a history of exposure to sick or dead poultry. It is also a reminder that biosecurity conversations with clients should not stop at the coop door. Rodent control around feed, water, and barns is not just about protecting grain, it may be protecting the whole farm from the next spillover event.

Source: Storms SM, Rathmann J, Hemker L, Vieson M, Wang L. Detection of highly pathogenic avian influenza A(H5N1) virus in cat and rats during outbreak in backyard poultry, United States, 2025. Emerging Infectious Diseases. 2026 Oct. DOI: 10.3201/eid3210.260418.