H5N1 found in a cat and rats at an Illinois farm where 100 of 105 birds died

Highly pathogenic avian influenza is a poultry and wild-bird problem that keeps finding its way into mammals. A diagnostic study published on 1 October in the US CDC's Emerging Infectious Diseases adds two more to the list, caught together on a single small farm: a domestic cat and farm rats, both carrying H5N1, both with the virus concentrated in the brain.

The investigation began with birds. On March 12, 2025, a farmer in southern Illinois with a small backyard poultry flock brought the remains of 6 animals to their primary veterinarian [s1]. The flock had been devastated — a mortality of 100 of 105 birds [s1]. The setting is the kind epidemiologists watch: the farm sat about 2 miles from a 25,000-acre reservoir lake within the Mississippi Flyway, and housed 4 turkeys, 1 goose, 20 Guinea fowl, 80 chickens, 17 cattle, 15 cats, and 2 dogs [s1] — birds, livestock, and domestic mammals sharing one environment under a major migratory-bird corridor.

The cat, then the rats

On March 16, 2025, the veterinarian collected the remains of a 13-year-old domestic shorthaired cat [s1], and on March 19 specimens were sent to the University of Illinois Veterinary Diagnostic Laboratory [s1]. The cat's lung tested presumptive positive, with a cycle threshold (Ct) value of 21.2 [s1] — a low Ct indicating a high viral load.

What the full workup showed is the striking part. All 12 cat tissue samples were influenza A virus-positive, with Ct values of 10.9–31.1, and 11 of 15 rat tissues were positive, with Ct values of 17.1–39.4 [s1]. The virus was not confined to the airway. Brain tissues of both animals had the highest viral loads, a cat Ct of 10.9 and a rat Ct of 17.1 [s1]. Tracheal and nasal swab samples from both animals were positive across a Ct range of 22.1–29.5 [s1]. Kidney tissues from both species were positive — cat Ct 20.9, rat Ct 34.6 — and urine from the rat also tested positive at Ct 32.0 [s1]. Nucleic acid in faeces from the cat registered a Ct value of 28.6 [s1].

Investigators performed whole-genome sequencing on 3 samples from the cat — brain, heart, and nasal swab — and 1 brain sample from the rat [s1], and found high sequence identity to an avian isolate from the same farm, tying the mammalian infections to the poultry outbreak rather than to a separate source [s1].

Why the brain finding matters

Respiratory-tract positivity in a cat near sick poultry is not, by itself, surprising; cats are known to be susceptible. The distribution here is what draws attention. Virus present in brain, kidney, urine, and faeces — with the heaviest loads in the brain of both species — describes systemic, not merely respiratory, infection, and it means an infected small mammal can shed virus by several routes. Rats are not bystanders in a farmyard: they move between buildings, feed, and water, and they are prey. The authors' conclusion is appropriately cautious but pointed — small mammals might contribute to H5N1 transmission [s1].

Where it fits in the bigger picture

WHO notes that A(H5N1) viruses have caused hundreds of millions of poultry infections and outbreaks in wild birds, and have spread to some mammals, including marine mammals, dairy cattle, and other wild and domestic animals [s2]. There have been hundreds of human cases, and many deaths, among people exposed to infected animals or contaminated environments [s2].

The reassurance, such as it is, remains intact: currently circulating zoonotic influenza viruses have not demonstrated sustained human-to-human transmission [s2]. The concern that keeps studies like this one in the surveillance literature is that every new mammalian host is another venue in which the virus can adapt. A farm that puts cats and rats in sustained contact with a heavily infected flock, under a flyway, is exactly the kind of interface where that adaptation would be most likely to go unnoticed.

What to watch

This is a single farm and a small sample, and it describes what the virus can do in mammals rather than how often it does it. The questions it raises are quantitative: how commonly peridomestic rodents are infected during poultry outbreaks, whether they can pass the virus onward, and whether routine outbreak response should sample the animals that live around a flock, not just the flock itself. On the evidence here, the rats around an infected barn deserve more attention than they usually get.

Sources

• Detection of Highly Pathogenic Avian Influenza A(H5N1) Virus in Cat and Rats during Outbreak in Backyard Poultry, United States, 2025 — Emerging Infectious Diseases (CDC), 1 October 2026
• Influenza (avian and other zoonotic) (fact sheet) — World Health Organization