H5N1 Spillover Reached Cats and Rats in an Illinois Backyard Poultry Outbreak
A backyard poultry outbreak in southern Illinois did not stop at birds. It moved into a domestic cat and two rats on the same property, where testing found highly pathogenic avian influenza A(H5N1) in multiple tissues and, in the cat and one rat, evidence of broad systemic infection.
The farm sat about 2 miles from a 25,000-acre reservoir lake within the Mississippi Flyway, a location that places domestic animals close to migratory bird traffic and the viral ecology that accompanies it. The flock itself was mixed and crowded: turkeys, geese, Guinea fowl, chickens, cattle, cats, and dogs shared the premises. After acute illness and near-complete loss of the birds, postmortem examination began with a chicken, turkey, goose, two Guinea fowl, and a rat. One chicken sample first screened presumptive positive for influenza A, was subtyped as H5, and was then confirmed by the National Veterinary Services Laboratories as H5N1 clade 2.3.4.4b.
The infection pressure on that farm was high enough to burn through species boundaries with little resistance.
Tissue-Level Evidence Pointed to Systemic Mammalian Infection
Follow-up testing on the cat and rat specimens gave the outbreak its most important signal. The cat, a 13-year-old domestic shorthaired animal, had a lung sample with a cycle threshold value of 21.2, and confirmatory testing identified H5N1 clade 2.3.4.4b. The rat, first negative on lung screening, later yielded H5N1-positive results from tracheal swab testing at the reference laboratory.
When investigators expanded testing beyond the initial screening samples, the viral footprint widened sharply. All 12 cat tissues tested positive for influenza A, with Ct values ranging from 10.9 to 31.1. Eleven of 15 rat tissues were positive, with Ct values from 17.1 to 39.4. In both animals, brain tissue carried the highest viral burden, a pattern that fits with neurotropic spread rather than a localized respiratory event. Tracheal and nasal swabs were also positive in both animals, kidney tissue was positive in both, and rat urine contained viral RNA as well. Cat feces also tested positive.
The distribution matters because it suggests more than incidental exposure. Viral RNA in brain, kidney, respiratory tissue, and excreta implies a level of dissemination that could increase opportunities for shedding and environmental contamination. In a farm setting, that creates a practical route for infection to reach other animals, surfaces, feed, or water sources without any obvious clinical warning from the birds alone.
Genomic Clues Favored Adaptation in Mammals
Whole-genome sequencing strengthened the link between the poultry virus and the mammalian cases. Sequencing was completed from three cat samples — brain, heart, and nasal swab — and one rat brain sample, and all yielded complete sequences for eight segments. The chicken sample submitted for confirmatory analysis was included in the phylogenetic comparison, and the bird, cat, and rat viruses clustered together with other avian and mammalian HPAIV strains.
A key finding was the presence of the polymerase basic 2 E627K mutation in all three species. In influenza biology, that substitution is widely associated with mammalian adaptation and more efficient replication in mammalian hosts. The cat and rat samples also shared two amino acid changes, neuraminidase G392D and nucleoprotein K372E, which were absent from the chicken virus. The rat strain carried an additional synonymous polymerase basic 2 C1552T mutation. The outbreak virus was identified as genotype D1.1.
These sequence differences do not by themselves explain every step of transmission, but they fit the pattern of a virus that was replicating in more than one mammalian host on the same property. The cat and rat did not simply encounter the same contaminated environment; the viral genomes show a common outbreak lineage with mammal-associated molecular features.
Pathology Showed Disease in the Respiratory Tract and Brain
Gross and microscopic pathology made the molecular findings harder to dismiss as contamination alone. The cat had multifocal bronchointerstitial pneumonia, mild multifocal nonsuppurative meningoencephalitis, and acute adrenocortical necrosis. Immunohistochemistry detected influenza A antigen in airway epithelium and in brain tissue, including glial and neuronal staining.
The rat showed severe cranioventral bronchopneumonia on a background of diffuse alveolitis, with a bacterial component likely contributing to the lung injury. Mild nonsuppurative periventricular encephalitis and myocarditis were also described as possible viral lesions, and influenza A immunoreactivity appeared in the brain. The upper respiratory tract signal in the rat lined up with the testing results, which found the lowest Ct values in nasal turbinates and trachea relative to lung. Experimental receptor biology offers a plausible explanation: rat upper airways preferentially express α2,3-linked sialic acid receptors, the receptor class avian influenza viruses bind most readily.
The cat and the rat were not passive dead-end samples in a single farm episode. Their tissues showed active viral spread across organ systems, and the cat in particular demonstrated a distribution of lesions compatible with fulminant H5N1 disease.
Rodents Add a Harder-to-Control Link at the Farm–Livestock Interface
The most consequential public health implication may lie with the rat findings. Rats live close to feed, barns, poultry houses, and waste streams, and they can move between contaminated and susceptible spaces without attracting much attention. Detection of viral RNA in rat kidney tissue and urine raises the possibility that rodent excreta could participate in transmission, although this study did not perform virus isolation because of biosafety constraints.
That gap is important. Viral RNA alone cannot prove infectiousness, and the absence of isolation leaves open the question of how much viable virus was present in urine or tissue. Even so, the combination of positive tracheal swabs, multi-tissue positivity, and compatible pathology argues that rats can be more than simple bystanders in backyard outbreaks. They may bridge avian exposure and mammalian susceptibility in a way that is difficult to detect until the outbreak is already underway.
Cats have already been implicated in HPAIV transmission in prior reports, and the current case adds more weight to that concern. What makes the rat signal especially concerning is the overlap with farm biosecurity. Rodent control is difficult even in well-managed poultry and swine operations, and the same animals that evade control measures can also move infection through hidden pathways.
Backyard Outbreaks Can Seed Larger Agricultural Risk
The outbreak was contained administratively through quarantine by state and federal authorities, and the farm was monitored for 4 months. Even with that response, the biological lesson is uneasy: a single poultry episode on a mixed-species property produced fatal disease in mammals that had no reason to be viewed as primary targets.
The study does not prove onward transmission from rats to other livestock. It does show that H5N1 can spread extensively in small mammals living at the wildlife–livestock interface, and it raises the likelihood that rodents deserve more attention in outbreak investigations, environmental surveillance, and farm biosecurity planning. The data also remind clinicians and diagnosticians that mammalian deaths on a poultry premises should not be treated as an afterthought when HPAI is circulating.
One limitation remains central: without virus isolation from tissues or urine, infectivity was not directly measured. Future investigations will need to determine whether rodent urine can carry viable virus and whether rats can serve as intermediate hosts in real-world farm transmission chains. Until then, the safest reading is straightforward. A backyard poultry die-off in Illinois was followed by widespread H5N1 detection in a cat and rats, and the mammalian findings were not superficial.