The ongoing outbreak of Highly Pathogenic Avian Influenza (HPAI) A(H5N1) clade 2.3.4.4b virus in the United States has taken a concerning turn, with domestic Cats now identified as highly vulnerable to this strain. Originally detected in wild birds and poultry, this virus has recently been found to cause illness in dairy cattle, and tragically, fatal infections in domestic cats. This report details the investigation into these cases, highlighting the severe impact of H5N1 on cats and the potential for cross-species transmission, raising significant alarms for pet owners and public health officials alike.
In early 2024, a mysterious illness emerged in dairy cattle across Texas, Kansas, and New Mexico. Farmers reported a sudden decrease in milk production, accompanied by nonspecific symptoms such as reduced appetite and lethargy in their cows. While initial tests for common bovine diseases came back negative, a parallel, more alarming situation unfolded: cats residing on these affected farms began to die.
These were not just any cats; they were domestic cats that had been fed raw, unpasteurized milk and colostrum from the sick cows. The symptoms observed in these cats were devastatingly swift and severe, including depression, uncoordinated movements (ataxia), blindness, circling behavior, and discharge from the eyes and nose. Neurological examinations revealed a lack of normal reflexes, indicating significant brain involvement. On one Texas dairy farm, over half of the cat population succumbed to this illness.
The Iowa State University Veterinary Diagnostic Laboratory (ISUVDL) launched an investigation into these cases, receiving samples from both cattle and deceased cats. The results were startling: both milk and tissue samples from the cows, as well as tissues from the cats, tested positive for Influenza A virus. Further confirmation by the US Department of Agriculture National Veterinary Services Laboratory identified the virus as the highly pathogenic avian influenza A(H5N1) virus. This marked the first reported instance of this HPAI strain causing significant illness in dairy cattle in the US, and more alarmingly, a high fatality rate in domestic cats linked to consumption of raw milk from infected cows.
Investigating the Outbreak: Methods and Findings
To understand the extent and nature of the infection, researchers conducted detailed examinations of both the cattle and cats. Samples collected from the cows included milk and tissues from euthanized or deceased animals. Postmortem examinations of the cows revealed mammary glands consistent with mastitis, though not consistently. Some cows showed ulcers and erosions in their gastrointestinal tracts, and signs of dehydration.
Two deceased adult domestic shorthaired cats from a Texas dairy farm were also examined. Externally, they appeared unremarkable, but internal examination revealed mild hemorrhages in the subcutaneous tissues of the skull and multifocal meningeal hemorrhages in the brain. Their gastrointestinal tracts were empty.
Microscopic analysis revealed key differences in lesions between cattle and cats.
Cattle Lesions:
The primary microscopic finding in cows was moderate acute multifocal neutrophilic mastitis, indicating inflammation of the mammary glands. Some cows also showed mild hepatitis. Immunohistochemistry (IHC) for Influenza A virus antigen confirmed the presence of the virus in the mammary glands of infected cows, specifically within the alveolar epithelial cells and lumina, as well as in lymph nodes in one cow.
Cat Lesions:
In stark contrast, the cats exhibited severe systemic viral infection. Microscopic lesions included severe necrotizing and lymphocytic meningoencephalitis (brain inflammation) with vasculitis and neuronal necrosis, necrotizing and lymphocytic interstitial pneumonia, necrotizing and lymphohistiocytic myocarditis (heart muscle inflammation), and lymphoplasmacytic chorioretinitis with retinal ganglion cell necrosis and retinal layer attenuation. IHC staining confirmed the presence of Influenza A virus antigen in the brain (neurons), lung, heart, and retina of the cats, indicating widespread viral dissemination and severe impact on multiple organs, particularly the nervous system and eyes.
PCR and Genetic Analysis:
Real-time reverse transcription PCR (rRT-PCR) tests confirmed high levels of Influenza A virus RNA in milk and mammary gland samples from cows, with lower and less consistent detection in other tissues. In cats, both brain and lung samples showed high viral RNA levels, further supporting the severity of infection in these organs.
Phylogenetic analysis of the hemagglutinin (HA) and neuraminidase (NA) gene sequences from both cow milk and cat tissues revealed a high degree of similarity, confirming that the virus infecting both species was closely related and belonged to the H5N1 clade 2.3.4.4b. This strongly suggests a shared source of infection and potential cross-species transmission. The HA sequences were almost identical (99.94% nucleotide identities) between milk and cat samples, and NA sequences were completely identical.
Implications and Concerns: Avian Flu and Cat Health
This investigation provides compelling evidence that domestic cats are highly susceptible to the HPAI H5N1 clade 2.3.4.4b virus through the consumption of raw milk from infected dairy cattle. The severe systemic illness and high mortality rate observed in the cats are consistent with previous reports of H5N1 infection in felines, typically linked to consuming infected wild birds. However, this case strongly suggests mammal-to-mammal transmission, with infected cows acting as an intermediary host, passing the virus to cats through their milk.
This finding is particularly concerning for several reasons:
- Cross-species Transmission: It underscores the ability of HPAI H5N1 viruses to jump between species and adapt to new hosts, including mammals.
- Mammal-to-Mammal Transmission Potential: The likely transmission via milk raises concerns about the potential for wider spread within mammal populations, not just through direct contact with infected birds.
- Risk to Domestic Cats: Cats, especially those in rural or farm environments with potential access to raw milk or contact with infected animals, are at significant risk.
- Public Health Implications: While commercial pasteurization of milk ensures safety for human consumption, the presence of the virus in raw milk highlights a potential route of exposure and the need for increased awareness about zoonotic risks.
The researchers emphasize the need for continued surveillance of HPAI viruses in domestic animals, including both livestock and companion animals like cats. Understanding the transmission pathways, pathogenesis, and evolution of these viruses is crucial for preventing further cross-species spread and mitigating potential public health risks. For cat owners, especially those in agricultural areas, this report serves as a critical reminder to never feed cats raw, unpasteurized milk, particularly in regions experiencing avian flu outbreaks.
The source of infection in cattle is presumed to be contaminated feed from wild birds, common carriers of avian influenza. The transmission among cattle seems to be horizontal, as evidenced by the spread to farms in other states that received cows from affected regions. Further research is needed to fully understand the transmission dynamics in cattle and the full range of mammals susceptible to this virus.
In conclusion, this outbreak highlights a significant and evolving threat posed by HPAI H5N1 viruses. The severe impact on domestic cats, coupled with the virus’s presence in dairy cattle and potential for mammal-to-mammal transmission via raw milk, demands heightened vigilance, continued research, and public awareness to protect both animal and human health.
