Monday, September 15, 2025

Virology: Highly Pathogenic Avian Influenza A H5N1 Virus Infection in an Immunocompromised Domestic Cat

  

PPE Recommendations For High Risk Settings

#18,873

We've known for > 2 decades that cats - both large and small - are susceptible to HPAI H5 infection (see 2004 CIDRAP Report) and have seen scattered reports from around the globe (see 2015's HPAI H5: Catch As Cats Can).

Until fairly recently, those reports were relatively rare, and mainly came from zoo/captive animals fed on a diet of raw chicken (see Fatal H5N1 Infection In Tigers By Different Reassortant Viruses - China).

Note: In 2016 we did see a large nosocomial outbreak of LPAI H7N2 in cats in New York City animal shelters, with spillover to humans, and in 2007 we saw a report from Indonesia suggesting 20% of stray cats in Jakarta may carry H5N1 antibodies.

Since the emergence of a new, more transmissible H5N1 virus in 2021, however, the number of companion animals (and other mammals) reported to be infected has risen sharply.

In 2023 we saw two high profile outbreaks; in Poland and in South Korea. The South Korean outbreak was quickly linked to contaminated pet food, while the results from Poland were less conclusive.

In the United States the USDA  began tracking H5N1 in cats in May of 2024, after receiving reports of multiple feline deaths around dairy farms, following the emergence of a new `bovine' B3.13 genotype.

While undoubtedly an undercount, as of today the USDA lists 145 domestic cats as infected (along with several dozen other feline species). 

The source of infection for these cats varies, with some exposed via contaminated (raw) cat food (see here, here, here, here, and here), while others were likely exposed via infected birds, cattle, rodents - or from contaminated environments. 

While many cats develop severe systemic illness and succumb to the virus, others appear to survive the encounter (see Viruses: The Seroprevalence of Influenza A Virus Infections in Polish Cats During a Feline H5N1 Influenza Outbreak in 2023), and some may even carry the virus asymptomatically.  

Last March we closely followed an outbreak of H5N1 in cats in New York City, which was linked to the consumption of contaminated cat food (see NYC DOH Update: H5N1 In Cats Linked To Raw Food & Suspected Cat-to-Cat Transmission).

Today we've a case report, published in the ASM journal Virology, which looks at what appears to be the result of nosocomial transmission of the H5N1 virus at a veterinary clinic during this outbreak. 

Phylogenetic analysis showed the cat-derived virus was >99% identical to strains circulating in U.S. dairy cattle and cats, with a noted N71S NA stalk mutation, which has previously been reported in other cats (see Emerg. Microbes & Inf.: Marked Neurotropism and Potential Adaptation of H5N1 Clade 2.3.4.4.b Virus in Naturally Infected Domestic Cats).

This is a lengthy and detailed report, so I've only included the abstract and some excerpts.  Follow the link to read it in its entirety.  

I'll have a postscript when you return. 

Highly pathogenic avian influenza A H5N1 virus infection in an immunocompromised domestic cat

Background

Highly pathogenic avian influenza (HPAI) H5N1 viruses of clade 2.3.4.4b have recently caused widespread outbreaks in mammals, including domestic cats that live closely with humans and other animals. In-depth molecular and pathological characterizations of naturally infected cats are urgently needed for developing better strategies to prevent interspecies transmission and further spreading of these viruses.

Case Summary

In this case report, we characterized a unique case of HPAI H5N1 virus infection in an immunocompromised domestic cat. The pet animal was a diabetic cat with a history of feline infectious peritonitis (FIP). In early 2025, the cat developed acute fever and rapidly worsening respiratory distress and liver dysfunction despite antibiotic treatment.
Due to severe clinical deterioration, the cat was euthanized. Postmortem examination revealed severe bronchointerstitial pneumonia, hepatic and lymphoid necrosis, bone marrow degeneration, and mild lymphohistiocytic meningitis. H5N1 viral RNA/antigens were specifically detected in the lung, brain, urine, or lymphoid tissues.
Whole-genome sequencing and phylogeny analysis identified that the virus belongs to influenza clade 2.3.4.4b (B3.13 subgroup), closely related to HPAI H5N1 strains that are currently circulating in domestic cats and cattle. The source of infection for this particular cat might be linked to a fomite/environmental transmission route.

Conclusion

The lethal HPAI H5N1 virus infection in an immunocompromised cat highlights the need for developing an improved prevention plan for pet animals. Clinicians should consider the possibility of H5N1 virus infection in cats with similar acute respiratory or neurologic signs, particularly in animals with chronic illness.

       (SNIP)

The exact source of infection for this cat has not been confirmed. Given no evidence of dairy or poultry consumption or contact with birds, cattle, or individuals working in poultry or cattle industries, we suspect that this cat may have been infected by fomite/aerosol transmission in the clinic by another cat with possible H5N1 virus infection. This highlights the importance of hand hygiene/decontamination between patients for healthcare.
Another important consideration in this cat is concurrent diabetes mellitus and prior infection with FIPV, both of which can be related to a weakened immune system that may promote transmission of the H5N1 virus. This indicates that animal health conditions need to be considered when assessing spillover risk. Animal handlers/healthcare providers should understand the potential risks for immunocompromised animals sharing the same space with other infected animals.

        (Continue . . . ) 


In the spring of 2024 the CDC released Guidance for Veterinarians: Evaluating & Handling Cats Potentially Exposed to HPAI H5N1, including the use of PPEs.

Earlier this year the CDC reported serological evidence of HPAI exposure in 3 veterinarians (see MMWR: Seroprevalence of Highly Pathogenic Avian Influenza A(H5) Infections among Bovine Veterinary Practitioners – United States, September 2024), from (n=150) samples collected last August.

None of the positive cases suspected they had contracted the virus, and two denied working with infected cattle (including one who worked in two states that have never reported bovine H5). A finding the authors suggested may indicate `. . . . there could be U.S. states with A(H5)-positive people and animals that have not yet been identified.'
Just over a month ago, in JAVMA: Companion Animals and H5N1 Highly Pathogenic Avian Influenza: Cause for Concern?, we looked at a study that warned:
The recent detections of genotype D1.1 in 3 humans with severe or fatal illness from Canada, the US, and Mexico and in a cat from the US in early 2025 are worrisome new developments. Concurrently, the recognition of highly related clade 2.3.2.1a viruses in humans and cats from India demonstrates how multiple pathways for reassortment could evolve simultaneously.
Although the risks of human infection from H5N1 infected cats is currently thought to be low - it is not zero - and that could increase as the virus picks up more mammalian adaptations. 

Meanwhile, the CDC offers the following advice to pet owners on how they may  keep their animals safe from avian flu, and what to do if they suspect infection. 

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