Avian Flu Diary: Pathogens: Clinical and Laboratory Findings in Cats with Confirmed Avian Influenza A/H5N1 Virus Infection During the 2023 Outbreak in Poland

#19,056

Three summers ago, when domestic feline infection with HPAI H5N1 was still considered a fairly rare event, we followed an unusually widespread outbreak in both indoor and outdoor Polish cats (see Media Reports Of Unusual Cat Deaths In Poland).

Two weeks later a government announcement (see Poland's National Veterinary Institute Genome Sequence Analysis Of H5N1 Viruses Detected In Cats), stated early testing suggested that the feline H5N1 avian influenza viruses analyzed all originated from a single, unidentified source.

While it was quickly and vehemently denied by both the government and the nation's poultry industry, the media and some local experts raised Concerns Over The Possibility Of H5N1 In The Food Chain (i.e. poultry).

Poultry production in Poland is a multi-billion dollar industry, with 20% of their earnings coming from exports (mainly to EU nations).

While Polish authorities pushed back hard against any suggestion that the virus might be in locally produced (and sold) poultry, on July 13th, 2023 the ECDC reported:

Among the affected cats, 13 were kept indoors with only occasional access to outdoor areas (i.e. balconies or terraces), whereas four were free-ranging and reportedly had contact with wildfowl. Raw poultry meat and offal were fed to 13 of the affected cats, but the exact time of feeding is unknown and no causal relationship has been established

The following advice was offered:

It is recommended to avoid exposure of domestic cats and dogs, and in general carnivore pets, to dead or diseased animals (mammals and birds), and to avoid feeding domestic cats and dogs offal and raw meat from wild or kept birds in areas where mortality in gulls or other potentially HPAI virus-infected animals are reported. Possible measures are keeping dogs on a leash, and confining cats indoors in areas where extensive circulation of HPAI viruses in wild birds has been confirmed.

In August of 2023, Eurosurveillance published Two Papers On HPAI H5N1 clade 2.3.4.4b virus in cats, Poland, June to July 2023, which described several mutations (PB2-E627K and PB2-K526R) considered to be mammalian adaptation markers, and warned `Although the most likely source appears to be poultry meat, no such meat has been identified to date.'

Officially, commercial poultry was never linked to this outbreak.

Last summer we looked at a study (see Viruses: The Seroprevalence of Influenza A Virus Infections in Polish Cats During a Feline H5N1 Influenza Outbreak in 2023) that reviewed 835 cat serum samples submitted for routine bloodwork during June of 2023 from cats not exhibiting influenza symptoms.

Out of those 835 cat serum samples - 68 cats (8.1%) tested positive for influenza A virus antibodies in the primary screening (with 3 more suspected). Of those 68 IAV positive samples, 23 were positive for H5-specific antibodies.

This suggests that some cats can be infected with H5 and survive, and that cats are also susceptible to a much wider range of influenza A viruses.

Since then we've seen pet-food related outbreaks in South Korea and in the United States, and today domestic cats are the most commonly reported (non-livestock) American mammal with H5N1.

Today we've got a highly technical report (of greatest interest to veterinarians) on the laboratory and clinical findings on 22 cats from the Polish outbreak of 2023. Some of the highlight, however, include:

These 22 cats came from different regions of Poland, and included both males and females, across a wide range of ages.

Many had outdoor access, but six were strictly indoor cats.

At least 13 of the 22 cats were known to eat raw meat, usually raw chicken or other poultry

The outbreak in cats happened currently with H5N1 reports in both poultry and wild birds in Poland, suggesting a link between infections in birds and spillover to cats.

The authors describe the infection as a `rapidly fatal respiratory and neurological disease', as most cats died or were euthanized within 2–3 days of the first signs, making the fatality rate 100%.

I've posted the abstract and a brief excerpt below. Those wanting a deeper dive can follow the link to read it in its entirety. I'll have a brief postscript after the break.

Clinical and Laboratory Findings in Cats with Confirmed Avian Influenza A/H5N1 Virus Infection During the 2023 Outbreak in Poland: A Retrospective Case Series of 22 Cats
Dawid Jańczak1,*, Anna Golke2, Karol Szymański3, Ewelina Hallmann3, Katarzyna Pancer3, Aleksander Masny3, Tomasz Dzieciątkowski4 and Olga Szaluś-Jordanow5,*

Pathogens 2026, 15(2), 200; https://doi.org/10.3390/pathogens15020200 This article belongs to the Section Viral Pathogens
Published:10 February 2026

Abstract

Highly pathogenic avian influenza (HPAI) A/H5N1 has emerged as a cause of severe disease in domestic cats, but clinical data from field outbreaks remain limited. We retrospectively reviewed medical records, laboratory results, and ancillary examinations from 22 domestic cats with RT-qPCR-confirmed A/H5N1 infection diagnosed in Poland in June 2023.

To the best of our knowledge, we report the first comprehensive retrospective case series from the 2023 Polish outbreak, combining 22 laboratory-confirmed cats with detailed clinical timelines and laboratory findings. For each cat, the temporal progression of clinical signs, hematology, serum biochemistry, and, when available, imaging findings were evaluated. Post-mortem examination data were not systematically available in this retrospective cohort.

Notably, six of these cats were strictly indoor cats that received raw poultry meat as part of their diet. Disease onset was acute, with fever, lethargy, and anorexia rapidly progressing to severe dyspnea and neurological signs, including ataxia, seizures, and paraplegia; case fatality was 100%, with a typical interval of ≤3 days from first signs to death or euthanasia. Hematologic changes were dominated by thrombocytopenia, lymphopenia, and marked eosinopenia, consistent with a systemic inflammatory/stress leukogram. Biochemistry indicated marked tissue injury, with increased AST, LDH, and CK activities, whereas creatinine and urea remained largely within reference intervals, arguing against primary renal failure. Imaging supported the presence of interstitial to diffuse pneumonia. These data characterize the clinical and laboratory phenotypes of feline A/H5N1 infection and underscore its importance as a rapidly fatal respiratory and neurological disease with One Health implications.

(SNIP)

5. Conclusions

Our findings confirm that A/H5N1 infection in domestic cats is associated with highly fatal, rapidly progressive respiratory and neurological disease, in which pneumonia and acute respiratory failure are central features. The clinical pattern, observed acute onset, fever, and depression, followed within 1–3 days by severe dyspnea, radiographic or sonographic evidence of lung involvement, and frequent neurological signs, should alert veterinarians to the possibility of HPAI infection, especially in cats with outdoor access, exposure to raw poultry meat, or contact with wild birds or their feces. Early recognition, targeted diagnostics, and the implementation of appropriate biosafety measures are essential for potential zoonotic and epidemiological risks.

(Continue . . . )

As we discussed last week in Several States Warn On Contact With Wild Birds/Mammals, there is a high level of H5N1 in the environment right now, which raises the risks to both humans and their pets.

Outdoor cats, or those fed a died of raw meat, are obviously at highest risk.

But when H5N1 is circulating at high levels, any pet that suddenly falls ill with fever, rapidly develops breathing problems or shows neurological signs, should be regarded as a possible H5N1 case until proven otherwise.

For more on pets and H5N1, you may wish to revisit: