H5N1 Virus in Cats: An Outbreak of Unprecedented Scale in Poland

The Polish outbreak in June–July 2023 changed the role of cats in the epidemiology of the H5N1 virus. These are no longer sporadic cases of feline infection with the A (H5N1) virus. In fact, this is the first time a country has reported a high number of cats infected with the avian influenza virus across a wide geographic area. As early as July 2023, the World Health Organization (WHO) documented the unusual scale of this outbreak in Poland: 47 samples, taken from 46 cats and one captive caracal, were analyzed. Of these 47 samples, 29 (62%) tested positive for influenza A(H5N1). The positive samples came from 13 different geographic areas of the country. In total, 20 cats exhibited neurological signs, 19 exhibited respiratory signs, and 17 exhibited both neurological and respiratory signs. Genomics analysis showed that the 19 sequenced H5N1 viruses all belonged to clade 2.3.4.4b and were closely related to one another, suggesting a common source of infection. These viruses are also similar to influenza A (H5N1) viruses of clade 2.3.4.4b circulating in wild birds that have recently caused outbreaks in poultry in Poland.  

Risk of Exposure for Owners and Veterinarians

The presence of similar viruses, with characteristics adapted to mammals, in such a large number of cats is a cause for concern. The source of the cats’ exposure to the virus remains unknown, and epizootic investigations are ongoing. Although no cases of human infection with this virus have been documented to date, the risk does exist. However, detection of the avian influenza A(H5N1) virus in humans remains unusual, and no sustained human-to-human transmission has been reported. Consequently, the WHO assesses that the risk of human infection following exposure to infected cats is low for the general population, and low to moderate for cat owners as well as for exposed professionals, such as veterinarians, in the absence of appropriate personal protective equipment. In this context, the recommendations from the CDC for veterinarians are consistent with this view: they aim to reduce exposure among staff in contact with cats or other animals with suspected or confirmed infection with the H5N1 virus. Recommended infection control measures include, in particular, raising owners’ awareness of the risk of exposure, confining cats when the environmental risk is high, and, above all, discontinuing raw diets (raw meat or raw milk, commercially produced raw foods) in areas where the H5N1 virus is circulating.  

Acute Clinical Course

*Pathogens* published the results of a retrospective case study from the 2023 Polish outbreak, including 22 cats with laboratory-confirmed infection. This case series expands upon existing data—which were initially limited to clinical reports of isolated cases published to date—and provides an overview of the clinical course, including hematological and biochemical parameters, within a larger cohort. The clinical picture was dominated by the sudden onset of fever, lethargy, and anorexia, rapidly followed by severe respiratory distress and neurological signs. Imaging confirmed the presence of pneumonia. The course of the disease was fulminant: all cats died or were euthanized within a few days of the onset of clinical signs. These findings reaffirm the strong respiratory tropism and high virulence of highly pathogenic avian influenza viruses in cats, thereby complementing previous data from Asia, Europe, and North America. When highly pathogenic strains are involved, influenza A viruses therefore represent a rarely diagnosed but clinically significant cause of severe pneumonia. The combination of acute onset, very high fever, marked dyspnea, and concomitant neurological signs should promptly raise suspicion of infection with highly pathogenic avian influenza virus, particularly in cats that have been in contact with wild birds, poultry, exposed to their droppings, or fed raw meat.   The incident in Poland warrants adopting new protocols: when a cat presents with an acute febrile syndrome that rapidly progresses to dyspnea and/or neurological signs—especially in cases of confirmed or potential exposure to the virus—avian influenza must be included in the differential diagnosis. In practical terms, the challenge is to reduce the time between suspicion and sample collection. Confirmation relies on PCR tests sent to approved national laboratories. The rapid characterization of strains originating from mammals must remain a priority, even when no human cases are detected.