Scientists called for urgent surveillance of bird flu in domestic cats after an analysis of 20 years of published data revealed a dramatic spike in feline infections.
The rise in cases coincided with the emergence of H5N1 clade 2.3.4.4b, which has also infected several other mammal species.
The study, published in the journal Open Forum Infectious Diseases, characterised the ongoing avian influenza virus (AIV) outbreak as a panzootic — an epidemic spanning multiple animal species — and warned of an increased risk of zoonotic spillover to humans.
“Infections among mammalian species in frequent contact with humans should be closely monitored. One mammalian family, the felidae, is of particular concern. Domestic cats are susceptible to AIV infection and provide a potential pathway for zoonotic spillover to humans,” the study published in Open Forum Infectious Diseases stated.
Kristen Coleman, the study’s lead author and assistant professor at the University of Maryland School of Public Health, warned of a looming risk. “The virus has evolved, and the way that it jumps between species — from birds to cats, and now between cows and cats, cats and humans — is very concerning. As summer approaches, we are anticipating cases on farms and in the wild to rise again,” Coleman said in a statement.
Coleman added that bird flu was often fatal in cats and called for immediate efforts to assess how widespread infections were in feline populations. Cats are not routinely screened for bird flu and most infections are discovered only after death, suggesting the real numbers may be much higher.
During the study, the scientists screened research papers to find 607 bird flu infections in cats with 302 deaths across 18 countries and 12 species, ranging from cats to tigers.
Researchers reviewed global scientific literature on avian influenza in cats between 2004 and 2024. They identified 607 recorded infections across 18 countries and 12 feline species, including domestic cats, tigers, and wildcats. Of these, 302 cases were fatal.
Of the total cases, 92.3 per cent were linked to highly pathogenic avian influenza (HPAI), while 7.7 per cent were associated with low pathogenic strains. Among positive cases confirmed by polymerase chain reaction (PCR) tests, HPAI accounted for 99.7 per cent of deaths.
Roughly 98 per cent of PCR-confirmed feline infections were caused by HPAI H5N1, and 33.8 per cent of those were linked to clade 2.3.4.4b. Domestic cats made up 96.4 per cent of all reported infections, with a case-fatality rate of 52.8 per cent for H5N1 and 89.6 per cent for clade 2.3.4.4b specifically.
Domestic cats made up 62.6 per cent of all cases. When tested by PCR, 423 cases were confirmed — and 71.3 per cent of those cats died.
Infection pathways frequently included ingestion of infected birds or raw poultry feed. The most common symptoms in cats were respiratory and neurological issues, often followed by death. Recent cases also reported eye inflammation and blindness.
The virus was believed to have spread to some cats through consumption of raw colostrum or milk from infected dairy cattle , a transmission route observed during the H5N1 clade 2.3.4.4b outbreaks in 2023 and 2024.
The authors also found mild, unnoticed infections and warned that how the virus enters the body and the amount of exposure could make the disease more severe.
Nearly 50 per cent of infections were reported from Asia, including Southeast Asia, followed by Europe (25 per cent) and North America (16.7 per cent). Cases of clade 2.3.4.4b infections in cats were reported in Finland, France, Poland, Italy, Peru, the United States and South Korea, spanning five feline species including 135 domestic cats, two bobcats, a lion, caracal and wild lynx each.
“We observed a drastic flux in the number of AIV infections among domestic cats in 2023 and 2024, commensurate with the emergence of H5N1 clade 2.3.4.4b. We estimate that this phenomenon is underreported in the scientific literature and argue that increased surveillance among domestic cats is urgently needed,” the study underlined.
Infected cats were most frequently found in zoos, farms, animal shelters, private rural properties, and other close-contact settings. In most cases, transmission appeared to be from infected birds to cats, often through scavenging of infected carcasses.
“Interestingly, cases of H5N1 clade 2.3.4.4b recently reported by the Colorado Health Department included two indoor-only domestic cats with no known exposure to infected animals,” the researchers said, adding the observation raises concerns on new and unknown transmission routes of bird flu to domestic cats.
Coleman said, “The virus has evolved, and the way that it jumps between species — from birds to cats, and now between cows and cats, cats and humans — is very concerning.”
As summer approaches, the authors expect increased cases on farms and wild. Coleman cautioned that animal shelters could be vulnerable to outbreaks. The virus could enter these environments and trigger widespread transmission, potentially involving humans, similar to what occurred in New York City in 2016 with a different strain of avian influenza, she said.
While there were no confirmed cases of human-to-human transmission of H5N1 to date, the authors warned that the virus’s ability to adapt and mutate might enable airborne transmission in the future.