University of Maryland scientists are calling for increased surveillance of avian flu in domestic cats after a global review of 20 years of published data reveals a dramatic uptick in feline infections—and the number of ways cats are being infected—after the emergence of H5N1 clade 2.3.4.4b in other mammals.
"Infections among mammalian species in frequent contact with humans should be closely monitored," the researchers wrote yesterday in Open Forum Infectious Diseases. "Domestic cats are susceptible to AIV [avian influenza virus] infection and provide a potential pathway for zoonotic spillover to humans."
The team conducted a systematic review of scientific literature from 2004 to 2024 to track the epidemiology and global distribution of AIV in cats.
'New and unknown transmission routes'
The review identified 48 articles that detailed 607 AIV infections in 12 feline species (ranging from pet cats to tigers), 302 of them resulting in death, in 18 countries. Half of the cases were from Asia, followed by Europe (25%) and North America (16.7%). H5N1 clade 2.3.4.4b infections were reported in Finland, France, Poland, the United States, Italy, Peru, and South Korea in five species (135 domestic cats, 2 bobcats, and 1 lynx, caracal, and lion).
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 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," which was consistent with the emergence and increased transmission of the clade in birds and mammals, the authors said.
Zoos, animal shelters, farms and private rural land were the most common settings of infections in cats. In total, 62.6% of the cases involved domestic cats, and 71.3% of the 423 polymerase chain reaction (PCR)-confirmed cases were fatal. Most infections were confirmed or suspected to result from bird-to-cat transmission, most often from eating dead pigeons, chickens, or other birds but also from contaminated raw chicken feed.
"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 authors wrote. "This observation raises concerns regarding new and unknown transmission routes of AIV to domestic cats."
High death rate
A total of 92.3% of feline cases were identified as highly pathogenic avian influenza (HPAI), and 7.7% were low-pathogenic avian flu. Among the PCR-positive infections, HPAI made up 99.7% of deaths.
Among the 98% of the PCR-confirmed feline infections identified as HPAI H5N1, 33.8% were clade 2.3.4.4b, and 96.4% were domestic cats. Of these cats, the case-fatality rate was 52.8% for H5N1, and 89.6% were clade 2.3.4.4b.
Of the studies that described symptoms, respiratory and neurologic illness were most common and often led to death. Blindness and chorioretinitis (inflammation of the choroid and retina of the eye) were also recently reported in two infected domestic cats that contracted the virus from drinking raw colostrum and milk containing high viral loads from dairy cattle infected with the clade 2.3.4.4b virus. Subclinical feline infections have also been reported.
"This clinical observation suggests that exposure route and dose of AIV might impact disease presentation and severity," the authors wrote.
Infections expected to rise
The avian flu outbreaks that started in February 2024 in dairy cattle are worrisome because most infections in mammals have been in carnivores or omnivores, they said: "The transmission to herbivores is interesting, as avian influenza is often foodborne in mammalian hosts, and tends to result from a new host eating an infected host."
The infection of ruminants rules out the predation or scavenging route of transmission in this case and suggests that other routes of transmission are occurring, in addition to cattle-to-cattle transmission.
"The infection of ruminants rules out the predation or scavenging route of transmission in this case and suggests that other routes of transmission are occurring, in addition to cattle-to-cattle transmission," they added.
Avian flu has infected 950 people worldwide and killed half of them. From April 2022 (when cumulative data on US human cases started being collected) and January 2025, the country has recorded 66 human infections and 1 death, the researchers noted.
"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," lead and senior author Kristen Coleman, PhD, said in a University of Maryland press release. "As summer approaches, we are anticipating cases on farms and in the wild to rise again."
Of particular concern, she said, is the potential for the virus to enter animal shelters, which could cause large outbreaks potentially involving people, similar to or worse than what happened in New York City in 2016 with a different avian flu strain.
Cases likely an underestimate
No cases of human-to-human transmission of avian flu have been reported, but the investigators worry that as the virus spreads and evolves, it could become transmissible through the air.
We estimate that this phenomenon is underreported in the scientific literature and argue that increased surveillance among domestic cats is urgently needed.
"Our future research will involve studies to determine the prevalence of HPAI and other influenza viruses in high-risk cat populations such as dairy barn cats," coauthor and doctoral student Ian Gill Bemis said in the release.
The number of new and unknown transmission routes is worrisome, because cats are not monitored for avian flu, and when testing is performed, it is usually done after death, the authors said. Also, infected cats often experience encephalitis (brain swelling) and other severe symptoms that are often misdiagnosed as rabies.
"We estimate that this phenomenon is underreported in the scientific literature and argue that increased surveillance among domestic cats is urgently needed," they concluded. "As feline-to-human transmission of AIV has been documented, and potential airborne and fomite-mediated transmission implicated, farm and free-roaming cat owners, veterinarians, zoo keepers, and animal shelter volunteers may have a heightened risk of AIV infection during epizootics among birds and mammals."
