Spike in avian flu cases in cats triggers worry about human spillover
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.
We let Joey yell at the birds but NEVER
to eat them (photo by Will Collette)
"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," 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,"
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.
"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."
