Feline avian influenza cases spark concerns

Since December 2022, the U.S. Department of Agriculture's (USDA) National Veterinary Services Laboratories (NVSL) had confirmed more than 130 feline cases of highly pathogenic avian influenza (HPAI) type A H5N1 infection across the country, from California to New Jersey. They range from barn and feral cats to pet cats, including those kept strictly indoors, to big cats in zoos and the wild.

Around 130 cases of highly pathogenic avian influenza type A (H5N1) have been confirmed in domestic cats, which are "exquisitely sensitive" to the virus, says Dr. Meghan Davis, an associate professor at Johns Hopkins Bloomberg School of Public Health.

Results from a recent review by the University of Maryland School of Public Health (UMDSPH) of more than 40 studies from over two decades suggest cats are potential reservoirs and vectors for avian influenza viruses, particularly highly pathogenic H5N1 strains, adding to growing calls for greater HPAI monitoring of domestic cats to mitigate pandemic risk.

Modes of transmission

Given the high prevalence and adaptability of H5N1, animal disease experts believe the virus is no longer an exotic disease in the United States but now is endemic in wild birds. Much of the current research is focused on protecting cattle, poultry, and the public from H5N1.

What little is known about the transmission dynamics of the H5N1 virus in cats comes mostly from investigations of infected cats. A Centers for Disease Control and Prevention (CDC) journal, Emerging Infectious Diseases, reported in April 2024 that H5N1 was found in cats that died after consuming raw colostrum and milk from infected cows. Since then, not only raw milk but also raw foods containing the virus have been linked to infections in cats.

The Food and Drug Administration (FDA) has been tracking cases of H5N1 in domestic and wild cats in California, Colorado, Oregon and Washington State that are associated with eating contaminated food products.

"Scientific information is evolving, but at this time it is known that H5N1 can be transmitted to cats and dogs when they eat products from infected poultry or cattle (e.g., unpasteurized milk, uncooked meat, or unpasteurized eggs) that have not undergone a processing step that is capable of inactivating the virus, such as pasteurizing, cooking or canning," the FDA information stated. "Cats (domestic and large felids) in particular can experience severe illness or death from infection with H5N1. Dogs can also contract H5N1, although they usually exhibit mild clinical signs and low mortality compared to cats. At present, H5N1 has not been detected in dogs in the United States, but there have been fatal cases in other countries."

On March 1, Washington-based pet food maker Wild Coast Raw recalled some of its food after an investigation into cat deaths found a link to the food. And on March 15, the New York City Health Department warned New Yorkers against feeding pets Savage Cat Food products after H5N1 infections in two, possibly three, cats were linked to the company's raw cat food.

Savage Cat Foods, a California-based company specializing in raw, prey-based diets for cats, issued a voluntary recall for Savage Cat Food poultry packets, lot number 11152026.

Another report, this one appearing in the February 20 issue of the CDC's Morbidity and Mortality Weekly Report, indicated the possibility of multiple modes of transmission to cats, such as from an infected person or contaminated fomites, like clothing and footwear, worn on affected premises and brought into the home.

Then in late February, New Jersey announced that six cats that had roamed the same property had become ill. One of the cats, which was feral, was confirmed to have H5N1 and was euthanized because of severe illness.

According to the New Jersey Department of Public Health, another cat was an indoor-outdoor cat. The living situation of the remaining four cats is unclear.

All six cats likely had contact with each other, according to public health officials. None of them had any known exposure to H5N1-infected livestock or ate unpasteurized dairy or raw poultry products. However, they may have encountered infected wild birds or other animals outdoors.

Notably, USDA Animal and Plant Health Inspection Service (APHIS) information states that cats, as well as other scavenging or carnivorous species, are typically dead-end hosts for this avian influenza virus, "meaning that, with rare exceptions such as transmission among family groups or where they roost together, they are not significantly involved in onward transmission."

Stepped-up surveillance

Kristen Coleman, PhD, an airborne infectious disease researcher at UMDSPH, led a systematic analysis of 41 studies published between 2004-24 to evaluate the global spread, epidemiology, and risks of avian influenza virus (AIV) infections in feline species, both domestic and wild. She found that a sharp spike in reports occurred in 2023 and 2024, largely tied to the emergence and spread of the H5N1 clade 2.3.4.4b strain responsible for ongoing outbreaks in dairy cattle.

HPAI infections were reported in a dozen wild and domestic feline species in 17 countries across seven global regions, most frequently from Asia (54%), Europe (24%), and North America (12%). Of the 486 confirmed AIV infections in felines, there were 249 deaths-a case fatality rate of 63%. For H5N1 clade 2.3.4.4b specifically, the fatality rate is even higher at 67%.

"Our numbers are definitely an underestimate of the bigger picture, and this is because we're just reporting whatever has been reported in the scientific literature. There are many other cases that have happened that don't end up being reported," said Dr. Coleman, an assistant professor at UMDSPH and affiliate professor in the university's Department of Veterinary Medicine.

Surveillance of AIV in domestic cats is critically lacking and urgently needed, especially in farm and shelter environments, according to Dr. Coleman, who worries cats could serve as a bridge for zoonotic spillover of H5N1 to humans. She is also concerned about the emergence of a mutated viral strain capable of spreading cat-to-cat. "This actually happened in New York City back in 2016," Dr. Coleman said. "It was a different strain of bird flu, but there were over 400 cats that were affected."

Dr. Meghan Davis, a molecular epidemiologist and environmental microbiologist and associate professor at Johns Hopkins Bloomberg School of Public Health, also believes the epidemiological data on AIV in cats to be in short supply.

She advocates for active testing by veterinarians, arguing that such surveillance is necessary given the rapidly evolving understanding of the multiple transmission routes for cats. However, such an aggressive measure has its own considerations, including questions over who would pay for the additional testing without reimbursement from the government currently.

In an article Dr. Davis co-authored for the website STAT, she explained why active testing was necessary: "Given the pandemic risk from H5N1, active surveillance of companion animals is needed to recognize the full scope of the problem and identify critical control points for intervention. It is most needed in instances where exposure could be highest, including pets of dairy and poultry workers and those fed raw milk or meat.

"Such surveillance is necessary given our rapidly evolving understanding of the multiple transmission routes for cats. One cat in the (MMWR report) reportedly rolled on the work clothes of the owner, who hauled unpasteurized milk and had contact with H5N1-infected farms. We need a clearer understanding of whether indirect routes of exposure like this can lead to infection in cats," the article states.

The government does not currently reimburse veterinarians for H5N1 testing for cats. Further, the volume of tests and man-hours required for active testing would be substantial. Not to mention, the potential stress on healthy cats associated with these increased visits is another factor.

To date, 70 human cases of H5N1 infection have been reported, including one death, according to the Centers for Disease Control and Prevention (CDC). With that in mind, the CDC encourages veterinarians to don personal protective equipment (PPE) when handling high-risk cats, which she defined as cats that have had contact with alive or dead animals confirmed or potentially infected with H5N1 or those that consume raw milk or raw meat pet diets.

"A respiratory or neurologic cat that consumes raw milk or raw meat pet food-or that goes outside in regions where H5N1 is known to be circulating-can be handled with the same intake infection prevention protocols used for other suspect transmissible diseases," Dr. Davis said.

To safely handle a high-risk cat, she recommended scheduling the appointment at the end of the day and having the cat enter the clinic through a side door. Use a designated room away from other exam rooms, and practice good hygiene by using gloves and changing scrub tops and coats after seeing the patient. And finally, clinic staff should consider using PPE for themselves, as recommended by the CDC and AVMA, for suspect cases.