With the plethora of news stories about the spread of H5N1 influenza making headlines each day, it can be hard to keep track of the latest updates. Finding the most relevant information for small animal practitioners can also be a challenge. To get you as much feline-specific information all in one place, we’ve gathered the latest information from a combination of federal agencies, research studies, news reports, and veterinarians studying the virus.

 

H5N1 in cats is a cause for concern 

Just this month, the first human fatality in the US attributed to H5N1 influenza infection was reported. The patient was an elderly individual with preexisting medical conditions who had been exposed to a backyard flock and wild birds. However, there is reason for individuals without those risk factors to be on alert. The CDC found mutations that could make the virus more infectious to humans in viral samples from the human patient but not from those taken from the backyard flock.   

In addition to the threats to human health and our food supply, this highly infectious virus has also proven to be a formidable threat to domestic cats. Most feline cases reported in the news result in swift progression of clinical signs and death. Cat owners and small animal veterinary practitioners are justifiably concerned and looking for answers as to how the disease is spread. Other concerns include how to diagnose it and how to reduce the risks of transmission to cats and other species.   

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From flocks to felines 

Although there have been avian influenza epidemics throughout the world for more than a century, the latest outbreak of H5N1 influenza, one of the strains commonly referred to as highly pathogenic avian influenza, started in 2022. Since then, this virus has affected over 130 million birds in the U.S..  While it has primarily been a disease of wild birds and poultry flocks, the USDA has also detected the virus in many wild mammalian species. These species include polar bears, dolphins, bobcats, racoons, and squirrels.  

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Where it all started

In March 2024, H5N1 influenza was first diagnosed in dairy cattle. These animals were likely infected through exposure to sick wild birds such as grackles and pigeons found dead or dying near the farms.  

Not long after that, domestic cats on two of the affected dairy farms started showing respiratory signs, lethargy, and neurologic deficits, before they subsequently died. They were confirmed to have H5N1 influenza as well, marking the first cases of H5N1 in domestic cats.  

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Other cases

Since the virus was diagnosed in those first few cats almost a year ago, additional cases have been identified throughout the U.S. and the world. 

According to Commander Lizette O. Durand, VMD, PhD, Chief Veterinary Officer, National Center for Immunization and Respiratory Diseases, Influenza Division, over 40 domestic cats were reported to have been infected with H5N1 influenza by the USDA Animal and Plant Health Inspection Service in 2024. Cats in Poland, South Korea, and France have also been diagnosed with the virus.

The true incidence of H5N1 in domestic cats is currently unknown. Durand said this is due to a “lack of standardized reporting and testing requirements for H5N1 in companion animals.” 

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How H5N1 in cats is transmitted

Cats can become infected with H5N1 through a variety of exposure routes. Exposure routes include the consumption of unpasteurized milk from infected cattle, preying on infected birds, and eating uncooked poultry, eggs, or beef from infected animals.  

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Dairy farms

Of the known infected cats in the U.S., most have been associated with dairy farms infected with H5N1 (where they may have consumed unpasteurized milk or had contact with infected birds), Durand said. But a few of the cats were indoor/outdoor cats who were not associated with a dairy farm. Rather, they’re assumed to have come in contact with infected wild birds.  

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Raw pet food

Cases of cats becoming infected with H5N1 after consuming commercial raw pet foods have recently been reported in California and Oregon, where the food manufacturer subsequently issued a voluntary recall of their product. Cats in California also became infected and died after consuming infected raw milk for which a recall had been issued.

 

Can H5N1 in cats be transmitted cat to cat?

According to Marta Mainenti, DVM, DACVP, Clinical Assistant Professor and Diagnostic Pathologist at Iowa State University Veterinary Diagnostic Laboratory, it is not yet known if H5N1 can be directly transmitted between domestic cats. There is evidence to suggest cat-to-cat transmission between tigers, she said, but as of now, not between domestic cats.  

There is also no known evidence that H5N1 can be transmitted from infected humans to cats. However, humans’ clothing and equipment could act as a fomite for transmission of the virus to cats. This is particularly concerning in the case of dairy and poultry farm workers who have pet cats. 

Morbidity and mortality 

Morbidity and mortality vary significantly between species. Birds have very high morbidity and mortality rates and cattle have much lower morbidity and mortality rates. The true morbidity and mortality rates in cats are still undetermined.  

While most reports of infected cats are tied to severe clinical signs and rapid death, some cats have survived infection. Chris Siepker, DVM, DACVP, Clinical Assistant Professor and Diagnostic Pathologist at Iowa State University Veterinary Diagnostic Laboratory, recalled that on the dairy farms where cats were first diagnosed with H5N1, roughly 50% of the cats with exposure history died.   

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More information needed

He added that it’s too soon to know which factors (route of exposure, dose of exposure, the health of each cat’s immune system, etc.) were most responsible for varying outcomes among exposed cats. However, the clinical signs presented did seem to correlate with the likelihood of death. For example, whereas infected cats with only upper respiratory signs seemed to fare better, those with neurologic signs had a more “negative prognosis,” according to Siepker.  

In terms of morbidity, some studies have found evidence of subclinical infection. This suggests that not every cat who is exposed will get sick. A study conducted in the Netherlands determined that 11.8% of stray cats sampled were seropositive for H5N1 influenza without showing any clinical signs of infection. The researchers suggested that cats’ morbidity and mortality with exposure to H5N1 is likely dependent on the dose of viral exposure they receive.  

Testing cats suspected of infection 

Durand recommends that veterinarians who suspect that their feline patient may be infected with H5N1—based on a documented history of exposure or risk of exposure coupled with relevant clinical signs—should contact their state public health veterinarian for guidance and testing recommendations. They should also consider rabies as a differential diagnosis for any cat presenting with neurologic signs, Mainenti added.  

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PCR testing

As part of their work at the Iowa State University Veterinary Diagnostic Laboratory, Mainenti and Siepker conduct PCR testing for H5N1 in cats and have recently published their research on ante- and postmortem diagnosis of H5N1 in cats. They explained that PCR testing for influenza A H5 subtype is offered by many Level 1 National Animal Health Laboratory Network (NAHLN) approved laboratories. Some of these are also state laboratories.  

Mainenti commented that they have received a variety of different sample types, including oropharyngeal swabs, urine, and whole blood. Oropharyngeal swabs, in which a sterile swab is used to obtain a sample of saliva and/or secretions from the area around the pharyngeal folds, have proven to be the most reliable samples for testing. Whenever possible, she added, it’s best to send more than one type of sample. Doing so improves the likelihood that the virus will be detected if it’s present.   

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Test reporting

Instead of reporting results as positive or negative, the PCR tests are reported as either negative or non-negative. That is because the National Veterinary Services Laboratories (NVSL) must perform an additional confirmatory test on any non-negative results for a cat to be considered positive for H5N1. As part of this, the NVSL will also determine if the strain of H5N1 is of bovine or avian origin, Siepker added, since cats are susceptible to both. 

Reducing the risk of transmission 

The various routes of exposure that are known to be risks to cats create multiple opportunities for animal health professionals and cat owners to reduce the risk of H5N1 transmission to cats.   

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Food safety

The CDC, FDA, USDA, and AVMA all strongly advise against the feeding of unpasteurized milk and milk products to pets. They also advise against uncooked beef or poultry products (including commercial raw pet foods) to pets. Both the FDA and the USDA have confirmed via studies that pasteurization inactivates the H5N1 virus in milk. The CDC has also confirmed that cooking poultry to 165 degrees and beef to 160 degrees effectively inactivates the virus in these foods.  

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Limit potential exposure

Limiting cats’ access to other birds and cattle is another opportunity to reduce their risk of exposure to the virus. Durand recommends not allowing cats to roam unsupervised outside to prevent wild bird exposure. The AVMA also recommends keeping cats “away from livestock, poultry, and their environments.” This reduces their risk of exposure to H5N1 influenza. 

Additional precautions should be followed by cat owners who work on dairy or poultry farms, Durand said. This includes keeping work clothing and shoes away from pets and thorough hand washing after work and before any interactions with pets. 

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Safety first

Veterinary professionals caring for cats who may have been exposed to H5N1 should utilize personal protective equipment (PPE) to reduce the likelihood of human and animal exposure to the virus. The CDC outlines recommended PPE, including fluid-resistant coveralls or gowns, an N-95 respirator, goggles or a face shield, boots or boot covers, a head covering, and gloves.