H5N1 and Cats: Growing Concerns About Cross Species Viral Transmission

By Ingrid R Niesman MS, PhD

In May of 2024, I told you not to panic about H5N1. I am now revising that statement to tell you that anxiety and concern are appropriate reactions to current events.

The reality is that cats are highly susceptible to H5N1 and some of our worst fears appear to be manifesting.

There is a lack of credible data reporting

 On Wednesday February 5, 2025, a much delayed Mortality and Morbidity Weekly Report (MMWA) from the Centers for Disease Control (CDC) describing probable human to cat and possible cat to human transmission was initially posted, then hastily removed by our government. The New York Times managed to grab the posting before it was deleted (CDC deletes H5N1 data).

Veterinarians, physicians and scientists have been watching these developments carefully worldwide. Until now, our public health expertise has been unparalleled. Without credible, timely and complete information, this virus will move quickly to infect more wildlife, pets and livestock.

The number of current reported domestic cat infections 

As of February 5, 2025, The USDA Animal and Plant Health Inspection Service (APHIS) has reported 83 domestic cats positively infected since late December of 2022 (APHIS, 2/5/25). The most recent update about new positive mammal results is from January 31, 2025. Whether the cats from the deleted CDC reports were included in this list is questionable, as the locations are unknown.

Although 83 dead cats may not seem like a large number, this is likely an undercount. As cat lovers know, sick cats hide when ill. Therefore, the number of community and feral cats is almost certainly underrepresented. Asymptomatic ferals would need to be trapped first.

As an example, in a January 24 news briefing, the Louisiana Department of Agriculture and Forestry reported a feral cat brought in by the public for neutering. Subsequently, the animal was sickened and eventually euthanized (Feral cat in Plaquemines Parish). The infection was only confirmed after further testing at the National Veterinary Services Lab (NVSL).

Given the cost of testing and a reluctance at all levels of surveillance to waste resources, few cats are tested specifically for H5N1, and usually, like our feral, only after death.

Loss of epidemiological data will affect animals and humans

Loss of important epidemiological data will eventually affect many aspects of human and veterinary medicine. As a cautionary example, the origins of the original SARS and SARS-CoV2 viruses lie in bats. What is questionable is how and when they became human pathogens. This happened in China and went unreported to the rest of the world.

We are currently seeing the potential development of a human-adapted H5N1 virus in real time. More importantly, due to complacency in the United States, our cats are at risk, too.

Since it appears that the CDC is currently prohibited from updating the public, use of state-level censuses or public reports of disappearing community cats and circulating conversations of practicing veterinarians may be the only way to prevent the United States from flying blind into a potential zoonotic pandemic.

Implications for veterinarians

Corey Cole, DVM, a San Diego veterinarian and owner of Community Mobile Veterinary Services and Telemedicine emphasizes the implications for primary care. “Right now, when a cat presents with respiratory symptoms, the big three, herpes, calicivirus, and bordetella are high on the list of differentials, and we recommend supportive care,” he explains. “Diagnostic testing, especially expensive PCR panels,  is not always needed or considered.”

However, with regularly reported H5N1 data, increases in cases locally or regionally are quickly recognized and disseminated. Clinicians can have a better snapshot of their patient. If H5N1 infections are locally rare, then why look to H5N1 first? If local or regional infections rates are high, look for unusual clinical symptoms like high fevers, neurological signs or jaundice in cats with respiratory symptoms, suggests Dr. Cole.

Moreover, understanding changing clinical signs to distinguish H5N1 from other neurological pathogens like rabies requires accurate medical assessments, investigations of the infectious environment and collated information. Without a repository for this data, key pieces will remain undetected.

Implications for cat parents 

If the data currently withheld shows the reported human to cat transmission, we as cat parents need to protect our cat’s lives.

In Part I of this series (Protecting Your Cats), I shared recommendations and guidelines for what you can do right now to keep your cats safe and prevent another pandemic, ranging from simple hygiene to avoiding feeding raw or undercooked poultry. There is irrefutable evidence that cats get severe disease from virus-infected foods. We may not get immediate recalls for contaminated pet foods. Report any suspect meals to your local public health and veterinary offices quickly.

As of January 6, 2025, there have been 66 confirmed human cases of H5N1 bird flu in the United States since 2024 and 67 since 2022. One person died. Today, there is no approved human clade 2.3.4.4b or feline H5N1 vaccine. The sad reality is that while you may get very sick if you contract the virus, your cat will almost certainly die. Other countries are heeding the calls for stockpiles of approved human vaccines. Finland is already vaccinating agricultural workers at risk (Finland June 2024).

We need to see the data deleted from the CDC website 

The very thought that data about CDC-verified transmission of H5N1 between species has been either suppressed or deleted from public view is an unbelievable development. We need to know what investigations were completed and by whom. Genotypes, timelines for estimated inoculation and onset of symptomology, where was active virus found, are just examples of the many unanswered questions. 

Moving forward without key data from the CDC

Using resources hobbled together, such as county and state-level reports from veterinarians, media data from the WHO and Public Health Agency of Canada (Canadian CDC), some important North American data can be archived. A boots-on- the-ground approach can provide more up to date data on infection activity in the US. The bottom line is that public awareness, vigilance and a willingness to report suspected cat illnesses and deaths are all helpful tools.

Citizen science has never been more important than right now. It will be up to each individual to be vigilant. Take precautions for yourself and for your cats. Especially, consider your sources for information carefully.

Coming next, Part Three: The pathological consequences of H5N1 infection in cats.

Currently the most frightening and enigmatic symptom is a preference for neurological tissues in cats. As a rule, viruses have a tough time crossing the blood-brain-barrier. This one appears to cross it.

Ingrid R. Niesman MS PhD is the Director of the SDSU Electron Microscope Imaging Facility at San Diego State University. She graduated from Utah State University and received her MS from the University of Illinois-Urbana-Champaign. After 30 years of technical electron microscopy, cell biology, neuroscience and infectious disease research, Dr. Niesman completed her PhD in the UK at the University of Sunderland. Her work experience includes time at LSU Medical School, Washington University, UAMS in Little Rock, UCSD, TSRI and a postdoctoral year at CALIBR in La Jolla, CA. She has worked for at least two National Academy of Science members and is credited with over 50 publications. She can be reached at [email protected]

Image by Karsten Paulick from Pixabay