In March 2024, dairy and poultry veterinarian Kay Russo, DVM, MAHM, DACPV, was contacted by a fellow veterinarian who was working up cattle on a dairy farm in Texas.

The cattle were displaying respiratory and gastrointestinal signs including high fevers, respiratory distress, decreased rumination, and significantly decreased milk production. They had high morbidity rates and low mortality rates—and they were testing negative for all the usual suspects.

Since the farm’s veterinarian had already ruled out bovine syncytial virus, coronavirus, salmonella, and bovine viral diarrhea, Russo knew to be on the lookout for something atypical. She reports that it was the combination of signs, including the mammary involvement, that really made her consider influenza.

That’s when she posed a question that would have an ominous answer. “What are the birds doing on the farms?” she asked, referring to wild birds such as pigeons and grackles.

Then answer came back, “the birds are all dead.”

And those who hadn’t died yet were showing severe neurologic signs.

Russo quickly advised those who were in the field to obtain as many samples as possible from the birds to send out for testing at the National Veterinary Services Laboratories.

When those samples came back positive for H5N1 (one of the two strains associated with highly pathogenic avian influenza or HPAI), Russo felt a pit in her stomach. She knew the cattle needed to be tested as well.

On March 21, 2024, the first sample from the dairy cattle came back positive for H5N1.

The rise of fowl plague
For almost 150 years, avian influenza has been circling the globe, causing epizootics (epidemics infecting nonhuman species) of varying severity and sometimes crossing over into nonavian species, including humans.

Originally called fowl plague, this type-A influenza virus typically infects poultry species and wild waterfowl. Depending on the pathogenicity of the strain, the results can range from mild respiratory and gastrointestinal signs to multisystemic signs including neurologic changes followed by death.1

The latest H5N1 influenza outbreak
The most recent outbreak of “avian” influenza started in 2022. Since that time, the USDA has detected the virus in many nonavian species, including:

Captive big cats
Mountain lions
Bobcats
Brown bears
Black bears
Polar bears
Bottlenose dolphins
Grey seals
Harbor seals
Red foxes
Coyotes
Fishers
American martens
North American river otters
Raccoons
Skunks
Virginia opossums
Abert’s squirrels2
Since Russo’s discovery of influenza in dairy cattle in March of this year, the list of mammals infected with H5N1 now includes dairy cattle, domestic cats, and humans.I asked Russo to explain more about the spread of H5N1 into domestic mammalian species, including cats, and to highlight areas of concern for small animal veterinarians.

How H5N1 spreads
H5N1 is an enveloped RNA virus whose genetic code is very susceptible to mutation as it replicates, Russo explains.

This ability to mutate easily (termed antigenic drift) allows the virus to become more virulent and infect new species through genetic reassortment.3
Once it infects a new species, it can spread laterally within that species, particularly when large numbers of animals are housed in close proximity to each other.
Birds also continue to spread the disease as they migrate.
Because it has jumped to other species, the term “avian influenza” is no longer appropriate, Russo said. It is best to refer to the virus by its subtype H5N1 until an official name is applied by the World Organisation for Animal Health (WOAH).

The emergence of H5N1 in cats and humans
Shortly after Russo received the H5N1 diagnosis in the farm’s cattle, barn cats on both affected dairy farms started showing respiratory and neurologic signs.

Between the two farms, five cats presented with:

Conjunctivitis
Periocular swelling
Respiratory signs
Dullness
Depression
Other neurologic signs (including circling and blindness)
Unfortunately, all five of these cats died. Samples from the cats were sent for testing and came back positive for H5N1.

At this time, it is not known whether the cats were infected from consuming diseased birds, drinking infected milk, direct contact with sick cows, or another mode of transmission.

A worker at one of the dairy farms also tested positive for H5N1 after showing clinical signs. One other human case of H5N1 has been reported in North America in a worker from a poultry farm infected with a highly pathogenic strain of H5N1.

Avian influenza outbreak protocols
Among avian species, the protocols for addressing influenza outbreaks involve mass depopulation. Russo reports that since this outbreak of H5N1 began in 2022, the US has had to cull over 82 million infected birds from domestic poultry flocks.

Other common biosecurity measures include:

Strict rodent control
Not allowing cats or other domestic species to be present
Not allowing poultry workers to raise their own flocks
H5N1 protocol for cattle
Since the spread of the disease into dairy cattle populations is so new, much is still being learned about how to address outbreaks in this species. Given that mortality rates seem to be much lower in cattle than they have been in some poultry flocks, depopulation may not be recommended for cattle farms.

Unlike poultry, however, it is common for cattle to be shipped around the country for various phases of their development and production.

At the very least, farms with affected cattle will need to institute strict quarantine procedures and avoid transporting their cattle from the farm.
Russo suggests that pre-transportation testing and vaccination will likely be needed in the future to reduce the impact of this disease on dairy cattle populations.
At this time, the FDA has indicated that pasteurization kills viruses such as influenza in cow’s milk, but that no milk from known infected cattle is being processed for human consumption.4
H5N1 protocol for cats and other birds
Prevention of spread of H5N1 among cats will likely involve:

Limiting cats’ contact with other possible vectors such as birds and cattle, and possible fomites such as milking equipment.
Not allowing cats to drink unpasteurized milk.
While pet birds have not been reported to be infected with H5N1, there could be a chance of exposure if they cohabitate with a cat who either goes outside or who eats a raw diet, Russo said.

Backyard flocks are also very susceptible to H5N1. Russo reports that at least 500 backyard flocks have been diagnosed with H5N1 since 2022.

Keeping wild birds and cats away from backyard poultry can help to reduce the risk of infection. Russo points out that it is also important to remember that humans can be a source of transmission to animals if they are exposed.

Veterinarians’ role watching H5N1
Russo stresses that we are learning new information on a daily basis about where this virus is and what it can do. It’s clear now that it’s no longer a disease that’s limited to wildlife and production animal species.

Therefore, it will be essential for all veterinary professionals to stay abreast of the latest updates on this disease and have influenza on their differential list where appropriate, no matter the species.