Over the past few months, a bird flu outbreak has spread swiftly through dairy cows in the United States, infecting more than 90 herds in 12 states. Along the way, the virus has caused collateral damage in several other species, spreading from dairies to poultry farms and from cows into at least three farmworkers, who developed symptoms of mild illness.

It has also caused mounting casualties in cats. On some dairy farms, sick or dead cats have provided an early signal that something was amiss. “They’re a bit of a canary in a coal mine,” Dr. Kammy Johnson, a veterinary epidemiologist for the Agriculture Department, said at a news briefing Thursday.

Since the dairy outbreak was first detected in late March, at least 21 cats in nine states have caught the virus, according to the department, which recently began tracking the feline cases.

Scientists have long known that cats are vulnerable to being infected by avian influenza, a group of flu viruses typically found in birds. In 2020, a new version of a bird flu virus, known as H5N1, emerged. It has spread rapidly around the world, infecting many wild birds and repeatedly spilling over into mammals, including cats.

“Domestic cats are actually highly susceptible to avian influenza, and especially H5N1,” said Kristen Coleman, an infectious disease researcher at the University of Maryland. “But there has been a recent uptick in domestic cat infections, a drastic uptick.”

There have been sporadic reports of infected dogs, too.

While bird flu infections of pets remain rare overall, they can be severe, especially in cats. “It results in very severe illness and oftentimes death,” Coleman said. “So it’s very serious, and it should be taken seriously.”

But a few “reasonable precautions” can help people keep their pets safe, she said.

Here’s what to know:

How are cats getting bird flu?

On dairy farms, cats were infected after drinking unpasteurized milk, also known as raw milk, which contains very high levels of the virus, from sick cows. More than 80% of affected dairy farms had cats on their premises, and more than half of those farms reported sick or dead cats, according to federal data released Thursday.

But even before the recent dairy outbreak, there were reports of infected cats, some of which probably caught the virus when they preyed upon infected birds. “As we get more dead, wild birds on the landscape, if we get more dead poultry on the landscape, these carnivores that may get into them and ingest them, even after they’re dead, are getting just a massive dose of virus,” said Dr. Justin Brown, a wildlife veterinarian at Penn State.

A few larger outbreaks have also been linked to contaminated raw poultry. In 2023, for instance, bird flu outbreaks hit two cat shelters in Seoul, South Korea. Government investigators there subsequently found the virus in raw duck meat fed to the cats.

How does bird flu affect cats?

Cats infected by the virus often become severely ill, and in some instances, the mortality rate has been “shockingly high,” said Dr. Younjung Kim, a veterinary epidemiologist at the French National Institute of Health and Medical Research. At one affected cat shelter in South Korea, he noted, 38 of 40 cats died.

In a recent review, which has yet to be published in a peer-reviewed journal, Coleman found that the new version of H5N1 had a 67% mortality rate in felines. (That calculation is primarily based on domestic cats, although it includes a handful of cases in other species, such as bobcats and lynx.)

Many infected cats develop fevers, loss of appetite and respiratory symptoms, which may include nasal discharge, difficulty breathing and pneumonia. Neurological symptoms, including stiffness, tremors and seizures, are common, too. “Sometimes this is confused for rabies,” Coleman said.

Infections in cats can also be asymptomatic, although it remains unclear how common that is, since mild or asymptomatic cases may escape detection.

Are cats spreading the virus to people or other animals?

It’s unclear. “We do not fully understand whether or not they are capable of transmitting,” Johnson said. There is not yet any evidence that cats have been contributing to the spread of the virus on dairy farms, she added.

In the case of the outbreaks at the Korean cat shelters, scientists aren’t sure whether all of the sick cats contracted the virus from contaminated food or whether some of the animals caught the virus from each other.

“It was a very good opportunity to study cat-to-cat transmission,” said Kim, who published a paper on the outbreaks. But by the time the virus was detected, many of the dead cats had already been disposed of, he said, making the epidemiological investigation difficult. “So we don’t know the extent of the viral transmission,” he said.

However, previous studies have shown that cats can spread some bird flu viruses to each other and to people. In 2004, a laboratory study revealed that cats infected with an earlier version of H5N1 were capable of shedding the virus and infecting other cats.

And in 2016, a different bird flu virus, known as H7N2, tore through the cats at a New York City animal shelter. At least two people — a veterinarian and a shelter worker — also became infected, investigators found. Both people had mild symptoms and recovered.

Are dogs at risk, too?

Yes. There have been a handful of cases, including serious ones, reported in dogs.

In the spring of 2023, a dog in Ottawa, Ontario, was infected with H5N1 and died “after chewing on a wild goose,” according to the Canadian Food Inspection Agency. That same year, an infected dog in Poland developed a severe cough and other symptoms but recovered.

Overall, however, dogs seem to be less susceptible to the virus than cats are and appear less likely to develop severe disease.

For a recent study, Brown and his colleagues analyzed blood samples from nearly 200 hunting dogs in Washington. The researchers looked for antibodies to the virus, which would indicate that the dogs had previously been infected by it.

“And we looked specifically at waterfowl hunting dogs because they are going to be the ones that are actively in infected habitats and retrieving infected birds in their mouth,” Brown said. “We figured that was about as high a risk group as you could get.”

Just four of the dogs had antibodies; none had developed any symptoms or passed the virus on to other dogs in their households, the researchers found. The results suggest that the virus is not well adapted to dogs, which may require very high doses of virus in order to become infected, Brown said.

Still, Brown cautioned, the virus can change quickly, and it could become a bigger threat to dogs in the future. “I think we always have to be ready for that change to occur,” he said.

How can I protect my pet from bird flu?

Pets that remain indoors should be at exceedingly low risk, experts said. But owners should avoid feeding their pets raw milk or raw meat, which both could potentially transmit the virus.

Of course, an entirely indoor lifestyle is not possible for many pets. “We’re not asking you to lock your pet up,” Coleman said. So owners should take common-sense safety measures with outdoor pets, including to try to keep them away from sick or dead birds and from places where wild waterfowl congregate, experts said. Removing shared food and water sources, such as bird feeders and baths, may also help curb the spread of the virus.

Paying attention to the local situation can help, too; owners may want to keep their pets indoors more, or supervise them more closely, if bird flu outbreaks have recently been reported in their area.

As always, pet owners should monitor their animals for signs of illness, especially if their pets have had recent exposure to sick or dead birds, and consult a veterinarian if they have concerns.

This article originally appeared in The New York Times.