Two dairy workers in Michigan may have transmitted bird flu to their pet cats last May, suggests a new study published on Thursday by the Centers for Disease Control and Prevention.
In one household, infected cats may also have passed the virus to other people in the home, but limited evidence makes it difficult to ascertain the possibility.
The results are from a study that was scheduled to be published in January but was delayed by the Trump administration’s pause on communications from the C.D.C.
A single data table from the new report briefly appeared online two weeks ago in a paper on the wildfires in California, then quickly disappeared. That odd incident prompted calls from public health experts for the study’s release.
The new paper still leaves major questions unanswered, including how the cats first became infected and whether farmworkers spread the virus to the cats and to other people in the household, experts said.
“I don’t think we can say for sure if this is human-to-cat or cat-to-human or cat-from-something-else,” said Jennifer Nuzzo, director of the Pandemic Center at the Brown University School of Public Health.
Officials in Michigan began investigating two households last May when exclusively indoor cats showed respiratory and neurological symptoms and, after death, tested positive for the virus, called H5N1. The officials interviewed the cats’ owners and household members and offered to test them for the virus.
The owners of both cats were dairy workers. The first farmworker did not work with cows directly, and the farm was not known to have infected herds. But the worker reported that many of the barn cats on the farm’s premises recently died. The worker also reported having experienced vomiting and diarrhea before the first household cat became ill.
The second farmworker reported being splashed in the face and eyes with milk and experiencing eye irritation. Both workers declined to be tested.
“This study provides yet more concerning evidence that farmworkers with high-risk exposures may refuse testing,” Dr. Nuzzo said.
“In order to protect people and stay ahead of this virus, we need to remove disincentives for patients to get tested,” she added. “People should not fear that testing positive will cause financial distress or other personal harms.”
In the household of the first farmworker, the first cat to become ill showed decreased appetite, lack of grooming, abnormal gait and lethargy, and quickly deteriorated. She was euthanized on the fourth day of illness.
A second cat in the household developed watery eye discharge, rapid breathing and decreased appetite four days after the first cat became ill. This cat recovered and was not tested for the virus. A third cat had no symptoms and tested negative for the virus 11 days after the first cat became ill.
Neither the cats nor the humans in the household drank unpasteurized milk. How the cats might have become infected is unclear, but experts said that the farmworkers were likely to have become infected with H5N1 at their workplace and to have brought the virus home to their cats.
“If you love your cat, you probably give it head kisses if it lets you,” said Kristen K. Coleman, an infectious disease researcher at the University of Maryland.
Three people in the household — an adult and two adolescents — tested negative for H5N1. Six days after the first cat became sick, one of the adolescents became ill with a cough, sore throat and body aches, and the other reported a cough that was attributed to allergies.
But because the adolescents were tested late — 11 days after the first cat became sick — it was not impossible that they became infected with H5N1 that they picked up from the cats, Dr. Coleman said.
Later in May, a pet cat in the second household developed severe neurological symptoms, including anorexia and minimal movement, and died within a day; the cat tested positive for bird flu after its death.
The cat’s owner transported unpasteurized milk, including from farms with known bird flu outbreaks. According to the study, the owner “did not wear personal protective equipment (PPE) while handling raw milk; reported frequent milk splash exposures to the face, eyes and clothing; and did not remove work clothing before entering the home when returning from work.”
The cat that became ill was known to “roll in the owner’s work clothes,” the study noted.
Virus in raw milk splattered on those clothes may be the source of infection in the cat, said Dr. Keith Poulsen, the director of the Wisconsin Veterinary Diagnostic Laboratory.
“At this point, I think the higher risk is their exposure from raw dairy products,” he said. “There’s so much virus in the milk.”
Of 24 veterinary staff members who were potentially exposed to the infected cats, seven reported symptoms such as nasal congestion and headache. Only five agreed to testing; all were negative.
Dr. Coleman recommended that veterinarians remain alert to the possibility of bird flu infections when they see sick cats. “Pet owners should not have to rely on postmortem sampling to get a diagnosis,” she said.
