Cats that became infected with bird flu might have spread the virus to humans in the same household and vice versa, according to data that briefly appeared online in a report from the Centers for Disease Control and Prevention but then abruptly vanished. The data appear to have been mistakenly posted but includes crucial information about the risks of bird flu to people and pets.
In one household, an infected cat might have spread the virus to another cat and to a human adolescent, according to a copy of the data table obtained by The New York Times. The cat died four days after symptoms began. In a second household, an infected dairy farmworker appears to have been the first to show symptoms, and a cat then became ill two days later and died on the third day.
The table was the lone mention of bird flu in a scientific report published on Wednesday that was otherwise devoted to air quality and the Los Angeles County wildfires. The table was not present in an embargoed copy of the paper shared with news media on Tuesday, and is not included in the versions currently available online. The table appeared briefly at around 1 p.m., when the paper was first posted, but it is unclear how or why the error might have occurred.
The virus, called H5N1, is primarily adapted to birds, but it has been circulating in dairy cattle since early last year. H5N1 has also infected at least 67 Americans but does not yet have the ability to spread readily among people. Only one American, in Louisiana, has died of an H5N1 infection so far.
The report was part of the C.D.C.’s prestigious Morbidity and Mortality Weekly Report, which, until two weeks ago, had regularly published every week since the first installment decades ago. But a communications ban on the agency had held the reports back, until the wildfire report was published on Wednesday.
Experts said that the finding that cats might have passed the virus to people was not entirely unexpected. But they were alarmed that the finding had not yet been released to the public.
“If there is new evidence about H5N1 that is been held up for political purposes, that is just completely at odds with what government’s responsibility is, which is to protect the American people,” said Jennifer Nuzzo, director of the Pandemic Center at the Brown University School of Public Health.
It was important that the C.D.C. immediately publish the full data and the context in which they were collected for other scientists to review, she said.
Scientists have long known that cats are highly susceptible to the virus. At least 85 domestic cats have been infected since late 2022, according to the U.S. Department of Agriculture. But there had not previously been any documented cases of cats passing the virus to people.
“Given the number of cats in the U.S. and the close contact with people, there is definitely a need to understand the potential risk,” said Dr. Diego Diel, a veterinarian and virologist at Cornell University.
Although cats may be infected when they prey on infected wild birds, cases among domestic cats in the United States began rising last year as the virus spread through dairy farms. On many farms, dead cats were the first signal that cows had been infected. Several recent cases in pet cats have also been linked to contaminated raw pet food or raw milk.
H5N1 is often fatal in cats, which may develop severe neurological symptoms.
Historically, H5N1 has primarily affected birds. But over the last several years, new versions of the virus have proved capable of infecting a wide range of mammals, including wild and domestic cats, seals and dairy cows. Infections in mammals give the virus more opportunities to evolve in ways that could allow it to infect humans more easily.