A genetic mutation of the H5N1 bird flu virus — a mutation associated with increased infectiousness and disease severity — has been found in two cats, in what scientists say is another indication of the risks posed by the virus.
The fact that the cats have the mutation “is a continued example of how this virus is evolving in nature and should concern everyone,” said Seema Lakdawala, a microbiologist at Emory University in Atlanta.
Henry Niman, founder of vaccine research firm Recombinomics Inc., reviewed the sequence data and reported the results to The Times.
The gene that Niman identified in the sequence data, known as PB2 E627K, has been associated with increased mammal-to-mammal transmission and disease severity in laboratory animals.
It is a similar mutation to the one found in San Bernardino dairy cows earlier this week, but has a slightly different origin. The cows were infected with the B3.13 strain of H5N1 — which has been circulating widely in dairy cows since last March. The cats were infected with the newer D1.1 strain, which is widespread in wild birds — and has also now appeared in a few cattle herds in Nevada and Arizona.
Niman said he believed the two cats were based in New Jersey and infected last month based on the scientific nomenclature used to label the genetic sequences.
The New Jersey Department of Public Health and Raritan Township, which reported a cluster of infected cats last month in Hunterdon County, couldn’t immediately be reached for comment.
Since the beginning of 2025, the U.S. Department of Agriculture has reported 51 H5N1-infected cats. They include both household pets and feral felines, and have been found in 13 states since the beginning of the year, including California, Montana, Colorado, Oregon, Washington and New Jersey. There have been more than 100 reported since last March, when the outbreak was first reported in dairy cows.
According to the New Jersey Department of Public Health, the infected cats all lived on the same property. One was feral, another was an indoor/outdoor cat. The living situation of the remaining four cats is unclear.
On Thursday, the genetic sequences of H5N1 virus taken from two infected cats were added to GISAID — the Global Initiative on Sharing All Influenza Data — a publicly-accessible gene data bank.
Richard Webby, an infectious disease expert at St. Jude’s Hospital in Memphis, Tenn., said the discovery of the mutation wasn’t alarming in and of itself.
“This mutation has sporadically popped up in other mammal infections over the past few years,” he said. “It’s an easy change for the H5 viruses to make and it does so relatively frequently.”
It’ll become concerning, he said, if it spreads more widely.
There have been no reports of infected humans in New Jersey, and a press release from the state said the people who interacted with the infected cats were asymptomatic.
That Feb. 28 release said that the infected cats had no known reported exposures to infected poultry, livestock, or consumption of raw (unpasteurized) milk or meat, “but did roam freely outdoors, so exposure to wild birds or other animals is unknown.”
Since the outbreak started last March, 70 people in the U.S. have been infected with H5N1; one person has died.