Mutated bird flu found in two cats, raising new concerns

Summary of key points

• Mutated H5N1 Bird Flu Found in Cats: Scientists have identified a genetic mutation in two infected cats that is linked to increased infectiousness and disease severity.
• Mutation Similar to One Found in Cows: The mutation, PB2 E627K, is similar to one found in San Bernardino dairy cows, though the cats were infected with a different strain.
• Rising Cases in Cats Across the U.S.: Since early 2025, 51 cats in 13 states have tested positive for H5N1, with over 100 reported cases since the outbreak began last year.

It's another sign of the virus's evolution

A genetic mutation of the H5N1 bird flu virus, known for increasing infectiousness and disease severity, has been identified in two cats. It's another sign of the virus’s continued evolution and its potential risks.

“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.

The mutation, known as PB2 E627K, has previously been linked to increased mammal-to-mammal transmission in laboratory animals. Henry Niman, a researcher at Recombinomics Inc., analyzed genetic sequences from the infected cats and reported the findings.

This mutation is similar to one recently detected in dairy cows in San Bernardino, California, but with a different origin. The cows were infected with the B3.13 strain of H5N1, which has been circulating widely since March 2024. In contrast, the cats were infected with the D1.1 strain, which has primarily affected wild birds but has now been detected in some cattle herds in Nevada and Arizona.

May have been New Jersey cats

Niman suggested that the two infected cats were located in New Jersey and likely contracted the virus last month, based on genetic sequencing labels.

Since the start of 2025, the U.S. Department of Agriculture has reported 51 cases of H5N1 in cats, including both domestic pets and feral felines. In total, more than 100 infected cats have been identified in 13 states since the outbreak was first detected in dairy cows in March 2024. Cases have been reported in California, Montana, Colorado, Oregon, Washington, and New Jersey.

New Jersey health officials confirmed that the infected cats had all lived on the same property. One was feral, another was an indoor/outdoor pet, while the living conditions of the other four remain unclear.

On Thursday, genetic sequences from the two infected cats were uploaded to GISAID—a global, publicly available influenza data-sharing platform. Richard Webby, an infectious disease expert at St. Jude’s Hospital, noted that the presence of the mutation wasn’t necessarily alarming.

“This mutation has sporadically popped up in other mammal infections over the past few years,” Webby said. “It’s an easy change for the H5 viruses to make and it does so relatively frequently.”

However, he cautioned that the situation could become concerning if the mutation spreads more widely.

There have been no reports of human infections in New Jersey. The Feb. 28 report also noted that the infected cats had no known contact with infected poultry, livestock, or raw milk, but had roamed freely outdoors, leaving the source of their exposure uncertain.

Since the H5N1 outbreak began in March 2024, 70 people in the U.S. have contracted the virus, with one reported death. Scientists continue to monitor the virus’s evolution and its potential risk to both animals and humans.