Bird Flu Viruses Raise Mounting Concerns Among Scientists

In the United States, the term bird flu has become synonymous with a particular virus that has devastated poultry and dairy farms over the past few years. But that virus, called H5N1, is not the only form of bird flu in circulation.

Concerned scientists are keeping a close eye other types, including a fast-changing flu virus called H9N2.

In a study published in November, researchers in Hong Kong showed that over the last decade, this virus has acquired mutations that allow it to spread more efficiently among people and to cause more severe disease.

H9N2 is often discounted as a threat, because it causes only mild symptoms in poultry. But in people, especially children, the virus can cause more severe illness than the seasonal flu.

There have been fewer than 200 reported cases of H9N2 in humans since 1998, but the number has been rising sharply. China reported 29 human cases of H9N2 last year, compared with 11 in 2024.

Those numbers, though, are likely to be underestimates, because many infected people are never tested and the virus may spread undetected, Dr. Kelvin To, a clinical microbiologist who led the November study, said.

“If it continues to be widely circulating in poultry, mammals and humans, it may certainly one day evolve into something very serious,” Dr. To said.

Researchers like Dr. To have long feared that the next pandemic will be caused by a flu virus, and the new report suggests that mutating H9N2 bears watching. But it is not the only one.

H5N1 is “the headline stealer,” said Richard Webby, an influenza expert at St. Jude Children’s Research Hospital in Memphis. It has infected poultry and mammals worldwide, leaving behind economic devastation.

But, he added, “some of these other subtypes may have just as much, if not more, capacity for human infection and spread than H5N1.”

In September, health officials in Mexico identified the first human infection of a highly pathogenic bird flu virus called H5N2. The patient was severely ill and hospitalized, but survived.

And in November, a resident of Washington State died of an infection with another type, H5N5, the first human known to have contracted the virus. H5N5 tends to circulate among birds along the Atlantic Coast, so its presence in the West surprised some scientists.

In the United States, H5N1 continues to be the primary threat. The virus is reported to have infected 71 people, killing one, and has affected nearly 185 million commercial, backyard and wild birds since January 2022, when it was detected in wild aquatic birds in the United States.