A new strain of bird flu — H5N9 — has been detected in California’s Merced County at a commercial duck operation.
According to the World Organization for Animal Health — an international consortium of medical, veterinary and wildlife professionals that researches and reports on animal diseases — testing of ducks at the farm showed the animals had been infected by both the H5N1 and H5N9 strains.
“This case is not unexpected or alarming,” said Annette Jones, California state veterinarian, who said the California Department of Food and Agriculture is “always on alert for any H5 virus.”
The new strain is a combination of the H5N1 strain that has been widely detected across the world and likely a “low-pathogenic” bird flu virus — a kind of mild virus that moves through birds without killing them, said Henry Niman, an evolutionary molecular biologist with Recombinomics Inc., a virus and vaccine research company in Pittsburgh.
“It is just a new serotype where the N1 segment is replaced by N9,” he said.
Flu viruses are composed of eight segments, including the haemagglutinin protein, or “H” part of the virus, and neuraminidase, or the “N” part. When an animal is infected by two different flu viruses, the genetic material of the two can switch or recombine to form a new version — which is what probably happened here: While the haemagglutinin — the part of the virus that allows it to attach to a host — remained the same, a new neuraminidase — the part that helps release the virus from infected cells — was substituted in.
Fortunately, said the World Health Organization’s Richard Webby, the combination that occurred in this population of commercial ducks is unlikely to increase the likelihood of a human pandemic. Webby, director of the WHO’s Collaborating Center for Studies on the Ecology of Influenza in Animals and Birds, said the virus was no bigger threat to human health than the current H5N1 strains circulating in dairy cows, commercial poultry and wild animals.
However, were there to be genetic reassortment between H5N1 and a human seasonal flu, that could be a problem.
Such a combination might provide the bird flu with the equipment it needs to pass more easily and efficiently between people.
“We know for sure this virus reassorts. Nature is a giant gain-of-function experiment,” said John Korslund, a former USDA scientist, referring to experiments researchers use to make viruses more lethal and transmissible.
So far, there has been no evidence that any of the strains of bird flu virus now circulating in North America have acquired that sort of ability.
According to the Centers for Disease Control and Prevention, 67 people in the U.S. have been infected with H5N1 bird flu. One person has died. Most cases have been connected to occupational exposure — via infected dairy cows or poultry. In three cases, the source remains unidentified.
Niman said there are four bird flu viruses in the H5 family circulating in North America. Two are part of the H5N1 strain. One of those, the H5N1 B3.13 version, has been found predominantly in dairy cows and dairy workers. The other H5N1, known as D1.1, has largely affected wild birds and commercial poultry. It is also the version of the virus that killed a person in Louisiana and severely sickened a teenager in British Columbia.
Then there’s H5N2, which is circulating in British Columbia. It is also a reassortment of the H5N1 virus with another bird flu virus. It was first discovered in November 2024, and has been detected in at least two commercial poultry operations.
And now there is H5N9, which so far has been detected only in Merced County, according to the U.S. Department of Agriculture, which performed the testing and genetic sequencing.
“This is the first confirmed case of HPAI (high pathogenic avian influenza) H5N9 in poultry in the United States,” the USDA said in a report to the World Organization for Animal Health. “The USDA animal and plant health inspection service (APHIS), in conjunction with state animal health and wildlife officials, are conducting comprehensive epidemiological investigations and enhanced surveillance in response to the HPAI related events.”
The virus was detected on the duck farm in late November. All the ducks — about 119,000 birds — were euthanized by Dec. 2.
Webby said he thinks the virus was wiped out in that poultry operation — there have been no reports of it appearing in other commercial operations or in wild birds.
So far, there have been no reports of any human becoming infected with H5N9.
“Big picture: There are multiple avian influenza viruses circulating so this is not horribly surprising,” said Maurice Pitesky, an associate professor who researches poultry health and food safety epidemiology at UC Davis. That said, researchers need a more robust surveillance system that provides information on “where waterfowl are, and where we suspect HPAI is present in wild birds relative to commercial facilities.”
Jones, the state veterinarian, said the agriculture department and the USDA “will continue to monitor for new or unusual viruses as a part of our ongoing testing strategy.”
