On Monday, the U.S. reported an outbreak of bird flu called H5N9 that is different from the H5N1 strain that is currently surging across the country. The outbreak occurred in a flock of commercial ducks in California, the epicenter of the bird flu crisis, according to a report from the World Organisation for Animal Health (WOAH).
"This is bad news," Dr. Angela Rasmussen, a virologist at the Vaccine and Infectious Disease Organization at the University of Saskatchewan, wrote on X, formerly known as Twitter. "It suggests reassortment of circulating H5N1 viruses with viruses containing N9 NA. Although this indicates reassortment with avian viruses, it's still bad. Reassortment makes pandemics. The last 3/4 flu pandemics (and maybe 1918 too) were reassortant viruses."
H5N9 is different from H5N1, though both are known as bird flu. Two strains of highly pathogenic avian influenza (HPAI) have been responsible for millions of cow and poultry deaths in the United States. One strain called B3.13 spreads mostly in cows and a strain called D1.1 spreads mostly in birds. Together, these strains have infected dozens of different species, including at least 67 humans, in an unprecedented spread. Earlier this month, the first reported human death in this bird flu outbreak was reported in Louisiana.
Meanwhile, health officials in the U.K. confirmed a human case of bird flu in which the person was infected with the DI.2 genotype of the virus, which is different from the ones circulating in the U.S. The circulation of various subtypes of the virus is concerning because the more the virus spreads, the higher the chances that something called viral reassortment, in which genes could swap and mutate in an organism to make the virus more infectious, could occur.
Health officials found H5N9 on a duck farm in Merced County, California, where the more common strain of the virus was also detected. Nearly 119,000 birds exposed to the virus have been killed there since Dec. 2.
The dominant H5N1 virus in the U.S. is more common than the H5N9 type, which is relatively rare. According to a 2015 study published in the Journal of Virology, which studied the virus when it was still new to science, H5N9 forms as a reassortment of other subtypes of the virus. Unlike chickens, which can have a 100% mortality rate with H5N9 virus, ducks usually only exhibit mild clinical signs following infection, according to a 2020 study in the same journal. While this built-in immunity is good news for the ducks, it can also allow for greater chance of reassortment — when viruses share genetic material — increasing the risk of another pandemic like COVID-19.
