Move Over, H5N1—Another Bird Flu Strain Has Arrived in the U.S.

H5N1 isn’t the only version of bird flu that we should be worried about. Animal health officials reported Monday that a highly pathogenic strain of H7N9 bird flu was recently detected on a poultry farm in Mississippi—a different strain from the H5N1 flu spreading among cows and poultry since last year.

The World Organization for Animal Health (WOAH) published its report on the H7N9 outbreak Monday, based on data provided to it by the U.S. Department of Agriculture and local officials. It’s the first poultry outbreak of H7N9 seen within the U.S. in almost a decade.

Highly pathogenic avian influenza viruses (HPAI) are a serious public health problem. These viruses spread quickly among wild birds and poultry, often causing widespread die-offs (and why they’re referred to as being highly pathogenic). They can also make the leap to other animal species, including humans. The fear is that one such strain will pick up the wrong mix of mutations that will allow it to spread widely and cause severe illness in people—the perfect recipe for a deadly pandemic.

The most well-known HPAI subtype circulating today is H5N1. Since early 2024, H5N1 has been causing outbreaks among cows across the country, and it has occasionally jumped into other mammals such as cats and people. But there are always other kinds of HPAI spreading around as well, including H7N9.

This particular subtype of bird flu was first discovered in China in 2013. One reason why H7N9 is worrying is its deadliness when it spills over into humans. According to the World Health Organizationmost reported cases of human H7N9 have resulted in severe illness. Of the 1,568 total H7N9 cases documented globally since 2013, 616 people have died—a 39% fatality rate.

The last known U.S. outbreak of H7N9 among poultry occurred in 2017. But in early March, it made its unwelcome return on a farm in Noxubee, Mississippi. According to WOAH, the virus was detected in a flock of nearly 50,000 commercial broiler breeder chickens. Genetic testing confirming the strain’s identity was completed late last week.

“The USDA Animal and Plant Health Inspection Service (APHIS), in conjunction with State Animal Health and Wildlife Officials, are conducting a comprehensive epidemiological investigation and enhanced surveillance in response to the detection,” WOAH said in its report of the outbreak.

As is standard procedure for containing these highly contagious infections, the farm’s population of exposed chickens is being culled. So it’s possible that this outbreak will remain an isolated incident. But the overall threat of bird flu is only growing. According to WOAH’s latest situation report, released last month, there have already been more poultry outbreaks of HPAI strains worldwide in the first five months of the current seasonal wave (which began October 2024) than there were total outbreaks during the entire previous season (October 2023 to September 2024).

These outbreaks have greatly contributed to surging egg prices, though their cost is beginning to drop back downaccording to the USDA. However, it’s not certain moving forward how effective health officials will be at containing H5N1 and other bird flu strains in the U.S., given the Trump administration’s general ineptitude. Just last month, the USDA had to scramble to rehire bird flu experts who were laid off under the orders of Elon Musk’s DOGE.