H5N1 Bird Flu: What You Need to Know – Yale Medicine

"Avian influenza A (H5N1)," "bird flu," and "H5N1 bird flu" all refer to an illness caused by influenza type A viruses, which primarily affect birds. H5N1 bird flu was first identified in geese in China in 1996 and in people in Hong Kong the following year. Almost 25 years later, in 2020, a new variant of H5N1, referred to as Highly Pathogenic Avian Influenza (HPAI), was detected in wild birds in Europe; it was first seen in birds in the U.S. and Canada in late 2021, and has since been detected in a variety of wild bird species in all 50 states.

(The term highly pathogenic relates to how the virus impacts birds, not necessarily humans. There is also a low pathogenic avian influenza [LPAI] that usually causes mild illness in birds and poultry; however, some LPAI strains can mutate into strains that can cause severe illness and even death in poultry.)

In February 2022, the virus began causing sporadic outbreaks of HPAI H5N1 in backyard and commercial poultry flocks in the U.S., causing serious illness and death in infected chickens. The number of outbreaks has increased and spread over timeas of May 2024, the CDC reported poultry outbreaks in 48 states.

In addition, there have been sporadic infections in mammals (including bears, bobcats, minks, mountain lions, raccoons, skunks, and others), according to the CDC. And now, as of early May, there have been outbreaks in dairy cattle in nine states.

In the two human cases in the U.S., neither involved person-to-person spreadboth people were infected after exposure to animals presumed to have bird flu. The most recent case, in April 2024, occurred in a dairy worker in Texas who became infected after being exposed to cows that were presumed to be infected, as described in a letter to the editor published in the New England Journal of Medicine in May.

The previous case, reported in April 2022 in Colorado, involved a person exposed to poultry also presumed to be infected, although this case may have been a contamination of the nasal passages with the virus as opposed to an actual infection, according to the CDC. Both cases in humans were mild, and both people recovered.

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H5N1 Bird Flu: What You Need to Know - Yale Medicine