The man, described as "very ill," was admitted to the hospital earlier this month with a high fever, confusion and breathing difficulties, and was confirmed to have H5N5, a subtype of bird flu usually carried by wild birds such as ducks and geese.
The Washington State Department of Health described the patient as "elderly" and suffering from "underlying health problems," noting that the man owns a "mixed flock of backyard poultry" at his home in Grays Harbor County on the state's southwest coast.
Two birds have recently died, according to the Washington Post, and wild birds can access the property. Health officials believe that any group of these birds is the most likely source of exposure to the virus.
The man remained hospitalized until last week while the investigation continues. Washington state health officials and the Centers for Disease Control and Prevention (CDC) have confirmed that the risk to the general population is low.
No further cases of the H5N5 strain have been recorded, and there is no evidence of person-to-person transmission, although experts warn that the virus's evolution could be "unpredictable".
The H5N1 strain is more common in the United States, but remains rare. The difference between the two strains lies in a protein on the surface of the virus. Since 2022, the H5N1 strain has continued to infect wild birds, domestic poultry, cattle, and occasionally humans.
Since 2024, there have been about 71 human cases of H5 avian influenza in the United States, with the majority of cases showing mild symptoms, although a patient in Louisiana died last January.
The infected man was raising chickens in his backyard, and these birds had come into contact with wild birds, which was considered the likely cause of the infection.