A resident of Washington state has become the first person in the United States confirmed to have been infected with the avian influenza strain H5N5, health officials announced on Friday. The patient, an older adult with underlying health conditions, remains hospitalised and the infection is under federal and state investigation.

Officials say the individual kept a mixed backyard poultry flock that had contact with wild birds, making either the domestic birds or wild avian exposure the most likely source. The new strain differs from the H5N1 variant previously responsible for around seventy human cases in 2024–25 in the U.S. The Centers for Disease Control and Prevention (CDC) and the Washington State Department of Health say there is currently no evidence of human-to-human transmission, and they continue to assess the case while monitoring more than one hundred contacts including health-care workers.

Virologists note that although H5N5 carries a different viral protein involved in cell-to-cell spread, it appears to behave similarly to H5N1 in known models, and the risk to the general public remains categorised as low. The confirmation comes as part of ongoing heightened surveillance of avian influenza viruses among wild birds, poultry and mammalian species across the country.

The patient developed symptoms in early November, was hospitalised soon after, and remained under care while investigators sampled poultry, wild birds and the backyard flock environment to trace the origin of the infection. While most human H5 cases in the U.S. have been mild and linked to agricultural exposures, this case is notable for involving a previously undocumented viral subtype in a human.

Authorities emphasise the significance of the incident for scientific monitoring of influenza viruses with zoonotic potential, but reaffirm that there is no change in public-health guidance: the general public need not change daily routines, but those working closely with poultry, cattle or wild birds should continue to use protective measures, report sick animals and avoid direct contact with dead birds.

Further laboratory testing and genome analysis of the virus are underway to assess whether any adaptations of concern have occurred. Meanwhile, the surveillance network for avian influenza will remain active and coordinated among agricultural, wildlife and human-health agencies as winter and migratory-bird patterns increase the risk of spread in avian populations.