Aviaria

H5n5 virus: first information on the human case in the US and public health implications

Compared to H5n1, the H5n5 virus does not increase the risk to public health according to experts. The origin of the infection could be the patient's home breeding

15 November 2025

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Galline ruspanti in un'azienda agricola di Marratxi, sull'isola spagnola di Maiorca. Nei giorni scorsi negli Usa si è registrato il primo contagio umano da virus H5n5. (Foto di Jaime REINA / AFP)

2' min read

Translated by AI

Versione italiana

2' min read

Translated by AI

Versione italiana

A Washington state resident has contracted the avian influenza caused by a different viral strain than that found in previous infections. This was announced on Friday by the US state health authorities.

This is the first recorded human case of avian influenza in the US since February and the first known human case of infection with the H5n5 virus, according to state and federal health authorities. The patient, an older adult with pre-existing health problems, remains hospitalised.

The health authorities had announced the preliminary diagnosis on Thursday, which was then confirmed the following day.

No Greater Threat

The H5n5 variant is not considered a greater threat to human health than the H5n1 virus, responsible for a wave of 70 human infections reported in the US in 2024 and 2025. Most of these cases involved mild illnesses in dairy and poultry farm workers.

Richard Webby, a leading influenza researcher at St. Jude Children's Research Hospital in Memphis, Tennessee, said that these viruses behave similarly. He added that his assessment is to consider it equivalent to H5n1 from a human health perspective.

The Centers for Disease Control and Prevention issued a similar statement on Friday, saying that no information suggested that the risk to public health was increased as a result of this case. The agency is waiting to receive a sample from Washington to conduct further tests.

L’aviaria, il burnout e la melatonina

The differences

The distinction between H5n5 and H5n1 lies in a protein involved in releasing the virus from an infected cell and promoting its spread to surrounding cells. Webby wrote in an email that the difference can be compared to that between different brands of car tyres: both perform the same function, but each is optimised for specific conditions that are not fully understood.

H5n5 might have a different preference as to the type of birds it most easily infects, he added.

The Grays Harbor County resident, whose identity was not disclosed, owns a domestic poultry farm in his backyard that had been exposed to wild birds, health authorities reported. They consider domestic poultry or wild birds to be the most likely source of exposure, but say the investigation is still ongoing.