In early November 2025, public health authorities in Washington state confirmed what appears to be the first ever human infection by the H5N5 strain of avian influenza.
The Washington State Department of Health announced that a resident of Grays Harbor County, already hospitalized with influenza-like symptoms, tested positive for influenza A H5.
Further laboratory testing identified the virus as H5N5 — a subtype previously seen in animals but never before documented in a human.
The Patient's Profile
The confirmed case involves an older adult with significant underlying health issues. According to state officials, the patient developed high fever, confusion, and respiratory distress.
The individual remains hospitalized, and authorities are closely monitoring health workers and others who had contact—though so far, no additional infections have been identified.
Likely Source of Infection
Investigators suspect that the patient’s backyard flock of domestic poultry was the source of the infection, given that these domestic birds had been exposed to wild birds — which often carry various avian influenza viruses.
The Washington State Department of Health is working alongside agricultural authorities to explore both animal and environmental pathways of exposure.
Public Health Assessment
Despite the novelty of this human H5N5 infection, public health experts do not currently consider it a major threat to the general population.
The Centers for Disease Control and Prevention (CDC) and Washington health officials have both stated that the risk to the public remains low. They emphasize that, to date, there is no evidence of sustained human-to-human transmission for this strain.
Comparison with Other Bird Flu Strains
H5N5 belongs to the same broader family of H5 avian influenza viruses, which have caused sporadic human infections in other forms — most notably H5N1.
The H5N1 subtype has been the most widely reported in humans over recent years, whereas H5N5 has been found in wild birds but was not known to infect people until now.
According to a flu researcher at St. Jude Children’s Research Hospital, the H5N5 strain “behaves similarly” to H5N1 in terms of its threat to human health.
Significance
The confirmation of H5N5 in a human is a noteworthy event for several reasons. It expands our understanding of the zoonotic potential of avian influenza: that is, the capacity of bird flu viruses to jump from birds into humans.
Such cross-species spillovers are rare, but when they happen, they merit close scientific scrutiny. The case underscores that even strains previously confined to animal reservoirs can, under the right conditions, infect humans.
At the same time, health authorities are urging calm. The fact that H5N5 has not yet been documented in more than this single case suggests that its ability to spread among people may be limited — at least in its current form, according to CDC.
Precautions and Ongoing Investigation
In response to this case, public health and veterinary authorities are intensifying their investigations. Epidemiological teams are interviewing the patient’s contacts, monitoring for symptoms, and conducting testing as needed.
On the animal side, efforts are underway to trace the infection route through the patient’s poultry flock and any possible contact with wild birds.
American authorities are also reinforcing general preventive advice: people who keep backyard poultry or who interact with wild birds are urged to use personal protective equipment especially when handling birds.
They also urge to report sick or dead animals promptly, and to avoid direct contact with potentially infected birds.
