CDC report details Washington state's fatal H5N5 avian flu case linked to backyard flock

The Centers for Disease Control and Prevention on Thursday published a detailed report on what it said was the world’s first laboratory-confirmed human infection with highly pathogenic avian influenza A(H5N5), a fatal case in Washington state tied to exposure around a backyard flock.

The report adds new detail to a death Washington state announced in November 2025 and highlights why the case stands out: it involved an H5 subtype not previously confirmed in a person, ended fatally, and appears not to have spread to anyone else. The CDC, Washington state and the World Health Organization said available evidence did not show person-to-person transmission, and health officials said the risk to the public remains low.

According to the CDC’s report, the patient was an older adult in Grays Harbor County, Washington, with underlying medical conditions. The state did not release the person’s name or gender. Symptoms began in late October 2025. The patient was hospitalized in early November, developed respiratory failure and died 28 days after symptom onset. Washington state publicly reported the death on Nov. 21, 2025.

Investigators said the patient owned a free-range backyard flock of about 25 ducks and 30 chickens. In the week before becoming ill, the patient regularly handled eggs and cleaned watering basins every day without personal protective equipment. Two ducks became ill on the second day of the patient’s illness and later died, although the patient did not handle the dead birds.

Lab testing eventually identified the virus as influenza A(H5N5), clade 2.3.4.4b. Washington Public Health Laboratories confirmed influenza A(H5), and sequencing by the University of Washington and the CDC determined the subtype. The CDC said this was the first laboratory-confirmed human infection with H5N5 reported anywhere in the world.

The field investigation pointed back to the property. Investigators found highly pathogenic avian influenza A(H5) in environmental samples, including sediment from a watering basin, and also found weak detections in apparently healthy ducks there. Genetic sequencing showed those samples were highly similar to the virus infecting the patient. Investigators concluded the most likely source of exposure was the backyard flock environment after the virus was introduced by wild waterfowl, with other animals on the property possibly helping amplify spread.

One of the clearest lessons from the report is for clinicians treating severe respiratory illness in people with bird exposure. Early upper-respiratory swabs, including nasal testing, were negative in this patient. Influenza A was detected only later, when a lower-respiratory sample was tested by RT-PCR, a common molecular test used to detect viruses. The CDC said that contributed to delayed and inconsistent use of airborne isolation precautions.

In plain terms, the report warns that suspected avian flu may show up mainly as lower-respiratory disease, meaning a negative nasal swab does not rule it out if a patient has severe pneumonia and a relevant exposure history.

Health authorities identified about 135 people with potential exposure to the patient: 124 health care workers, seven known family contacts, and four state or federal employees. Fifteen monitored contacts developed symptoms that could fit flu, but none tested positive for influenza A(H5) or for any other influenza virus.

Washington’s health department said when it announced the death in November that “the risk to the public remains low” and that “No other people involved have tested positive for avian influenza.”

H5 clade 2.3.4.4b viruses have circulated in U.S. wild birds since 2022 and have also been detected in poultry and some mammals. The United States has recorded dozens of human H5 infections since 2024, most linked to animal exposure and many of them mild. This case was notable both because it was the first confirmed human H5N5 infection and because, despite antiviral treatment and intensive supportive care, it was fatal.