CDC confirms first severe H5N1 case in US patient

The US Centers for Disease Control and Prevention (CDC) today announced that it has confirmed the nation’s first severe H5N1 avian flu case, which involves a probable illness reported by Louisiana health officials a few days ago.

The CDC said the illness also marks the nation’s avian flu infection linked to exposure to backyard poultry. At a briefing today, a top CDC official said the severe clinical picture and the backyard poultry exposure matches the global pattern seen over the years in other countries.

Initial sequencing identifies D1.1 genotype 

Partial genetic sequencing of virus from the patient’s sample reveals that the virus belongs to the D1.1 genotype, which is different than the B3.13 genotype circulating in dairy cattle. That genotype has mainly caused mild symptoms such as conjunctivitis in affected dairy workers.

Demetre Daskalakis, MD, MPH, who directs the CDC's National Center for Immunization and Respiratory Diseases, said the D1.1. genotype is known to circulate in wild birds and was recently implicated in a similar severe illness in a British Columbia teen and poultry cullers in Washington.

He said the Louisiana patient was hospitalized for flu infections specifically, unlike a hospitalized Missouri patient who was hospitalized in late August for chest pain and gastrointestinal symptoms and was tested for flu as part of the diagnostic work-up. The Missouri patient’s exposure source was never determined, but genetic testing on traces of the virus showed similarity to the dairy cow strain.

Daskalakis referred questions about the patient’s symptoms and health status to Louisiana health officials, who aren’t yet releasing details due to patient privacy concerns. In its December 13 announcement about the presumptive positive test, the Louisiana Department of Health said the patient is from southwestern Louisiana.

The CDC said more tests on the patient’s D1.1 sample are underway to determine if there are any mutations that would point to higher pathogenicity or transmissibility in humans. Investigators in Louisiana are conducting contact tracing, and no other related illnesses have been reported.

New developments don’t change risk assessment

The CDC said the D1.1 detection and the first infection due to contact with backyard poultry don’t change its public health risk assessment, which remains low.

“A sporadic case of severe H5N1 bird flu illness in a person is not unexpected; avian influenza A(H5N1) virus infection has previously been associated with severe human illness in other countries during 2024 and prior years, including illness resulting in death. No person-to-person spread of H5 bird flu has been detected,” the CDC said in a statement today.

The case raises the CDC’s number of confirmed human H5N1 cases for the year to 61 from 8 states. The agency is classifying it under a new category called other animal exposure.