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#USA, #Influenza A #H5N1 in #Humans: #Epidemiology & #Laboratory #Update {as of Feb. 26 '25}

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By G.M.



{Excerpts}

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Epidemiology Updates

As of February 24, CDC has confirmed three human cases of H5 bird flu in people who became ill in 2025: 

-- a dairy worker with exposure to infected dairy cows (Nevada), 

-- a poultry worker with exposure to infected commercial poultry (Ohio), and 

-- the owner of an infected backyard poultry flock (Wyoming). 

These are all considered higher-risk exposures. While the dairy worker was not hospitalized, both people with poultry exposures experienced severe illness and were hospitalized. Both hospitalized cases were confirmed positive from lower respiratory specimens, including a bronchoalveolar lavage and sputum. To date, there has been no evidence of onward spread from any of these people to anyone else.

The dairy worker in Nevada had conjunctivitis (eye redness and irritation) and has recovered. Most infections associated with U.S. dairy cows to date have involved mild respiratory symptoms or conjunctivitis. This person was exposed to infected dairy cows and tested positive for avian influenza A(H5N1) virus.

The poultry worker in Ohio had respiratory symptoms and is home and recovering. This person participated in culling activities on a farm with infected poultry. The initial upper respiratory specimens could not be confirmed as positive for avian influenza A(H5) virus at CDC, so CDC initially reported this as a probable case; a subsequent specimen from the person was confirmed positive for avian influenza A(H5) virus at CDC.

The backyard flock owner in Wyoming had respiratory symptoms and is reported to have underlying health conditions that can make people more vulnerable to severe influenza illness. This person has been discharged from the hospital and is recovering. This person had direct contact with poultry infected with avian influenza A(H5) virus that died on their property. Initial upper respiratory specimens were negative for influenza viruses; a lower respiratory specimen collected several days later in the hospital was positive for avian influenza A(H5N1) virus.


Laboratory Updates

CDC has successfully sequenced the viruses from the Nevada and Wyoming cases. Genetic data have been posted in GISAID (Wyoming: EPI_ISL_19749443, Nevada: EPI_ISL_19726293) and GenBank. Sequencing data are not yet available for the Ohio case.

CDC's analysis of the genetic sequence of the virus isolated from the patient in Nevada identified the virus as an avian influenza A(H5N1) virus from clade 2.3.4.4.b (genotype D1.1). The nucleotide sequence was nearly identical to that of the viruses that USDA reported from dairy cows in Nevada that the person worked with. The virus had a genetic mutation in its polymerase basic 2 (PB2) protein that has previously been associated with more efficient virus replication in mammalian cells (i.e., change of PB2 D701N). This change was previously identified in a human case in Chile in 2023. No other changes associated with mammalian adaption were identified in the sequence data. CDC also did not identify any changes that might impact effectiveness of influenza antiviral medications or existing clade 2.3.4.4b H5 candidate vaccine viruses.

CDC's analysis of the genetic sequence of the virus from the patient in Wyoming identified an avian influenza A(H5N1) virus from clade 2.3.4.4.b (genotype D1.1). The virus had a genetic mutation in its PB2 protein that has previously been associated with more efficient virus replication in people and other mammals (i.e., change of PB2 E627K). This change was previously identified in a human case in Texas during 2024. No other changes associated with mammalian adaption were identified in the sequence data. CDC also did not identify any changes in the sequence data that might impact effectiveness of influenza antiviral medications or existing H5 candidate vaccine viruses. Virus was isolated from the case and will undergo further testing and analysis.

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Source: US Centers for Disease Control and Prevention, https://www.cdc.gov/bird-flu/spotlights/h5n1-response-02262025.html

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