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

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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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#USA, Confirmed #human #H5N1 #influenza virus case #summary since 2024, by state and exposure source: One new case in #Nevada (Feb. 10 '25)

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[State - Exposure Source: Exposure Associated with Commercial Agriculture and Related Operations: Dairy Herds (Cattle) - Poultry Farms and Culling Operations - Other Animal Exposure† - Exposure Source Unknown‡ - State Total]

1) California - 36 - 0 - 0 - 2 - 38

2) Colorado - 1 - 9 - 0 - 0 - 10

3) Iowa - 0 - 1 - 0 - 0 - 1

4) Louisiana - 0 - 0 - 1 - 0 - 1

5) Michigan - 2 - 0 - 0 - 0 - 2

6) Missouri - 0 - 0 - 0 - 1 - 1

7) Nevada - 1 - 0 - 0 - 0 - 1 {+1}

8) Oregon - 0 - 1 - 0 - 0 - 1

9) Texas - 1 - 0 - 0 - 0 - 1

10) Washington - 0 - 11 - 0 - 0 - 11

11) Wisconsin - 0 - 1 - 0 - 0 - 1

-- Source Total - 41 - 23 - 1 - 3 - 68 {+1}

NOTE: One additional case was previously detected in a poultry worker in Colorado in 2022. Louisiana reported the first H5 bird flu death in the U.S.

{†} Exposure was related to other animals such as backyard flocks, wild birds, or other mammals

{‡} Exposure source was not able to be identified

Probable human case summary during the 2024 outbreak, by state and exposure source

When a case tests positive for H5 at a public health laboratory but testing at CDC is not able to confirm H5 infection, per Council of State and Territorial Epidemiologists (CSTE) guidance, a case is reported as probable.

-- Probable cases with commercial poultry exposure (e.g., poultry farms or culling operations):

1) Washington (3)

2) Arizona (2)

-- Probable cases with commercial dairy (cattle) exposure:

1) California (1)

-- Probable cases with exposure source unknown:

1) Delaware (1)

Confirmed and probable cases are typically updated by 5 PM EST on Mondays (for cases confirmed by CDC on Friday, Saturday, or Sunday), Wednesdays (for cases confirmed by CDC on Monday or Tuesday), and Fridays (for cases confirmed by CDC on Wednesday and Thursday). Affected states may report cases more frequently.

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Source: US Centers for Disease Control and Prevention, https://www.cdc.gov/bird-flu/situation-summary/?CDC_AAref_Val=https://www.cdc.gov/flu/avianflu/avian-flu-summary.htm

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#USA, APHIS Confirms {Avian #Influenza #H5N1} #D11 #Genotype in Dairy #Cattle in #Nevada

On January 31, 2025, the USDA Animal and Plant Health Inspection Service (APHIS) National Veterinary Services Laboratories (NVSL) confirmed by whole genome sequence the first detection of highly pathogenic avian influenza (HPAI) H5N1 clade 2.3.4.4b, genotype D1.1 in dairy cattle. 

This confirmation was a result of State tracing and investigation, following an initial detection on silo testing under the USDA’s National Milk Testing Strategy (NMTS) in Nevada. 

USDA APHIS continues to work with the Nevada Department of Agriculture by conducting additional on-farm investigation, testing, and gathering additional epidemiological information to better understand this detection and limit further disease spread. 

This is the first detection of this virus genotype in dairy cattle (all previous detections in dairy cattle have been HPAI H5N1 clade 2.3.4.4b, genotype B3.13). 

Genotype D1.1 represents the predominant genotype in the North American flyways this past fall and winter and has been identified in wild birds, mammals, and spillovers into domestic poultry. 

The detection does not change USDA’s HPAI eradication strategy and is a testament to the strength of our National Milk Testing Strategy (NTMS). In the interest of sharing information of import to the scientific community, APHIS will publish a technical brief on the findings on our website and post the sequence data on GenBank in the coming week. 

Source: Department of Agriculture, https://www.aphis.usda.gov/news/program-update/aphis-confirms-d11-genotype-dairy-cattle-nevada-0

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