Key facts
- On 26 February, the US CDC has published the analysis of the genetic sequence of the avian influenza A(H5N1) virus isolated from the case in Wyoming (case reported in CDTR on 20 Feb 2025). The isolated virus belongs to clade 2.3.4.4.b, genotype D1.1, with the mutation E627K in the PB2 segment, previously associated with more efficient virus replication in people and other mammals. No additional mutations linked to mammalian adaptation were found in the sequence data.
- Since 1 April 2024, and as of 27 February 2025, a total of 70 human cases of avian influenza A(H5) have been reported from 13 states in the United States (US). Of these, 41 were individuals exposed to dairy cattle known or presumed to be infected with A(H5N1) and 24 were workers exposed to outbreaks of HPAI A(H5) at poultry farms. Three people had no known animal exposure and two had exposure to other animals, such as backyard flocks, wild birds, or other mammals.
- According to the US CDC, the risk to the general population remains low, while farmers and workers who work with infected animals or their by-products, backyard bird flock owners, animal care workers (e.g. veterinarians, wild animal facility workers), and animal health and public health responders are at increased risk of infection with A(H5N1).
Overview
Update:
On 26 February 2025, the US CDC has published the analysis of the genetic sequence of the avian influenza A(H5N1) virus isolated from the case in Wyoming. The case was reported in the CDTR on 20 Feb 2025. The isolated virus belongs to clade 2.3.4.4.b, genotype D1.1, the clade and genotype as the virus isolated from the case in Nevada. Mutation analysis of the viral sequence revealed the presence of mutation E627K in the PB2 segment, previously associated with more efficient virus replication in people and other mammals. This mutational change was previously reported in a virus from a different clade isolated from the case in Texas in April 2024. No additional mutations linked to mammalian adaptation were found in the sequence data. The CDC also did not detect any genetic changes that could affect the effectiveness of influenza antiviral medications or existing H5 candidate vaccine viruses. The isolated virus will undergo further testing and analysis.
Currently, there is no evidence of human-to-human virus spread or of any additional human cases. According to the US CDC (H5 Bird Flu Response | Bird Flu | CDC), the risk to the general population remains low, while farmers and workers who work with infected animals or their by-products, backyard bird flock owners, animal care workers (e.g. veterinarians, wild animal facility workers), and animal health and public health responders are at increased risk of infection with A(H5N1).
Background:
Since 1 April 2024, and as of 25 February 2025, 70 human cases of avian influenza A(H5N1), including one death, have been confirmed by the US CDC from 13 states. 41 cases reported exposure to dairy cattle in the following states: California (36), Colorado (1), Michigan (2), Nevada (1) and Texas (1). 24 cases reported exposure to poultry farms and culling operations in the following states: Colorado (9), Iowa (1), Ohio (1), Oregon (1), Washington (11), and Wisconsin (1). Two cases reported exposure to other animals such as backyard flocks, wild birds, or other mammals: one case in Louisiana, who later died, and one in Wyoming. Three additional cases with unknown exposure have also been identified: two in California and one in Missouri.
ECDC assessment
To date, there have been no confirmed human cases of influenza A(H5N1) infection and no reports of A(H5N1) infection in cattle in the EU/EEA. The genotype B3.13, identified in cattle and several of the human cases in the US has not been detected in Europe. The genotype D1.1, identified in one cow to date, has been found in poultry and a small number of human cases exposed to poultry in the US and Canada.
ECDC has assessed the risk from the circulating HPAI A(H5N1) clade 2.3.4.4b viruses as low for the general population and low-to-moderate for those whose activities expose them to infected animals or contaminated environments (e.g. occupational exposure to infected animals).
Actions
ECDC is monitoring the situation together with partner organisations in Europe and will continue to update its assessment of the risk for humans in the EU/EEA as new information becomes available.
In addition to enhanced surveillance, active monitoring and testing of exposed individuals is recommended for early detection of human cases and to assess the possibility of human-to-human transmission, according to relevant ECDC guidance documents ('Testing and detection of zoonotic influenza virus infections in humans'; 'Investigation protocol of human cases of avian influenza virus'; 'Enhanced surveillance of severe avian influenza virus infections in hospital settings').
It is important to raise awareness, including among all primary care workers, of the need to enquire about animal exposure and symptoms compatible with avian influenza infections and to test symptomatic individuals with a history of exposure, following a risk-based approach. It is also important to communicate the epidemiological situation so as not to miss or delay diagnosis of potential human cases.
Given the uncertainties related to mammal-to-mammal transmission and depending on the epidemiological situation, a low threshold can be considered for testing individuals exposed to potentially infected mammals (e.g. symptomatic individuals with conjunctivitis or respiratory symptoms). Due to the higher risk of infection for individuals exposed to infected animals and contaminated environments, appropriate personal protective measures and other precautionary measures should always be used to mitigate the risk.
Relevant ECDC publications:
- 'Testing and detection of zoonotic influenza virus infections in humans in the EU/EEA, and occupational safety and health measures for those exposed at work'
- 'Investigation protocol of human cases of avian influenza virus infections in the EU/EEA'
- 'Surveillance and targeted testing for the early detection of zoonotic influenza in humans during the winter period in the EU/EEA'
- 'Joint ECDC-EFSA Drivers for a pandemic due to avian influenza and options for One Health mitigation measures'
ECDC is in contact with the US CDC for further information and is closely following any updates on the event. ECDC monitors zoonotic avian influenza strains through its influenza surveillance programme and epidemic intelligence activities in collaboration with the European Food Safety Authority (EFSA) and the EU Reference Laboratory for Avian Influenza in order to identify significant changes in the virological characteristics and epidemiology of the virus. Together with EFSA and the EU Reference Laboratory for Avian Influenza, ECDC produces a quarterly updated report on the avian influenza situation.