Key facts
- On 14 February, the A(H5N1) influenza case reported earlier in Nevada, was confirmed as caused by the virus clade 2.3.4.4b, genotype D1.1.
- The case is an adult with occupational exposure to infected dairy cattle.
- The person experienced conjunctivitis and is now recovering.
- This is the first human case of A(H5N1) infection in Nevada and the first instance of transmission of genotype D1.1 from dairy cattle to humans.
- Since 1 April 2024, and as of 10 February 2025, a total of 68 human cases of avian influenza A(H5) have been reported from 11 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 23 were workers exposed to outbreaks of HPAI A(H5) at poultry farms. Three people had no known animal exposure and one 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 10 February 2025, the US CDC (H5 Bird Flu: Current Situation | CDC) and the Central Nevada Health District (Community Alerts - Central Nevada Health District) reported one human case of A(H5N1) infection in the state of Nevada, US. The case is an adult who had occupational exposure to infected dairy cattle. The individual experienced conjunctivitis and is now recovering. In response to this event, health authorities are monitoring close contacts of the person for symptoms and are offering personal protective equipment (PPE), testing and antiviral treatment. 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).
On 14 February 2025, the sequence of the human case from Nevada was released on GISAID EpiFlu assigned to clade 2.3.4.4b and genotype D1.1. Out of the 302 influenza A(H5N1) strains of the same genotype available as of 18 February 2025, the majority (93%, n=282) are collected from birds. There are eight virus sequences assigned to genotype D1.1 from humans, all collected in the United States, including the strain A/Nevada/10/2025 from the recent human case of influenza A(H5N1) from Nevada. Mutation analysis of A/Nevada/10/2025 revealed the presence of mutation D701N in the PB2 segment, which is associated with several mammalian adaptations such as increased virulence and contact transmission in guinea pigs and increased virulence in mice. This is the first observation of PB2:D701N in genotype D1.1 from a human infection and the mutation is not present in any of the D1.1 strains from birds. The USDA Animal and Plant Health Inspection Service (APHIS) announced the first detections of genotype D1.1 in dairy cattle in Nevada, US, on January 31 2025, with presence of PB2:D701N in sequenced specimens. The PA segment of A/Nevada/10/2025 has the change S421I, which, although has been associated with increased virulence in mice, is common in isolates from birds.
We gratefully acknowledge all data contributors, i.e., the Authors and their Originating laboratories responsible for obtaining the specimens, and their Submitting laboratories for generating the genetic sequence and metadata and sharing via the GISAID Initiative, on which this research is based.
Background: Since 1 April 2024, and as of 17 January 2025, 68 human cases of avian influenza A(H5N1), including one death, have been confirmed by the US CDC from 11 states. 41 cases reported exposure to dairy cattle in the following states: California (36), Colorado (1), Michigan (2), Nevada (1) and Texas (1). 23 cases reported exposure to poultry in the following states: Colorado (9), Iowa (1), Oregon (1), Washington (11), and Wisconsin (1). One case, the patient reported in Louisiana who died, had exposure to backyard flocks and other wild birds. Three additional cases have been identified with unknown exposure: 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.