Key facts
- Three new cases of zoonotic avian influenza A(H5) have been reported in Washington state, United States, bringing the total number to five confirmed cases in the state since 24 October 2024.
- The cases are individuals working with infected poultry.
- According to the US CDC, the risk to the general population remains low, while people with exposure to infected poultry, cattle or other potentially infected domestic or wild animals have a higher risk of infection.
Overview
Update: US CDC has reported three human cases of avian influenza A(H5) in Washington state, US (CDC, Bird Flu Response Update). All cases were in people with reported exposure to infected poultry. At this time, no further information is available on the latest confirmed cases. According to US CDC, there is no evidence of human-to-human transmission at the moment.
US CDC is performing genetic analysis of viruses isolated from other confirmed cases. Genetic sequencing confirmed that six of the cases reported previously in California were infected with avian influenza virus A(H5N1) from clade 2.3.4.4b. All six sequences are closely related genetically to the virus causing infections in domestic dairy cattle. Whole genome sequencing was performed on viruses isolated from two people in California and both viruses were confirmed to be a B3.13 genotype. In addition, partial sequences were obtained from samples collected from four other California individuals (CDC A(H5N1) Bird Flu Response Update October 18, 2024). Sequencing analysis did not identify any changes associated with increased ability to infect or transmit between humans. Similarly, no changes associated with mammalian adaptation in other gene segments were identified in the analysed samples. In addition, no mutations associated with reduced susceptibility to antivirals (e.g. neuraminidase inhibitors or polymerase acidic inhibitors) were identified.
US CDC is performing additional testing, including antigenic characterisation of the isolated viruses. This will reveal if existing candidate vaccine viruses (CVVs) are well-matched to the new reported cases of avian influenza A(H5).
CDC has posted in GISAID and submitted to GenBank the haemagglutinin (HA), neuraminidase (NA), and non-structural (NS) gene segments for A/California/134/2024 (GISAID EPI_ISL_19463619; NCBI_PQ435213- PQ435215) and the whole genome sequences for A/California/135/2024 and A/California/147/2024.
On 24 October US CDC also published results of a retrospective investigation of individuals who had close contact with an H5N1 patient reported in Missouri in September 2024 (CDC Report on Missouri H5N1 Serology Testing). Seven people, including five healthcare workers, the reported patient and one family member of the patient have been identified for further serological testing. The CDC supported this investigation by testing blood samples from six of these contacts and the original patient to detect possible prior exposure to H5N1. None of the healthcare workers showed signs of past H5N1 infection, ruling out human-to-human transmission between the individual and healthcare workers. The Missouri patient and a household contact had some inconsistent signs suggesting possible exposure to, or a previous infection with, H5N1. According to US CDC, "immunologic results coupled with the epidemiologic data that these two individuals had identical symptom onset dates support a single common exposure to bird flu rather than person-to-person spread within the household."
As of 28 October 2024, a total of 34 human cases of avian influenza A(H5) have been reported in the US during 2024. Of these, 19 were individuals exposed to dairy cattle that were infected, or presumed to be infected, with A(H5N1) and 14 were workers exposed to outbreaks of HPAI A(H5) at poultry farms. One person had no known animal exposure.
The US CDC’s current assessment of the human health risk of A(H5N1) to the general public in the US has not changed and continues to be considered low.
ECDC assessment
To date, there have been no confirmed human cases of 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.
ECDC has assessed the risk of infection from the circulating HPAI A(H5N1) clade 2.3.4.4b viruses as low for the general population and low-to-moderate for those with activities that expose them to infected or dead animals or a contaminated environment (e.g. occupational exposure to infected animals). ECDC will revisit the risk assessment once more information becomes available from the ongoing sequencing and investigations of the most recent human cases in the US.
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 the 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; Enhanced influenza surveillance to detect avian influenza virus infections in the EU/EEA during the inter-seasonal period). Raising awareness – including about the need to enquire about animal exposure and symptoms compatible with avian influenza infections and testing of symptomatic people with a history of exposure following a risk-based approach – among all primary care workers and communicating on the epidemiological situation is important in order to not 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 taken to mitigate the risk.
ECDC relevant 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
- Enhanced influenza surveillance to detect avian influenza virus infections in the EU/EEA during the inter-seasonal period
- Investigation protocol of human cases of avian influenza virus infections in the EU/EEA
- Joint ECDC-EFSA Drivers for a pandemic due to avian influenza and options for One Health mitigation measures
Actions
ECDC is in contact with the US CDC for further information and is closely following any updates on the event. ECDC monitors 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.