Key facts
- Two human cases of avian influenza A(H5) in California were confirmed by US CDC on 3 October 2024.
- Both cases were in individuals working with dairy cattle infected with avian influenza.
- To date, there have been no confirmed cases of A(H5N1) infection in humans and no reports of A(H5N1) infection in cattle in the EU/EEA.
- The risk of zoonotic influenza transmission to the general public in EU/EEA countries is considered low. The risk to occupationally exposed groups, such as farmers and cullers, is considered low-to-moderate.
Overview
Update: On 3 October 2024, California Department of Public Health (CDPH) reported two cases of avian A(H5) in individuals working at a dairy facility in Central Valley (www.cdph.ca.gov). Both cases were in contact with dairy cattle infected with avian influenza. They reported mild symptoms, including conjunctivitis, but no respiratory symptoms. Neither of the cases required hospitalisation. According to CDPH news release, the first case is being treated with antiviral medicine and is self-isolating at home. Details about second case treatment are not available at the moment. No link or contact between the cases has been established, suggesting animal-to-human transmission.
After testing for A(H5N1) at the local public health laboratory, the first case was notified to CDPH as "presumptive positive". Both cases were later confirmed as positive for avian influenza A(H5) by US CDC (www.cdph.ca.gov). The neuraminidase (N) designation has not been yet confirmed.
These are the first reported human cases of avian influenza A(H5) in California. In August 2024, A(H5N1) was detected in dairy cattle in the state. As of 4 October 2024, a total of 16 human cases of avian influenza A(H5) have been reported in the United States during 2024. Of these, six were workers exposed to dairy cattle infected, or presumed to be infected, with A(H5N1) and nine were workers exposed to outbreaks of HPAI A(H5) at commercial egg farms. One case had no known animal exposure.
Where genetic analysis has been available, the virus has been characterised as genotype B3.13 clade 2.3.4.4b of highly pathogenic avian influenza (HPAI) A(H5N1) and has been closely related to viruses identified in recent poultry outbreaks and infected dairy cattle herds in the US. The virus maintains avian genetic characteristics. However, mutations associated with mammalian adaptation have been observed in viruses from some cases. No markers of antiviral resistance were found in viruses from human cases and they remain antigenically similar to the two existing HPAI A(H5) candidate vaccine viruses.
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. However, findings from the ongoing investigation will determine whether the assessment needs to be updated.
ECDC assessment
To date, there have been no confirmed cases of A(H5N1) infection in humans 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 enquiring 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 (for example 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.