Key facts
- Five new cases of avian influenza A(H5N1) have been reported in California, United States, bringing the total number to 11 confirmed cases since 3 October 2024.
- All 11 cases were in individuals with exposure to infected dairy cattle.
- According to the United States Centers for Disease Control and Prevention (US CDC), the risk to the general population remains low, while people with exposure to infected animals have a higher risk of infection.
Overview
Update: On 17 October 2024, the US CDC reported an additional five human cases of avian influenza from Central Valley in individuals with exposure to infected cattle (www.cdc.gov). Six cases reported between 3 and 11 October 2024 were in individuals who experienced only mild symptoms, including eye redness or discharge (conjunctivitis) and did not require hospitalisation. Four of these individuals had contact with infected cattle at different farms in Central Valley, while two were from the same farm. According to the US CDC, the two individuals worked at different parts of the farm and were not close contacts. US CDC considers all cases to be due to sporadic animal-to-human transmission. No additional information is currently available for the last five confirmed cases.
US CDC has previously reported that the virus isolated from the samples collected from the first two cases confirmed on 3 October and one case reported on 9 October belong to clade 2.3.4.4b, closely related to viruses isolated from infected dairy cows (CDC A(H5N1) Bird Flu Response Update). Whole genome sequencing was performed on one of the isolates and the virus was confirmed to be a B3.13 genotype. 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 sequence for A/California/135/2024 (GISAID EPI_ISL_19463618; NCBI_PQ435216-PQ435223).
The HAs of the three analysed viruses contained new amino acid mutations compared with candidate vaccine viruses (CVVs). Two of the changes were in A/California/135/2024, three in A/California/134/2024, and three in A/California/146/2024. However, none of the changes associated with increased ability to infect or transmit between humans have been identified. As for susceptibility to antivirals, there was no reduced susceptibility to neuraminidase inhibitors or polymerase acidic inhibitors. Similarly, no changes associated with mammalian adaptation in other gene segments were identified in the analysed samples.
US CDC is performing additional testing, including antigenic characterisation of the isolated viruses. This will inform if existing CVVs are well-matched to the new reported cases of avian influenza A(H5).
US CDC also performed neutralisation assays with the use of reverse genetic engineered virus containing HA P136S and A156T changes identified in a case reported in Missouri on 6 September 2024. The tests showed that the mutated virus had reduced cross-reactivity to ferret antisera generated to viruses without those changes. The generated virus will now be used for serology testing of serum samples from the confirmed case in Missouri and the individual's close contacts.
As of 18 October 2024, a total of 25 human cases of avian influenza A(H5) have been reported in the US during 2024. Of these, 15 were individuals exposed to dairy cattle that were 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.
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 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 environments (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.