Key facts
- On the 20th of September the US CDC provided an update on the recent avian influenza A(H5N1) case in a human in Missouri including further details on the follow-up investigation. A second health care worker who experienced mild respiratory symptoms has been identified through contact tracing. They were not tested for influenza as they had already recovered at the time of investigation but will be offered serological testing.
- In 2024, and as of 24 September 2024, a total of 14 human cases of avian influenza A(H5) have been reported in the United States. From these, four cases have been reported in workers exposed to dairy cattle infected or presumed to be infected with A(H5N1) and nine cases have been reported in workers exposed to commercial egg layer farms with outbreaks of HPAI A(H5). One (the most recent case) had no known animal exposure identified.
- 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 the 20th of September, the US CDC provided an update on the recent avian influenza A(H5N1) case in a human in Missouri, including further details on the follow-up investigation. Previously, two contacts with symptoms had been reported, one household contact who developed symptoms on the same day as the patient but was not tested at the time and one healthcare worker with mild respiratory symptoms who tested negative for influenza virus. Blood samples have been collected for serological testing from the case and the household contact to test for presence of
antibodies to avian influenza A(H5) virus as an indication of previous exposure to infection. A second health care worker who experienced mild respiratory symptoms was also identified through contact tracing. They were not tested for influenza as they had already recovered at the time of investigation but will be offered serological testing.
Background On 13 September 2024, the US CDC published additional information on the recent human case of avian influenza A(H5) in the state of Missouri. The case was in an adult person with serious comorbidities, who on 20 August developed symptoms of chest pain, nausea, vomiting, diarrhoea, and weakness. The case was hospitalised on 22 August and received antiviral treatment. The patient was not severely ill and has subsequently recovered. One household contact developed symptoms on the same day as the case, but was not tested and has since recovered. As the symptoms developed at the same time, the two people were likely exposed to the same source and were not part of a person-to-person transmission chain. A healthcare worker, who was a close contact of the case, developed mild symptoms but tested negative for influenza. No additional cases have been identified within a 10-day follow-up period.
The clinical specimen contained low amounts of genetic material (viral RNA) and only partial sequencing data was generated. Full-length gene sequences were Because of low amounts of genetic material (viral RNA) in the clinical specimen, sequencing produced limited data for analyses. Full-length gene sequences were produced for matrix (M) and non-structural (NS) genes only. Analysis of partial sequences of the haemagglutinin (HA) and neuraminidase (NA) genes (GISAID: EPI_ISL_19413343) showed a close match to A(H5N1) viruses from US dairy cattle. Similar HA and NA genes sequences have been found in birds and other animals around dairy farms, as well as in raw milk, and poultry.
Analysis of the HA gene sequence shows that the virus belongs to clade 2.3.4.4b, and the NA sequence was confirmed as N1. Compared to previous human cases, the HA has two amino acid differences not observed before. The two amino acid changes are not known to modify virus ability to infect and spread among people. However, the location of the two amino acid changes may impact the cross-reactivity of clade 2.3.4.4b candidate vaccine viruses (CVVs). Further antigenic testing is planned. Additional antigenic testing is planned. The first amino acid change, HA A156T, has been found in fewer than 1% of viruses identified in dairy cows, while the second mutation, HA P136S, has been observed in one virus sequence from dairy cows.
Outbreaks of avian influenza A(H5) have been reported in both commercial and backyard poultry in Missouri in 2024 but not in cattle. Avian influenza A(H5N1) has also been previously detected in the state in wild birds.
In relation to previously reported cases associated with poultry exposure in Colorado, US CDC informed that all virus specimens were determined to be susceptible to antivirals (oseltamivir, zanamivir, peramivir, and baloxavir) following antiviral susceptibility testing.
In 2024, and as of 24 September 2024, a total of 14 human cases of avian influenza A(H5) have been reported in the United States. From these, four cases have been reported in workers exposed to dairy cattle infected or presumed to be infected with A(H5N1) and nine cases have been reported in workers exposed to commercial egg layer farms with outbreaks of HPAI A(H5). One (the most recent case) had no known animal exposure identified.
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 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 does not change and continues to be considered low. Nevertheless, findings from the ongoing investigation will inform 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 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 case 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 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 of the avian influenza situation.