Key facts
- On 22 July 2024, the US CDC reported six human cases of A(H5N1) avian influenza virus among workers exposed to infected poultry as part of the response to the ongoing outbreak at a commercial egg layer operation in Colorado. The cases showed mild respiratory symptoms and none have required hospitalisation to date.
- The virus was characterised as genotype B3.13 clade 2.3.4.4b HPAI A(H5N1) and it was closely related to the recent poultry outbreaks and infected dairy cattle herds.
- The virus maintains avian genetic characteristics and lacks adaptations for human infection. However, a specific change at PB2 M631L suggests mammalian adaptation.
- No markers of antiviral resistance were found and the HA sequence closely resembles to the two existing HPAI A(H5) candidate vaccine viruses.
- On 25 July 2024, the US CDC confirmed three human cases of highly pathogenic avian influenza (HPAI) A(H5) among workers exposed to infected poultry as part of the response to an outbreak at a second poultry farm in Colorado. The cases had mild symptoms and have been offered antiviral treatment.
- In 2024 and as of 25 July 2024, a total of 13 human cases of HPAI A(H5), including 10 human cases of A(H5N1), have been reported in USA. From these, four cases have been reported in workers exposed to dairy cattle infected with A(H5N1) and nine cases have been reported in workers exposed to commercial egg layer farms with outbreaks of HPAI A(H5).
- 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
Weekly update: On 22 July 2024, the US CDC confirmed six human cases of A(H5N1) avian influenza virus among workers exposed to infected poultry as part of the response to the ongoing outbreak at a commercial egg layer operation in Colorado. The cases showed mild respiratory symptoms and none have required hospitalisation to date. Sequencing results available from one case show a close match with the first human case exposed to cattle in Michigan, including the absence of antiviral resistance and close resemblance to the two existing HPAI A(H5) candidate vaccine viruses (IDCDC-RG78A and IDCDC-RG71A), which have only 2 to 4 amino acid changes compared to the Colorado virus. The virus was characterised as genotype B3.13 clade 2.3.4.4b HPAI A(H5N1), closely related to recent poultry outbreaks and infected dairy cattle herds. Notably, the virus maintains avian genetic characteristics and lacks adaptations for human infection. A specific change at PB2 M631L suggests mammalian adaptation, similar to dairy cow sequences. However, unlike the Texas case, this virus lacks the PB2 E627K change associated with human adaptation.
Partial gene segments were obtained from two other confirmed human cases associated with the poultry farm depopulation. Only internal genes (not HA or NA) provided sufficient data for analysis.
On 25 July 2024, the US CDC confirmed three human cases of highly pathogenic avian influenza (HPAI) A(H5) among workers exposed to infected poultry as part of the response to an outbreak at a second poultry farm in Colorado.
These three cases have mild symptoms and have been offered antiviral treatment.
The US CDC has deployed a multidisciplinary field team to support the response in Colorado to the ongoing poultry outbreak of H5 avian flu in the state.
A seroprevalence study from Michigan analysed blood samples from 35 individuals who worked on dairies with confirmed HPAI A(H5N1) outbreaks. None of these participants showed neutralising or hemagglutination inhibition (HI) antibodies specific to the avian influenza A(H5N1) virus. However, many participants had neutralising antibodies to seasonal flu, indicating prior infection or vaccination. These results suggest that the seroprevalence of HPAI A(H5N1) among workers with known exposures is low.
Summary: In 2024 and as of 25 July 2024, a total of 13 human cases of HPAI A(H5), including 10 human cases of A(H5N1), have been reported in the US. From these, four cases have been reported in workers exposed to dairy cattle infected with A(H5N1) and nine cases have been reported in workers exposed to commercial egg layer farms with outbreaks of HPAI A(H5).
The cases among workers exposed to dairy cattle have been reported in the states of Texas (1), Michigan (2) and Colorado (1). All cases among workers exposed to commercial egg layer farms have been reported in Colorado (9).
Background: On 1 April 2024, the first human case of HPAI A(H5N1) was reported in Texas, USA. The individual had prior exposure to dairy cattle presumed to be infected with HPAI A(H5N1). Genetic analysis revealed the virus belonged to HA clade 2.3.4.4b, genotype B3.13, and displayed the PB2 E627K mutation, indicating mammalian adaptation. The virus retained avian genetic characteristics and showed no markers for antiviral resistance.
On 22 May 2024, a second human infection with avian influenza A(H5N1) was reported in a farm worker in Michigan, USA, associated with infected dairy cattle. The virus identified belonged to the same clade and genotype as the Texas case but lacked the PB2 E627K mutation. Instead, it had the PB2 M631L change, also associated with mammalian adaptation. The genome of the virus was closely related to viruses found in dairy cattle, suggesting cow-to-human transmission.
On 30 May 2024, a third human case of HPAI A(H5N1) was reported in Michigan, USA. The dairy farm worker exhibited upper respiratory tract symptoms and eye discomfort. The virus was closely related to those found in US cattle, with a high similarity to the virus sequences from US cattle confirmed.
Michigan Department of Health and Human Services further mentioned that neither of the two cases reported in Michigan were wearing full personal protective equipment (PPE). The case reported on 22 May 2024 developed eye symptoms after having milk splashed in their eye, highlighting the importance of appropriate PPE. Since the three reported cases had direct contact with infected cattle, the
US CDC's current assessment of the human health risk of A(H5N1) to the general public in the US does not change.
On 3 July 2024, a fourth human case of HPAI A(H5N1) was reported in Colorado, USA. The dairy farm worker had exposure to infected cows and reported eye symptoms. Genetic sequencing of the virus is pending.
On 19 July 2024, the Colorado Department of Public Health and Environment, in coordination with other agencies, reported six human cases of avian influenza in workers responding to an outbreak at a commercial egg layer operation. The workers exhibited mild symptoms, including conjunctivitis and respiratory infection symptoms. Sequencing results from one case showed a close match with the first human case exposed to cattle in Michigan, including the absence of antiviral resistance and close resemblance to two existing HPAI A(H5) candidate vaccine viruses. On 20 July the US CDC confirmed these six human cases of and the virus was characterised as genotype B3.13 clade 2.3.4.4b HPAI A(H5N1), closely related to recent poultry outbreaks and infected dairy cattle herds. The virus maintains avian genetic characteristics and lacks adaptations for human infection, with no markers for influenza antiviral resistance found in the patient’s specimen.
On 25 July 2024, the US CDC confirmed three human cases of highly pathogenic avian influenza (HPAI) A(H5) among workers exposed to infected poultry as part of the response to an outbreak at a second poultry farm in Colorado.
The United States' Department of Agriculture (USDA) Food Safety and Inspection Service (FSIS) has detected viral particles of HPAI A(H5N1) in tissue samples, including muscle, from a culled dairy cow that had been condemned (declared unfit for consumption) during a post-mortem inspection due to signs of systemic disease. Dairy cattle slaughtered for beef production undergo routine inspection before and after slaughter by the FSIS. Meat from dairy cattle condemned at slaughter due to systemic disease do not enter the human food supply.
A study of ground beef inoculated with A(H5N1) suggested cooking burgers medium to well done (to a temperature of 63C to 71C) inactivated the virus, while cooking burgers to 49C (rare) also substantially inactivated the virus.
HPAI A(H5N1) virus has been detected in dairy cattle in several states in the US. As of 3 July 2024, the USDA reports the detection of HPAI A(H5N1) in 12 states, affecting 138 dairy farms/herds. The most recently reported detection was on 1 July 2024 in Colorado. Markers of influenza A(H5) have also been detected in wastewater in Texas (Tisza et al., 2024; Wolfe et al., 2024). Furthermore, cats fed unpasteurised milk and colostrum from affected cows developed systemic, fatal infection (Burrough et al., 2024).
HPAI A(H5N1) has also been reported in some other mammals in the US, including goats and alpacas at farms with infected poultry. The virus genotype identified in the infected goats was different to the one identified in cattle (B3.13), while a virus from the same genotype B3.13 had infected the alpacas.
Genetic material of HPAI A(H5N1) has been detected in milk samples. Studies performed by US FDA have shown that pasteurisation inactivates HPAI in milk and dairy products, with no viable virus being detected following pasteurisation. The USDA and US FDA highlight that commercially produced, pasteurised milk is safe for consumers and recommend that milk from cattle with clinical signs of infection is removed from the human food chain. For further information, please refer to the US FDA update. Samples of ground beef from states with affected dairy herds tested negative for HPAI A(H5N1) (USDA).
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 the human case in the US has not currently been detected in Europe. In addition, HPAI A(H5N1) has not been detected in European (waste)water.
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. occupationally exposure to infected animals). The implementation of personal protective measures mitigate the risk.
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). 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 Centers for Disease Control and Prevention (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.