Avian Influenza and Food

May 2025

Avian influenza (‘bird flu’) is a viral disease of birds with naturally occurring avian influenza type A viruses. These viruses are globally widespread in aquatic birds (from the orders Anseriformes and Charadriiformes), such as shorebirds and waterfowl, and are highly contagious. Avian influenza can infect all species of domestic and wild birds, as well as mammals such as pigs, pets, and zoo animals. The viruses normally circulate in animals but can also infect humans via contact with infected animals or contaminated environments. To date, no avian influenza viruses have demonstrated sustained human-to-human transmission, and the likelihood is currently low.

Influenza A viruses exist in multiple forms (there are 16 H subtypes and 9 N subtypes in birds). The highly pathogenic H5N1 subtype of influenza crossed the species barrier from avian to human in 1997 for the first time, and from 2003, has been recirculating in Europe, Africa, and Asia since re-emerging in China. In the UK, in January 2025, the UK Health Security Agency (HSA) reported a human case of H5N1 in England¹, which was acquired from a farm with a large number of infected birds. There are multiple subtypes and genotypes, all linked to the ancestral virus reported in 1997. Outbreaks in poultry and wild birds have been reported in over 65 countries², including the UK. Human cases of avian influenza H7N9 have been reported in China since 2013, and occasional cases have been reported with a novel H5N6 strain since 2014. This zoonotic variant of the H5N6 virus has been associated with some spread amongst poultry but is limited to China and South-East Asia³.

Globally, poultry production and trade continue to expand, and as production increases, so does the potential opportunity for pathogenic viruses such as avian influenza to continue to evolve, including the acquisition of host adaptation and the ability to cross species barriers.

In 2022, the UK had the largest outbreak of avian influenza seen in the country and across Europe, with record cases. In 2025 the H5N1 and H5N5 strains of highly pathogenic avian influenza circulated in the UK wild bird population, with cases reported in England, Scotland, and Northern Ireland, and a risk assessment reported as very high. Furthermore, the first case of H5N1 in a single sheep was reported in March 2025. The Department of Environment, Food and Rural Affairs (Defra) and Animal and Plant Health Agency (APHA) set out a notifiable avian disease control strategy for Great Britain, supported by a mitigation strategy for avian influenza in wild birds in England and Wales^4,5,6. However, it has also been necessary to put in place mandated biosecurity measures to prevent further spread of the disease.

Dairy farms in the USA have had cases of H5N1 in cattle since March 2024, which led to increased surveillance of various milk products and cattle. In the UK, a risk assessment was updated in February 2025 by the Food Standards Agency (FSA) considering the risks to UK consumers from H5N1 (serotype B3.13) in USA dairy and beef products^7,8,9. Pasteurisation and cooking have been shown to reduce the infectivity of H5N1 in these products, and overall, the probability of UK consumers receiving infectious exposures to avian influenza was reported as negligible or very low in raw beef.

The UK HSA advises that the risk to the general public is very low. The FSA has said poultry, poultry products, and eggs can be safely consumed if cooked properly^10. Human cases of illness have been strongly linked to the slaughtering and/or handling of diseased or dead birds in countries experiencing outbreaks of avian influenza, typically in areas where humans live and work alongside infected poultry. Home preparation of live birds in these areas must be conducted using good hygiene practices to prevent contamination of the handler, such as wearing personal protective equipment.

Biology of Avian Influenza

Three of the five viruses (A, B, and C) belonging to the Orthomyxoviridae family, specifically the genus influenza virus, cause influenza and are single-stranded RNA viruses¹¹, which do not encode messenger RNA (a key intermediary in gene expression) with segmented genomes. Influenza A viruses derive from wild aquatic avian hosts, while types B and C are typically isolated from humans. Type A viruses exhibit cross-species transmission and can infect warm-blooded mammals, including cats¹², swine, dogs, horses, sea mammals, and mink¹³, and are classified according to the surface glycoproteins into subtypes. There are at least 16 hemagglutinin (HA) and 9 neuraminidase (NA) proteins with the type of hemagglutinin and neuraminidase determining the subtype; for example, a virus with the protein types 5 HA and 8 NA has the subtype H5N8¹⁴.

Genetic reassortment and gene mutation result in highly diverse influenza A viruses, with the former potentially leading to antigenic shifts, resulting in a new virus that is not affected by existing immunity in birds and mammals, including humans. The 1918 influenza pandemic is likely to have been caused by a virus that developed entirely from an unknown avian source, following modification through an unknown mechanism of the eight genome segments, allowing human infection¹⁵.

Infectivity and Pathogenicity

Most avian influenza HA subtypes are of low pathogenicity for birds and typically cause few clinical symptoms in poultry. However, the H5 and H7 viruses may mutate into highly pathogenic viruses after circulation in poultry flocks, with the potential to cause pandemics or epidemics^16,17. Highly Pathogenic Avian Influenza (HPAI) strains can be present in poultry (especially gallinaceous, i.e., chickens and turkeys) with severe clinical signs and high mortality rates. The H5 and H7 viruses that have caused illness in humans have been demonstrated to have unstable hemagglutinin, an obstacle in the viruses having efficient airborne transmission¹⁸, necessary for spread between humans.

The infectivity, pathogenicity, and transmissibility of avian influenza viruses and their adaption to new hosts depend on the capability of the viral HA to use and bind to sialic acid receptors¹⁹. When bound, the enzyme neuraminidase destroys receptors and cleaves sialic acid, so releasing the virus. The specificity of the binding to receptors influences the infectivity and pathogenicity of the viruses; HA in avian influenza viruses prefer to bind sialic acid occurring at the end of sugar chains in glycoproteins with an α2-3 linkage, whereas human influenza strains typically bind to the α2-6 linkage. In birds, the α2-3 linked sialic acids are found in the intestinal mucosa, with α2-6 links being prevalent in the upper human respiratory tract. The severe infection of humans by H5N1 may be explained by the α2-3 links to sialic acid in their lower respiratory tract as viral replication in this area, as opposed to the upper respiratory tract, is more likely to cause severe diseases such as pneumonia²⁰. This may also mean that avian influenza subtypes with the highest human pathogenicity are less likely to be transmissible along the human-to-human route, as it is likely that replication in the upper respiratory tract is important for transmission.

Cases and Outbreaks

Avian influenza in animals

Highly Pathogenic Avian Influenza (HPAI) viruses have been described in birds since 1878, in Italy, then in other parts of Europe, and globally.

In 2022, the UK had the largest outbreak of avian influenza seen in the country and across Europe, with record cases and significant consequences for agriculture and the environment²¹. In the UK, two strains of HPAI (H5N1 and H5N5) are currently increasing in both kept poultry and wild birds.

The first case of HPAI H5N1 of the 2024-onwards outbreak was confirmed in:

England on 17 November 2024
Scotland on 10 January 2025
Northern Ireland on 12 February 2025.

Whilst no cases of HPAI have been confirmed to date in Wales during this outbreak, in line with World Organisation for Animal Health (WOAH) rules, the UK is no longer free from highly pathogenic avian influenza.

Up to 30 May 2025, there were a total of 64 cases of HPAI in this outbreak. Sixty-three of these cases were of the HPAI H5N1 strain, with 57 cases reported in England, two in Scotland, and four in Northern Ireland. There has been one case of the HPAI H5N5 strain reported in England. In addition, the first case of H5N1 in a single sheep was reported in March 2025 in Yorkshire.

Avian influenza in humans

In 1997, 18 cases of H5N1 were reported in Hong Kong, the first time the virus crossed the species barrier to humans, with 18 cases being confirmed and 6 of those cases resulting in deaths. Since 1999, there have been sporadic cases of H9N2, generally in association with mild illness typified by influenza-like respiratory signs or conjunctivitis. Cases of H5N1 HPAI zoonotic infection steadily increased from 2005, associated with expansion in the virus range in poultry. Cases of H5N1 HPAI have reduced in recent years and have been limited to a few countries where infection is endemic in poultry. In January 2025, one human case of H5N1 in England¹ was reported, which was acquired from a farm with a large number of infected birds. In 2003, in The Netherlands, for the first time, a large poultry outbreak with a different subtype (H7N7) was associated with human cases of exposed personnel, resulting in mild illness in more than 80 people and one death. The following year, H7N3 was reported in British Columbia, Canada. The H5 HPAI viruses of multiple subtypes continue to cause significant global problems with ongoing outbreaks in poultry with some occasional impact on other populations, including wild birds and humans.

Infections of the novel avian influenza H7N9 subtype were first reported in humans in China in 2013 and have since been reported in both poultry and humans. The majority of human cases reported exposure to poultry markets or live poultry. Human-to-human spread is thought to have occurred on rare occasions; however, there is no evidence of sustained person-to-person infection. A programme of vaccination in poultry in China has led to a large reduction in the number of human cases.

Animal and Food Handling

Infected domestic and wild birds shed avian influenza viruses in their oral secretions, including high concentrations in their faeces. Transmission of the virus to humans is likely to occur by airborne (when a person breathes in droplets or dust containing the virus) or indirect contact with dust, feathers, clothes, etc., with the eyes or nose potentially being important initial infection sites. While human-to-human transmission is currently limited, there is the possibility that H5N1 or related viruses may be transmitted through the human faecal-oral route²²in humans, and this route is known as a route between waterfowl and domestic poultry.

Globally, most human illness has been caused by two Asian lineage strains (H5N1 and H7N9), often with severe illness and mortality²³. Symptoms in humans with avian influenza A virus infections include subclinical symptoms such as conjunctivitis, fever and muscle aches and severe symptoms due to complications including severe respiratory illness, multi-organ dysfunction and secondary bacterial and fungal infections²⁴.

Outbreaks have raised concerns regarding the risk to food from avian influenza. However, to date, there is no convincing evidence that avian influenza can be transmitted to humans through the consumption of contaminated food. Avian influenza in domestic birds has been reported in over 65 countries to date, primarily in Europe, Africa and Asia. Since 2013, over 120 million birds have died or been killed (exact estimates lacking, and this figure is probably a gross underestimate; numbers vary and may be as high as 300 million) and disposed of due to HPAI avian influenza outbreaks²⁵. However, as shown by surveillance data in the USA, HPAI H5N1 can still enter the human food chain, as demonstrated by being detected in frozen duck meat imported from Asia, as well as more recent reports of the virus in duck carcasses imported into Korea and Japan²⁶. There has been no research confirming that the disease can be transmitted to humans through food that is correctly handled and cooked. However, there is evidence potentially linking human and carnivorous animal illness to the consumption of food made with raw contaminated poultry blood²⁷.

Diseased or dead birds should not be handled in outbreak areas without personal protective equipment (PPE). Measures must be taken to prevent personnel from being contaminated by blood, meat, and bones and ensure that such birds are not used for human consumption. Some species can be asymptomatic carriers and may not show clinical signs of the disease, as can vaccinated poultry in some countries.

Virtually all parts of a bird infected with H5N1 HPAI or closely related viruses can be impacted, and the virus can survive through the marketing and distribution chain in fresh and frozen meat since it is viable at cold chain temperatures (35 days at 4°C in faeces and at ambient temperatures on surfaces for several weeks)²⁸.

In areas not impacted by outbreaks, there is a very low likelihood of the virus being present in fresh or frozen meat in the food chain, and the risk to food handlers or consumers is very low. Correctly prepared and cooked poultry and poultry products are safe to eat. In the UK, the FSA has said avian influenza poses a very low food safety risk for UK consumers. Poultry, poultry products, and eggs can be safely consumed if cooked properly.

On-farm Biosecurity

Practising good biosecurity at all times protects the health and welfare of kept birds, and commercial keepers will help protect their businesses from HPAI and other diseases. In the UK, to prevent the spread of disease, an Avian Influenza Prevention Zone (AIPZ) mandating enhanced biosecurity is in force across England and Scotland. The AIPZ means that all bird keepers need to take extra precautions, such as restricting access for non-essential people on site, ensuring workers change clothing and footwear before entering bird enclosures and cleaning and disinfecting vehicles regularly to limit the risk of the disease spreading. In addition, in various locations in England, there are mandatory housing measures for all kept birds.

The Department of Agriculture, Environment and Rural Affairs (DAERA) has the below measures listed in guidance for bird keepers to reduce the risk of introducing or spreading avian influenza:

Minimise movement by staff and visitors in and out of bird enclosures and between sites/other poultry farms
Ensure staff and visitors/contractors understand the site’s biosecurity requirements
Ensure clean clothing, footwear and hands before and after visiting a poultry house, using approved disinfection at all entrances and exits
Clean and disinfect any vehicles and equipment that come into contact with poultry
Limit or prevent contact with non-commercial poultry and birds, including live bird markets. Ensure feed and water cannot be accessed by wild birds
Buy healthy stock from reliable sources and feed from a reputable mill or supplier
Inspect the flock daily for signs of disease and mortality. Know the signs of disease and how to report suspected notifiable illnesses. Separate and isolate new or ill stock. Keep records of all birds moving on or off the premises. Keep an up-to-date contact list of key people and develop a contingency plan
Provide good ventilation and maintain litter in a relatively dry condition to prevent growth environments for pathogenic microorganisms
Minimise the congregation of waterfowl and wild birds
Monitor wild bird detections as these may proceed with farmed poultry outbreaks²⁹.

Handling

Good hygiene practices are essential after handling live poultry or poultry products pre- and post-slaughter to prevent cross-contamination.

Wash hands thoroughly before preparing, cooking or eating with warm, soapy water
Wash hands after using the toilet, handling animals or their waste, emptying bins, touching phones, door handles, etc. and after touching eyes, nose or mouth
Staff should not work or enter food-handling areas if they are suffering from a disease likely to be transmitted through food or carrying a disease that may be transmitted through food, have skin infections, infected wounds or sores, or have diarrhoea
Do not wash raw meat to prevent the spread of bacteria and viruses onto hands, workstations, and utensils
Separate raw and cooked or ready-to-eat foods and use separate chopping boards and utensils for raw foods
Thoroughly clean all equipment and surrounding areas after handling raw poultry products
Do not use raw or undercooked eggs from outbreak areas that will not be subjected to further cooking methods
Use potable water that has been heat-treated and/or disinfected, including for poultry production, since avian influenza may contaminate drinking water sources such as reservoirs and rivers.

Cooking

Poultry meat and products should be cooked to at least 70^oC. Eggs from poultry where outbreaks are occurring should be cooked until the whites and yolks are firm.

Heath Measures for Trade

The World Organisation for Animal Health (OIE) Terrestrial Animal Health Code (Terrestrial Code) specifies safe commodities for trade^30,31.

References

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