Avian Influenza

Avian influenza (AI) is a highly contagious viral disease affecting several species of food producing birds (chickens, turkeys, quails, guinea fowl, etc.), as well as pet birds and wild birds. Occasionally mammals, including humans, may contract avian influenza. There are many AI virus strains, which are usually classified into two categories, low pathogenic (LPAI) strains, which typically cause few or no clinical signs in poultry and highly pathogenic (HPAI) strains, which can cause severe clinical signs and potentially high mortality rates among poultry. Wild birds are natural hosts and reservoirs for all types of avian influenza viruses, and play a major role in the evolution, maintenance, and spread of these viruses. Avian influenza has captured the attention of the international community over the years, with outbreaks in poultry having serious consequences on both livelihoods and international trade in many countries. Monitoring and controlling avian influenza at its poultry source are essential to decrease the virus load in susceptible avian species and environment. Implementation of biosecurity measures, in line with OIE international standards, is key in securing the production sector and trade, to safeguard food security and the livelihoods of farmers, and to limit the risk of human infection with avian influenza strains that have zoonotic potential.

What is avian influenza?

AI is a highly contagious viral disease that affects both domestic and wild birds. AI viruses have also been isolated, although less frequently, from mammalian species, including rats, mice, weasels, ferrets, pigs, cats, tigers, dogs and horses, as well as from humans.

Circulation of avian influenza (AI) viruses is not a new phenomenon. There are many descriptions of historical outbreaks of avian influenza disseminating within domestic poultry flocks in the literature. AI occurs worldwide and different strains are more prevalent in certain areas of the world than others.

Economic consequences

Avian influenza outbreaks can lead to devastating consequences for the poultry industry as well as at national level.

Experience has shown that:

  • farmers might experience a high level of mortality in their flocks, with rates often around 50%
  • due to the labour intensive nature of the poultry industry, especially in developing countries, job losses can be significant
  • to contain outbreaks, healthy birds often need to be culled, resulting in risks to animal and human welfare, and concerns regarding protein wastage and economic impacts
  • the presence of HPAI restricts international trade in live birds and poultry meat
  • public opinion may be damaged, reducing both travel and tourism in affected areas.

Transmission and spread

Several factors can contribute to the spread of AI viruses, such as: 

  • globalisation and international trade
  • farming and sale (live bird markets)
  • wild birds and migratory routes.

In birds, AI viruses are shed in the faeces and respiratory secretions. They can all be spread through direct contact with secretions from infected birds, especially through faeces or through contaminated feed and water. Because of the resistant nature of AI viruses, including their ability to survive for long periods when temperatures are low, they can also be carried on farm equipment and spread easily from farm to farm.

Wild birds normally carry AI viruses in their respiratory or intestinal tracts but they do not usually get sick, which allows them to carry the viruses long distances along their migration flyways.

Wild birds and the global epidemiology of AI viruses

Wild birds are natural hosts and reservoirs for all types of avian influenza viruses, so play a major role in the evolution, maintenance, and spread of these viruses. The main wild species involved are waterfowls, gulls, and shorebirds; however the virus seem to pass easily between different bird species.

The incidence of infection appears to be seasonal, with the highest isolation rate being in juvenile birds in the fall of the year.

Several routes of exposure of wild bird viruses to poultry have been documented or suspected of being the origins of outbreaks. Direct exposure to wild birds is the most likely transmission factor.

Therefore, limiting exposure of poultry to wild birds through confinement rearing and other biosecurity measures provides an opportunity to reduce the risk of introduction of avian influenza virus from wild birds, and consequently is key to decrease the risk of evolution into highly pathogenic forms, exposure and infection of humans, and recombination with human viruses components to form viruses that can not only infect humans but readily transmit among humans.

Public health risk

People who are in close contact with infected birds are at risk for acquiring avian influenza.
While many human cases are limited to conjunctivitis or mild respiratory disease, some viruses tend to cause severe illness.

However, there is no evidence to suggest that the consumption of poultry or eggs fit for human consumption could transmit the AI virus to humans. As a precautionary and regulatory measure, animals that have been culled as a result of measures to control an AI outbreak should not be allowed to enter the human food and animal feed chain, and precautionary measures for the cleaning and cooking process should be respected

Due to ongoing circulation of various strains (e.g. H5N1, H5N2, H5N8, H7N8), outbreaks of avian influenza continue to be a global public health concern. The OIE’s objectives of promoting transparency and understanding of the global animal disease situation continue as a priority of our organisation in the face of this situation, in order to protect public health, and to ensure the safety of world trade in animals and animal products.

More information is available on the World Health Organization website: Influenza at the human-animal interface


The Center for Food Security and Public Health, Iowa State University

Questions and answers

Updated 1 March 2021

What is avian influenza?

Avian influenza (AI) is a viral disease affecting birds, including several species of domestic poultry, as well as pet and wild birds. While AI viruses are highly species-specific, on certain occasions they have crossed the species barrier and have been isolated from mammalian species, including humans.

The many strains of AI viruses can generally be classified into two categories according to the severity of disease in poultry: 

  • low pathogenic AI (LPAI) that typically causes little or no clinical signs in birds
  • highly pathogenic AI (HPAI) that can cause severe clinical signs and possible high mortality rates in birds.

What is the current situation with avian influenza?

Updated information on the current situation of AI is available based on the data reported by countries through the OIE World Animal Health Information System (OIE-WAHIS).

Since August 2020, there has been a substantial increase in the number of AI outbreaks caused by various subtypes, notably the H5N8, reported by many countries in Europe, Asia and recently in Africa, which reflect a  period of heightened risks. This 2020–21 epizootic is marked by the significant involvement and mortality of wild birds. As the risk of  virus introductions via migratory wild birds remains high, countries should stay vigilant, and implement effective surveillance and monitoring measures for early detection and response.

What are the causes of the current wave of avian influenza cases and is it worse than in previous years?

Molecular characterization of the currently circulating AI virus subtypes shows genetic variability from previous years. This might be explained by multiple reassortments with low pathogenic viruses circulating in wild birds. The evolution of the viruses and these recent events need to be closely monitored and further studied in order to assess the risks.

It is most likely that migratory wild birds, followed by local farming practices, have contributed to the dissemination and introduction of the virus into new countries and farms. A similar situation of H5N8 epizootics in many countries associated with wild bird migration occurred in 2016–17.

The H5N8 virus has also reassorted with other wild bird influenza viruses to form new subtypes, which were also reported by countries.

What factors can facilitate the spread of avian influenza?

The dynamic of the spread of influenza viruses is extremely complex and difficult to predict. Several factors can influence it, such as the wild bird migration pattern, unregulated trade, farming systems, biosecurity and immunity status.

During the Northern Hemisphere winter, the wild bird movements may increase, and lower temperatures may facilitate the environmental survival of AI viruses, increasing exposure of infection in poultry. Additionally, the mixing of wild birds from different geographic origins during migration can increase the risk of virus spread and genetic reassortment resulting in changes in viral properties. At local level, as the AI viruses can survive for long periods in the environment, they can be easily transmitted from farm to farm by the movement of infected animals, as well as contaminated boots, vehicles and equipment if the adequate biosecurity measures are not implemented.

Has the COVID-19 pandemic hindered the implementation of the preventive measures for avian influenza?

Sustaining veterinary activities amid the COVID-19 pandemic is essential to avoid the detrimental impacts of animal diseases, which could exacerbate the current sanitary and socio-economic crises.

Despite the challenging context, Veterinary Authorities in the affected countries have responded to contain AI outbreaks in poultry with stamping out measures, heightened surveillance, and recommendations to poultry owners to increase biosecurity.

What is the impact of avian influenza?

AI poses a major threat to animal health and welfare and can lead to devastating consequences at different levels of society.

The disease severely affects the livelihoods of small and large-scale poultry producers and many other actors involved in the supply chain. Farmers might experience a high level of mortality in their flocks, and therefore job losses can also be significant.

Additionally, to contain outbreaks and control the disease, contact and suspected bird populations often need to be culled, threatening food security worldwide.

The presence of HPAI also restricts international trade in live birds and poultry meat, heavily impacting national economies.

What is the risk of avian influenza for human health?

The epidemiology of AI viruses is complex: They can change and evolve by mutation and reassortment, with the emergence of new subtypes causing significant impacts on animal health and production. Some subtypes (not all) can be zoonotic, and therefore pose a threat to human health.

Transmission of AI viruses to humans occurs when there is close contact with infected birds or heavily contaminated environments.

In February 2021, human cases of AI subtype H5N8 were reported by Russian authorities to the World Health Organization (WHO). The OIE/FAO network of experts on animal influenza (OFFLU) analysed the preliminary information of the event and stated that, to date, there is no evidence to suggest that severe human infections or human-to-human transmission of this virus have occurred. This situation likely represents an isolated spillover event of the virus as genetic evidence suggests that these viruses were derived from chickens with no adaptation to humans.

More information is available in the OFFLU statement (February 2021). 

What are the food safety recommendations?

There is no evidence to suggest that the consumption of poultry meat or eggs could transmit the AI virus to humans. However, as a general precautionary measure, animals that have been culled as a result of the implementation of control measures in response to an AI outbreak, including the H5N8 subtype, should not enter the human food and animal feed chain.

What are the key elements to prevent the further spread of avian influenza?

Controlling avian influenza at its animal source is essential to decrease the risk and, consequently, the impact of the disease.

In this context, continued surveillance of avian influenza virus in wild birds and poultry combined with timely generation and dissemination of data are crucial. This early warning system enables the international community to follow the virus evolution and to promptly detect changes in the virus properties, such as introductions, reassortments or genetic mutations, that are relevant for animal and public health.

When cases are detected in animals, control measures should be implemented at the level of the infected farm and within a short radius around the infected premises, in an effort to rapidly contain and eradicate the disease.

More information on these measures is available on the OIE Terrestrial Animal Health Code – Infection with avian influenza viruses.

What prevention measures are recommended at farm level?

It is essential for poultry farmers to maintain biosecurity practices to prevent the introduction of the virus. Some of these measures include:

  • prevent contact between poultry and wild birds
  • minimise movements around poultry enclosures
  • maintain strict control over access to flocks by vehicles, people and equipment
  • clean and disinfect animal housing and equipment
  • avoid the introduction of birds of unknown disease status
  • report any suspicious case (dead or alive) to the veterinary authorities
  • ensure appropriate disposal of manure, litter and dead animals
  • vaccinate animals, where appropriate.

What is the World Organisation for Animal Health (OIE) doing to tackle avian influenza?

As the leading world organisation on animal health, the OIE works with its OFFLU network of experts on animal influenzas, as well as with its partners, notably the World Health Organization (WHO) and Food and Agriculture Organization (FAO), to assess the risks of AI viruses and provide the needed guidance and recommendations.

To support countries in the fight against this disease, the OIE developed international standards on AI, which provide the framework for the implementation of effective surveillance and control measures. These standards follow a responsive, science-based and transparent process and are published in the OIE Terrestrial Animal Health Code, after being adopted by our Members.

Additionally, the OIE World Animal Health Information System (OIE-WAHIS) provides a window on the disease situation worldwide. Through its online platform and a mobile application, the system disseminates information about AI outbreaks and sends alerts on events in real time. This allows the international community to follow the evolution of the virus and, therefore, to implement appropriate and timely responses.

Global collaboration

The OIE works closely with the Food and Agriculture Organization of the United Nations (FAO), the World Health Organization (WHO), and others to develop international recommendations aimed at greater intersectoral collaboration and global implementation of the most appropriate strategies.

OFFLU: OIE/FAO global network of expertise on animal influenza

Visit the OFFLU website here.

OFFLU is the OIE/FAO global network of expertise on animal influenza working to reduce the negative impacts of animal influenza viruses by promoting effective collaboration between animal health experts and the human health sector. Originally launched in April 2005 to support avian influenza, OFFLU was expanded in 2009 to include all animal influenza and to further support to Veterinary Services in their efforts to reduce risks to animals and the public from animal influenza viruses.

The objectives of OFFLU are to:

  • exchange scientific data and biological materials (including virus strains) within the network, and to share such information with the wider scientific community
  • offer technical advice and veterinary expertise to Member Countries to assist in the prevention, diagnosis, surveillance and control of avian influenza
  • collaborate with the WHO to contribute to the early preparation of human vaccines
  • highlight avian influenza research needs, promote their development and ensure co-ordination.

Tripartite activities

The worldwide spread of H5N1 avian influenza at the beginning of the 2000s, with its host of economic and health consequences, intensified the joint work of the FAO, OIE and WHO (the Tripartite). Since then, the three organisations regularly exchange follow-up information on the global animal influenza situation, as one of their priority topics.

OIE international standards and networks

The OIE provides science-based standards, guidelines and recommendations for the control of the disease in animals and to prevent its spread as well as standards for the diagnosis of avian influenza and the production of high quality veterinary vaccines.

The science-based standards, guidelines and recommendations issued by the OIE are designated as the international reference in dealing with avian influenza.

The OIE’s scientific standards are updated regularly, and include:

  • OIE Terrestrial Animal Health Code Chapter 10.4 ‘Infection with avian influenza viruses’
  • OIE Terrestrial Animal Health Manual Chapter 3.3.4 ‘Avian influenza (infection with avian influenza viruses)

OIE global scientific network

Through its global network of more than 300 Reference Laboratories and Collaborating Centres (collectively, ‘Reference Centres‘) the OIE provides policy advice, strategy design and technical assistance for the diagnosis and control of avian influenza.

The OIE’s specialist avian influenza Reference Laboratories across the world are centres of expertise and standardisation of diagnostic methods. OIE reference experts – internationally renowned researchers – are committed to enabling the Reference Laboratories to provide the required technical and scientific expertise and to form opinions regarding the monitoring, control, and eradication of these viruses.

They also propose scientific and technical training for OIE Members and coordinate scientific and technical studies in collaboration with other laboratories and organisations.

OIE situation reports for avian influenza

The OIE draws on its network of expertise to produce analyses of the current global situation for avian influenza. The production frequency of these situation reports is largely driven by the number and severity of notifications for avian influenza received in OIE-WAHIS.

The documents briefly present the key risks driving current events – how the strains are interacting with hosts (both wild birds and poultry, and sometimes humans) and the environment (season and climate, livestock husbandry systems, ecosystems) – and how the events may evolve in the months ahead.

2021 reports
2020 reports
2019 reports


Prevention and control

Monitoring and controlling avian influenza at its poultry source is essential to decrease the virus load in susceptible avian species and environment.

Implementation of biosecurity measures, in line with OIE international standards, is key to securing the production sector and trade, to safeguard food security and the livelihoods of farmers (in particular in developing countries) as well as to limit the risk of human infection with avian influenza strains that have zoonotic potential.

Surveillance and reporting

The first line of defense against avian influenza is the early detection of disease outbreaks followed by a rapid response. This is strongly linked to a high level of awareness among veterinarians and animal owners, and high quality Veterinary Services. Putting in place accurate warning systems as well as prevention measures is essential as part of an effective strategy to prevent and control avian influenza. This needs to be coupled with similar efforts placed on preparing for a potential outbreak.

Avian influenza is a notifiable disease listed by the OIE. As detailed by the OIE Terrestrial Animal Health Code, Members must report:

  • all highly pathogenic avian influenza viruses, irrespective of their strain, detected in birds (domestic and wild)
  • all low pathogenic viruses of subtypes H5 and H7 detected in poultry.

Unusual mortality among wild birds should also be reported to the OIE through its World Animal Health Information System (WAHIS).

Prevention at animal source

Because of the stability of the virus in the environment and highly contagious nature, strict biosecurity measures and good hygiene are essential in protecting against disease outbreaks.

Measures include:

  • keep poultry away from areas frequented by wild fowl
  • do not keep on the premises elements that may attract wild birds, including poultry feed products placed outside the building
  • maintain strict control over access to flocks by vehicles, people and equipment
  • ensure the sanitation of property, poultry houses and equipment
  • avoid the introduction of birds of unknown disease status into the flock
  • report any bird illnesses and deaths to the Veterinary Services
  • ensure appropriate disposal of manure, litter and dead poultry
  • vaccinate animals where appropriate.

Control strategies and compensation

If the infection is detected in animals, a policy of culling infected and contact animas is normally used in an effort to rapidly contain, control and eradicate the disease.

Requirements include (and are described in the OIE Terrestrial Animal Health Code):

  • humane destruction of all infected and exposed animals (according to OIE animal welfare standards)
  • appropriate disposal of carcasses, litter and all animal products
  • surveillance and tracing of potentially infected or exposed poultry
  • strict quarantine and controls on movement of poultry and any potentially contaminated vehicles and personnel
  • thorough cleaning and decontamination of infected premises
  • a period of at least 21 days before restocking.

When outbreaks are detected, stamping out is generally applied at the level of the infected farm or within a short radius around the infected premises in conjunction with active surveillance.

Controlled elimination of infected poultry, movement restrictions, improved hygiene and biosecurity, and appropriate surveillance should result in a significant decrease of viral contamination of the environment. These measures should be taken whether or not vaccination is part of the overall strategy.

Systems of financial compensation of farmers and producers who have lost their animals as a result of mandatory culling ordered by national authorities vary around the world; unfortunately they may not exist at all in some countries. The OIE encourages national authorities to develop and propose compensation schemes because they are a key incentive to support early detection and transparent reporting of animal disease occurrences, including avian influenza.

Should vaccination be used?

It is important that vaccination alone is not considered the solution to the control of avian influenza if eradication is the desired result. Without the application of monitoring systems, strict biosecurity and depopulation in the face of infection, there is the possibility that these viruses could become endemic in vaccinated poultry populations. Long-term circulation of the virus in a vaccinated population may result in both antigenic and genetic changes in the virus and this has been reported to have occurred in several countries.

Vaccination should be implemented for a limited duration when culling policies cannot be applied because either the disease has become endemic and therefore widespread, or the infection in affected animals is too difficult to detect.
When appropriate vaccines complying with OIE quality standards are available, vaccination is used to protect susceptible poultry populations from potential infection. Vaccination strategies can be effective as an emergency measure in an outbreak or as a routine measure in an endemic area.

Any decision to use vaccination must include an exit strategy, i.e. conditions to be met to in order to stop vaccination.

Media resources

The communication tools developed by the OIE are freely accessible and available to everyone for downloading and distribution.

Informative documents


  • Scientific and Technical Review 28(1) Avian influenza
  • Bulletin 2006-1 Avian influenza: International community takes action
  • Bulletin 2006-4 Transparency on avian influenza
  • Report, Agriculture and Rural Development (World Bank), 2006: Enhancing control of highly pathogenic avian influenza in developing countries through compensation: issues and good practice
  • Vaccination: A tool for the control of avian influenza (OIE)

Virus information


Influenza viruses are grouped into three types; A, B, and C. Only type A is known to infect animals and is zoonotic, meaning it can infect animals and also humans. Types B and C mostly infect humans and typically cause mild disease.

Avian influenza viruses are extremely variable and are widespread among birds where they all belong to the Orthomyxoviridae family and are placed in the genus influenzavirus A.

Influenza A viruses are classified into subtypes based on two surface proteins, the hemagglutinin (HA) and neuraminidase (NA). For example, a virus that has HA 7 protein and NA 9 protein is designated as subtype H7N9.

At least 16 hemagglutinins (H1 to H16), and 9 neuraminidases (N1 to N9) subtypes have been found in viruses from birds, while two additional HA and NA types have been identified, to date, only in bats.

AI virus strains are usually classified into two categories according to the severity of the disease in poultry:

  • Low pathogenic (LPAI) strains, which typically cause few or no clinical signs in poultry, and may go undetected due to the lack of symptoms in some species of birds.
  • Highly pathogenic (HPAI) strains, which can cause severe clinical signs and potentially high mortality rates among poultry.

To date, naturally occurring highly pathogenic influenza A viruses that produce acute clinical disease in chickens, turkeys and other birds of economic importance have been associated only with the H5 and H7 subtypes.

Molecular epidemiology and characterisation of the genotypes of AI virus in poultry and wild birds is important to understand the distribution of different viral strains in various hosts. National and OIE Reference Laboratories use molecular diagnostic techniques in the surveillance and detection of highly pathogenic strains and emergence of novel subtypes from unknown hosts or hosts that have not been previously reported.

Strain differentiation, mutation, and reassortment

Differentiation between low and high pathogenicity AI viruses is based on the results of laboratory tests, which are described in the OIE Manual of Diagnostic Tests and Vaccines for Terrestrial Animals. This characterisation of AI viruses as low or high pathogenicity is specific to poultry and other birds and does not necessarily apply to other species that can be susceptible to AI viruses, including humans.

The viral HA, and to a lesser extent the NA, are major targets for the immune response. Influenza A viruses are very diverse, and two viruses that share a subtype may be only distantly related. The high variability is the result of two processes: mutation and genetic reassortment. Once these proteins have changed enough, immune responses against the former HA and NA may no longer be protective and this can provide the virus with the ability to rapidly adapt to new hosts.

Consequently, the ‘avian influenza’ reportable to the OIE is defined as an infection of poultry and other birds, including wild birds, caused by any influenza A virus with high pathogenicity (HPAI), or by all influenza A viruses of H5 and H7 subtypes with low pathogenicity when detected in poultry.

The reason that all H5 and H7 subtypes are reportable when detected in poultry is because there is a risk for them to become highly pathogenic by mutation.