Questions and answers on avian influenza

What is avian influenza (bird flu)?

Influenza is a large family of different viruses, some of which affect humans and many of which affect animals and especially birds.

Avian influenza, or bird flu, is a common term used to refer to the many types of influenza viruses that usually infect birds exclusively. Many different bird species are susceptible to avian influenza. Waterfowl represents a reservoir for avian influenza, mostly carrying the virus without showing any symptoms. Migratory waterfowl can distribute these viruses across large geographical regions in the course of autumnal and spring migrations.

The introduction of avian influenza viruses into poultry farms occurs most likely through direct contact with wild birds, or indirect exposure e.g. through contaminated material. The incubation period can vary between several hours to a week, and is usually estimated to be around five days.

Flu in birds is quite common. Most strains of bird flu are relatively harmless to their natural hosts, and do not infect humans. Viruses that cause only minor or no disease in chickens and do not have multiple basic amino acids at the haemagglutinin (HA) cleavage site, are called low pathogenic avian influenza viruses (LPAIV). However, avian influenza viruses that are low pathogenic to poultry can be transmitted to humans causing severe disease.

Infection with low pathogenic avian influenza virus (LPAIV) is defined in the Commission delegated regulation (EU) 2020/689 as ‘any influenza A virus of H5 or H7 subtypes that are not HPAI viruses’. 

One such example is the A(H7N9) avian influenza virus in China, that was identified in 2013. This subtype presented as asymptomatic (caused no obvious symptoms) or presented only as mildly symptomatic in poultry, while transmission to humans (as observed between 2013 and 2019) caused severe disease with fatal outcomes.

LPAIV can mutate into highly pathogenic avian influenza virus (HPAIV), which is an extremely infectious form that leads to a systemic infection causing high mortality in poultry or other captive or wild birds. HPAIV outbreaks are characterised by a sudden onset of high mortality in chicken or turkey farms.

In the case definition for HPAIV, the disease agent responsible for HPAI must be either:

(a), an influenza A virus of H5 and H7 subtypes or any influenza A virus with an intravenous pathogenicity index (IVPI) greater than 1,2;


(b), an influenza A virus of H5 and H7 subtypes with a sequence of multiple basic amino acids present at the cleavage site of the haemagglutinin molecule (HA0) that is similar to that observed for other HPAI isolates.’ [Commission delegated regulation (EU) 2020/689]. Highly pathogenic viruses cause outbreaks in poultry farms and other commercial bird populations with high morbidity and mortality rates in affected poultry, but high morbidity and mortality was also observed in wild birds. Examples of such avian influenzas are A(H5N1) and A(H7N7).

Importantly, the term ‘highly pathogenic’ is not related to the disease in humans. Some of these avian viruses do not cause disease in humans, or are known to only cause mild disease when transmitted to an exposed person. Some, however, are known to cause severe disease in humans, for example, A(H5N1) or A(H5N6).

Any detection of avian influenza viruses of the H5 and H7 subtypes in poultry holdings needs to be notified and precautionary measures should be implemented to prevent potential avian-to-human infection; see Regulation (EU) 2016/429.

Why are we concerned about bird flu outbreaks?

Bird flu viruses represent two types of risks for humans:

  • There is a risk that avian influenza viruses may transmit from birds to humans and result in severe human disease. The risk of transmission is higher in areas where people and domestic birds reside closely together, or for occupational exposure to infected birds e.g. during culling operations.  
  • Influenza viruses evolve and can increase the risk of human transmission either through acquiring mutations within the genome that confer to mammalian adaptation or also through the exchange of genome segments between different viral subtypes from different species (reassortment). Both situations could lead to the generation of new pandemic strains that are transmissible to and among humans.

What are the control measures in birds and animals?

Avian influenza monitoring systems (active or passive) have been designed to identify outbreaks of highly pathogenic avian influenza of the H5 or H7 subtype. Outbreak detection requires the immediate implementation of control measures e.g. culling to avoid further spread according to Regulation (EU) 2016/429. Restriction and surveillance zones around the affected premises are immediately implemented to reduce the risk of further spread and investigations into the source of the outbreak are initiated.

How does avian influenza spread?

Avian influenza viruses are spread through local or regional wild bird movements as well as via bird migration over longer distances. Introduction of the avian influenza viruses can happen through wild birds, but also through illegal trade of birds and bird products.

Avian influenza viruses can be transmitted directly from wild birds to domestic poultry or indirectly e.g. through contaminated material. The virus spreads directly from bird to bird via airborne transmission or indirectly, through faecal contamination of material, feathers or feed. Large amounts of virus are secreted in bird droppings, contaminating the soil and water supply. Large gatherings of various wild bird species on lakes create an environment where reassortment of different avian influenza viruses can occur. Viruses can also spread between the animals and then further on through migratory routes to new areas.

Contaminated equipment, vehicles, feed, cages or clothing - especially shoes - can spread the virus in between farms. Furthermore, there is a possibility of contaminated dust particles spreading via wind from one farm to another, in close proximity. The virus can also be mechanically carried by other animals, such as rodents. In Asia, so called ‘wet’ markets or live bird markets, where live birds are sold, can be another source of spread and mixing of different viruses between bird species.

How are humans infected by avian viruses?

Humans are usually infected through close contact with infected birds, bodily fluid droplets, such as those generated during defeathering processes, or other contaminated material. Birds shed influenza viruses in their faeces and therefore contact with bird droppings is also a possible transmission route. The upper respiratory tract and the eyes can serve as an entry point for viruses to infect people. Wearing goggles and face masks reduce the risk of infection.

How easy is it to kill avian influenza viruses? Will cooking destroy them?

Avian influenza viruses are killed by heat and common alcohol-based disinfectants. Heat treatment, such as cooking, will destroy the viruses. Ultraviolet (UV) light can also inactivate the viruses.

How severe is the illness caused by avian influenza viruses?

Most avian influenza viruses do not cause disease in humans, or cause only mild illness, such as fever or conjunctivitis. A few avian influenza viruses are known to cause severe disease with mortality in humans, notably, A(H5N1), A(H5N6), and A(H7N9) with mortality rates of up to 50%. Sporadic human cases infected with other avian influenza viruses such as A(H6N1), A(H7N2), A(H7N3), A(H7N4), A(H7N7), A(H9N2), A(H10N7) or A(H10N8) have been reported with varying severity.

The emergence of different subtypes over time and ongoing evolution processes challenge the estimation of more general mortality rates.

What are the protective measures against avian influenza?

The best protective measure against avian influenza is to avoid direct contact with dead and potentially infected wild birds or poultry. Sites where deaths in wild birds have been observed should not be accessed. Transmission to other wild mammals such as foxes, seals, etc. has also been reported which reemphasises the general recommendation to not touch any sick or dead animals. Places with large concentrations of birds should be avoided, especially in affected areas.

When exposed to potentially infected, dead wild birds (or animals) and particularly when occupationally exposed to avian influenza during culling and cleaning operations in outbreaks, personal protective equipment should be used to minimise direct exposure.

Are any drugs available for prevention (prophylaxis) and treatment against avian influenza?

Antiviral drugs such as oseltamivir and zanamivir are considered effective against several avian influenza viruses. Baloxavir marboxil is also licenced for use against influenza in the EU.

Is there a vaccine against avian influenza in humans?

There is no single vaccine against avian influenza. A specific vaccine is needed for each specific avian influenza strain, and they need to be adapted as the virus continues to change. During the Vaccine Composition Meetings at the World Health Organization (WHO) twice a year, proposed candidate vaccine viruses belonging to different phylogenetic clades for pandemic preparedness are reviewed and updated. Such viruses are not available for ad hoc use, but are prepared for emergency situations.

Page last updated 9 Feb 2023