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 exclusively infect birds.

Flu in birds is quite common. Most strains of bird flu are relatively harmless to their natural bird hosts and do not infect humans. Viruses that cause only little or no disease in chickens, infected experimentally and without multiple basic amino acids at the HA0 cleavage, are called low pathogenic avian influenza viruses (LPAIV). However, avian influenza viruses that are low pathogenic to poultry can be transmitted to human causing severe disease. One such example is A(H7N9) avian influenza, that was identified in 2013 in China, which is asymptomatic (causes no obvious symptoms) or shows only mild symptoms in poultry while transmission to human causes severe disease with a sometimes fatal outcome.

LPAIV can mutate into highly pathogenic avian influenza virus (HPAIV), which is an extremely infectious, systemic disease that produces high mortality in poultry. HPAI show a sudden onset of high mortality rates in chickens or turkeys.

The definition for HPAIV is outlined in the COUNCIL DIRECTIVE 2005/94/EC:

(a) avian influenza viruses of the subtypes H5 or H7 with genome sequences codifying for multiple basic amino acids at the cleavage site of the haemagglutinin molecule similar to that observed for other HPAI viruses, indicating that the haemagglutinin molecule can be cleaved by a host ubiquitous protease;


(b) avian influenza viruses with an intravenous pathogenicity index in six-week old chickens greater than 1.2;

Those viruses cause outbreaks in poultry farms and other commercial bird populations with high morbidity and mortality rates in affected poultry. Examples of such avian influenzas are A(H5N8), A(H7N7) or quite a few others. 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. 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 subtype in poultry holdings need to be notified and precautionary measures are applied to prevent potential avian-to-human infection.

Why are we concerned about bird flu outbreaks?

Bird flu viruses represent two types of risks for humans:

- The risk that avian influenza virus 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 professions exposed 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 mammal adaptation or through the exchange of genome segments between different viral subtypes also 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 to avoid further spread. The affected birds/poultry are culled and safely destroyed. Restriction and surveillance zones around the affected holding 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 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 soil and water supply. Contaminated equipment, vehicles, feed, cages or clothing - especially shoes - can spread the virus in between farms. The possibility of spread of contaminated dust particles via wind from one farm to another, in close proximity, is discussed. 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 virus in their faeces and therefore contact with bird droppings is also a possible transmission route.

How easy is it to kill the virus? Will cooking destroy it?

Avian flu virus is killed by heat and common disinfectants. Heat treatment such as cooking will destroy the virus.

How severe is the illness caused by avian influenza?

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 rate 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.

What are the protective measures against avian influenza?

The best protective measure is to avoid direct contact with dead and potentially infected wild birds or poultry. Visits to live bird markets and similar places with large concentrations of birds should be avoided, especially in affected areas. If occupationally exposed, personal protective equipment should be used to minimise direct exposure.

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

Antiviral drugs such as oseltamivir and zanamivir are considered effective against several avian influenza viruses. 

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 WHO twice a year, proposed candidate vaccine viruses belonging to different phylogenetic clades for pandemic preparedness are reviewed and updated.