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 other 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. These strains are called low pathogenic avian influenza. However, some avian influenza viruses that are low pathogenic to birds can be transmitted to human causing severe disease. An example of such influenza virus is the A(H7N9) avian influenza that was identified in 2013 in China which is asymptomatic (causes no obvious symptoms) or shows only mild symptoms in birds while transmission to human causes severe disease with sometimes fatal outcome.
There are other groups of avian flu viruses that are referred to as highly pathogenic. 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(H5N1) and 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).
Any detection of avian influenza viruses of the H5 and H7 subtype in poultry holdings during regular surveillance 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.
- The variability of the influenza virus genome and possibility that different avian virus subtypes exchange (reassort) their genetic material could lead to the generation of new pandemic strains that are transmissible among humans.
What are the control measures in birds and animals?
An established avian surveillance system has been designed to be able to identify an outbreak of avian influenza of the H5 or H7 subtype. This initiates measures to control the outbreak and 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 do outbreaks of 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 footwear or feed. The so called "wet" markets or live bird markets, found especially in Asia, where live birds are sold can be another source of spread and mixing of different viruses between avian species. 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 virus can also be mechanically carried forward by other animals, such as rodents.
How are humans infected by avian viruses?
Humans are usually infected through close contact with infected birds. 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. Thorough cooking of any poultry meat 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) and A(H7N9).
What are the protective measures against avian influenza?
The best protective measure is to avoid direct contact with live poultry. Visits to live bird markets and similar places with large concentrations of birds should be avoided, especially in affected areas of Asia.
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. WHO has suggested candidate vaccine viruses for particular avian reference viruses belonging to different phylogenetic clades for pandemic preparedness.