Avian influenza (Bird Flu) - key facts
This fact sheet provides general information about influenza viruses, avian influenza (bird flu) and information about one type of bird flu, called avian influenza A (H5N1), that has caused infections in birds and in humans.
Avian influenza refers to influenza A viruses found mainly in birds but which can occasionally cause infections in humans. There are two ways in which human health is put at risk by avian influenza viruses:
- Firstly, by their ability to occasionally cause infections in humans, and
- Secondly, and more importantly, through the potential for the emergence of new human pandemic strains either directly from avian viruses, or from their recombination with human or other animal viruses.
There are three main types of influenza virus: A, B and C.
- Type A strains of influenza virus can cause more severe illness and are the only types to have caused human pandemics. The H5N1 strain is a type A influenza virus.
- Type B strains cause sporadic human cases and occasionally cause small-scale outbreaks.
- Type C strains only rarely cause human infection and have not caused outbreaks.
Of the influenza A viruses, only subtypes H1, H2 and H3 have been transmitted easily between humans. Only the H1 and H3 subtypes are currently circulating in humans.
New influenza subtypes can occasionally emerge with the ability to cause infections within a particular animal species for the first time. Human pandemics result when a new influenza A virus emerges with the capacity to efficiently infect and spread between humans.
New influenza subtypes can emerge in one of two ways:
- When an animal is infected with two different influenza strains at the same time, the genetic material of the viruses may mix (reassortment) to produce a new strain which has some of the characteristics of both strains.
- When an influenza virus undergoes gradual mutation which gives it the capacity to infect a completely different animal species (adaptive mutation).
The 1918 influenza pandemic strain (H1N1) appears to have been as a result of adaptive mutation of an avian influenza A virus. The pandemic strains of 1957-58 (H2N2) and 1968-69 (H3N2) both probably involved reassortment events between avian and human influenza A strains.
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Avian influenza viruses
The term 'avian influenza' is used to describe influenza A subtypes that primarily affect poultry, migratory waterfowl, and other bird species.
Wild aquatic birds are the major reservoir for influenza A viruses but generally do not develop severe disease. Domestic poultry can contract disease from wild birds and are susceptible to severe and potentially fatal influenza infections. The virus spreads through bird faeces and contaminated water or dust.
Outbreaks of avian influenza have been recognised in poultry flocks in most countries of the world for many years. Strains that cause a high proportion of deaths in affected flocks are called highly pathogenic avian influenza (HPAI).
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Avian Influenza H5N1
The World Health Organization
(WHO) has reported an outbreak of a highly pathogenic avian influenza (HPAI) in birds affecting a number of countries in Asia, the Middle East, Europe and Africa. The HPAI strain involved in the current outbreak is called H5N1.
The H5N1 strain was first identified in 1997 in Hong Kong, where it caused poultry outbreaks and led to infections in 18 people with 6 deaths. Fortunately, the outbreak was halted in Hong Kong by strict control measures in the poultry industry.
The strains of H5N1 that have emerged in Asia since 2003 are slightly different to the 1997 H5N1 strain. These strains have now spread to many parts of the world by migratory birds and possibly through trade in poultry. Some countries have reported only isolated cases in wild birds while other countries have had extensive outbreaks in commercial and backyard poultry.
Although the H5N1 virus can cause severe and sometimes fatal infections in humans, the actual number of human cases around the world has been small relative to the number of outbreaks in birds. Human cases have almost all been in people who had close contact with infected poultry, usually from their own farms.
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H5N1 and the Risk to Human Health
There have been reports of a few rare cases of human to human infection with H5N1 but the virus found has been shown to be identical to the virus caught from birds. This means that the bird flu virus has not mutated to a human-human spread pandemic virus.
Pandemic influenza is caused by a subtype of the influenza A virus that has not circulated in the human population for sometime, and is capable of causing severe disease and spreading easily from human to human. To do this the virus must change from one that only occasionally infects humans to one that is easily spread between people. There is concern that the H5N1 virus may make this change.
It is also important to note that the H5N1 strain is not the only possible source for a human influenza pandemic. Other influenza A subtypes, such as H7 and H9, have caused human infections and so, also pose a threat. There are also other influenza strains that primarily affect other animals, such as swine influenza viruses, and these also have the potential to mutate into new human strains.
There has been no evidence of effective human-to-human spread of H5N1 infection, and so no evidence that a new influenza pandemic is starting. Nevertheless, the Department of Health and Ageing is continuing to closely monitor the avian influenza situation.
This information was issued on 05 December, 2008