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.

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Avian influenza

Pandemic Influenza

Pandemic Influenza

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.

Influenza 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.

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.

H5N1 and the Risk to Human Health

The risk of H5N1 infection of humans is currently very low. Although many thousands of individuals are likely to have been exposed to the virus during poultry outbreaks of the disease, there have been only a small number of cases of human infection recorded since 2003. This shows that the H5N1 virus is not well adapted to causing infection in humans. However, for the few people that have become infected the virus has caused severe and often fatal illness.

Of greater concern is the potential for avian influenza viruses such as H5N1 to mutate into a new human pandemic virus. This would be a new influenza virus which is able to easily infect humans and which can readily spread from person to person.

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.

For more information on the Australian Government’s response to the threat of an influenza pandemic see the Pandemic influenza website.