Types of influenza

There are three main types of influenza virus: A, B and C.

Type A strains of influenza virus can cause severe illness and are the only type to have caused human pandemics. The H5N1 strain is a type A influenza virus.
Type B strains cause sporadic human cases and small-scale outbreaks.
Type C strains only rarely cause human infection and have not caused large 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 causing winter influenza 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.

How is pandemic influenza different to seasonal influenza ('the flu') and avian influenza ('bird flu')?

Seasonal influenza

Influenza, or 'the flu', is a viral infection of the nose, throat and lungs (the respiratory system). In Australia it usually affects people during the winter months from June to September. The flu viruses that circulate every winter are often similar to those from the preceding winter, so there is already a level of immunity (body defences) in the community. Seasonal flu most commonly affects the very young or the elderly.

Bird flu

Avian influenza - also known as 'bird flu' - is used to describe influenza virus A subtypes that primarily affect birds. Strains that cause a high proportion of deaths in affected flocks are called 'highly pathogenic avian influenza'. One of these strains is H5N1

The H5N1 strain of bird flu is presently causing disease in water birds and domestic poultry such as chickens, geese and ducks in many countries across Asia, Europe and Africa. While there is no evidence that the H5N1 strain of bird flu is in Australia, it is possible that H5N1 could arrive in migratory birds. The risk of H5N1 infecting Australian poultry flocks, however, is considered low. Should an outbreak of bird flu occur, Australia has well-tested plans to contain and eradicate the disease. For more information see the AusVet Plan (external link).

Avian influenza H5N1

The H5N1 strain was recognised as a potential human health issue in Hong Kong in 1997, where it caused outbreaks in poultry 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.

H5N1 and the risk to human health

It is currently very difficult for the H5N1 virus to be transmitted from birds to humans - it requires very close contact with sick or dead birds.

Although many thousands of individuals are likely to have been exposed to the virus during outbreaks in poultry of the disease, there have been a small but regular number of human infection cases recorded each year since 2003. This shows that the H5N1 virus is not, at this stage, well adapted to causing infection in humans. However, in cases where it has been transmitted, it has caused severe illness and the death rate has been high. Human cases have almost all been in people who had close contact with infected poultry, usually from their own farms.

The World Health Organization website (external link) provides the latest updates, news and answers to frequently asked questions about human cases of bird flu.

If you are planning overseas travel, see the Australian Government's travel advisory service (external link) to find out if the country you intend travelling to is affected by bird flu.

Pandemic influenza

An influenza pandemic is a disease outbreak that occurs worldwide when:

• a new subtype of influenza virus emerges in humans to which most people have not been previously exposed and are, therefore, highly susceptible
• the virus causes serious disease in humans
• the virus is easily and rapidly spread between humans, infecting large numbers of people worldwide and causing many deaths.

Vaccines can be developed to boost the community's immunity against a pandemic flu strain. But this takes time and cannot be started before the new virus appears. Therefore the virus can spread quickly between humans before a vaccine is available; and can cause disease in humans that is more severe than the usual seasonal influenza.

It is possible the H5N1 virus causing infection in birds could change (mutate) in the future to spread more easily between humans. If this virus gains the ability to transmit between humans efficiently, it could potentially cause an influenza pandemic.

Although the current risk of an influenza pandemic is from the H5N1 virus, it is not the only possible source for a human influenza pandemic. Other influenza A subtypes, such as H7, H9 and H10 have caused human infections and so also pose a threat. There are also other influenza strains, such as swine influenza viruses, that primarily affect other animals and these also have the potential to mutate into new human strains. Authorities are monitoring the pandemic potential of all circulating influenza strains in humans and birds.

This information is current for 09 February, 2010
This information was issued on 05 December, 2008