Avian Influenza (Bird Flu)

AN OVERVIEW OF AVIAN INFLUENZA

Avian influenza (AI) is an infectious disease of birds caused by type A strains of the influenza virus.

There are 16 sub-types of AI virus, some of which cause severe mortalities in birds. The disease occurs worldwide but, at present, a particular strain of AI called H5N1 is causing great concern as it is spreading globally. This strain was first discovered in Hong Kong in 1997 and has now spread further in Asia (nine countries have reported outbreaks) and to Europe and Africa, resulting in the death or destruction of over 150 million birds. As well, there have been over 240 human cases, with over 120 deaths, almost all in people in close contact with sick or dying birds or their tissues. In its present form, this virus does not jump easily from birds to humans nor spread readily from human to human, but this situation could change if the virus were to mutate.

Two forms of the disease

While all birds are thought to be susceptible to infection with AI viruses, many wild bird species carry these viruses with no apparent signs of illness. In poultry, AI viruses cause two distinctly different forms of disease - one common and mild, the other rare but with high mortality. In the mild form, non-specific signs of illness may be seen (such as ruffled feathers, mild respiratory disease and a drop in egg production). Outbreaks can be so mild that they escape detection. In contrast, the highly pathogenic form of AI is characterised by a sudden onset of severe disease, rapid spread within a flock, and a mortality rate that can approach 100% within 48 hours. In this form of the disease, the virus not only affects the respiratory tract, as in the mild form, but also invades multiple organs and tissues.

How the disease is spread

Wild waterbirds can be an important vector for the international spread of AI viruses. It is likely that migratory waterbirds were responsible for the spread of the H5N1 virus to Eastern Europe, and then to other European countries and to Africa. Australia has suffered five outbreaks of AI over the past 30 years (the latest in 1997), probably due to migratory waterbirds coming into contact with domestic poultry, but the disease has been eradicated in each case. Countries that lie along the flight pathways of birds migrating from central Asia may face a persistent risk of introduction or re-introduction of the virus to domestic poultry flocks but few if any migratory ducks and geese fly to Australia. Another route for the introduction of the virus could be via smuggled birds or poultry products.

Impact of the disease on humans

The virus is relatively easily killed by cooking and even in countries in which the disease is endemic, it is safe to eat cooked eggs and poultry meat. As Australia is free of the disease and we do not import fresh eggs or raw poultry meat, there are no AI risks from eating poultry products. As well, all live imported birds are thoroughly screened for AI.

Humans are an 'accidental' host for the AI virus. Although the consequences of a human pandemic should not be under-estimated, Australia has appropriate plans in place and it is important that the actual risks be kept in perspective.

What action is being taken in Australia

The Australian Government has increased surveillance at airports, seaports and mailing facilities. All quarantine inspectors are on heightened alert for any live birds and poultry products, especially from countries in which AI has been reported. The Australian poultry industry is well aware of the serious consequences of an AI outbreak, and biosecurity plans have been implemented at most commercial enterprises. Finally, there has been intense activity by health and veterinary authorities at international, national and state/territory levels, to ensure that Australia is well-prepared to deal with a possible outbreak.

Australia has a series of well developed plans that describe the proposed response to an outbreak of AI. These Australian Veterinary Emergency Plans (AUSVETPLAN) include a manual describing AI that can be downloaded from the following link: Avian Influenza