Avian Influenza
Bird Flu Transmission Routes
Australian commercial poultry flocks are currently free of the bird flu outbreaks which have been associated with a number of human fatalities following outbreaks of the disease in Asia, Europe, Africa and Middle East since the end of 2003 (DAFF, 2006).
In the past there have been five limited outbreaks of high pathogenic avian influenza (HPAI) in Australian poultry. These were all linked to wild bird populations. Other methods of transmissibility include bird-to-human contact and food consumption.
Wild or imported birds
Two hypothesised routes through which bird flu may enter Australia are the arrival of infected migratory birds or through the traffic of birds from countries with outbreaks of bird flu.
Although there are instances of wild birds being infected with bird flu, it has been much debated whether they play an active role in the geographic spread of the disease around the world (Olsen et al., 2006; Thomas, 2005). Some ague that infected birds in the wild would be too sick to migrate and therefore spread the virus (Thomas, 2005). However, experiments have demonstrated that some wild bird can carry bird flu without showing apparent signs of the disease (Olsen et al., 2006). This issue of migratory wild birds as an infection route for bird flu remains unclear and further research is required to clarify the precise risk (Capua & Alexander, 2006; Olsen et al. 2006; Tracey et al., 2004).
Australia’s unique geography and ecology does, however, present a number of barriers to the arrival of bird flu that are not found in countries that have experienced recent outbreaks. Firstly, the prevalence of the low pathogenic form of avian influenza virus in wild birds surveyed in Australia is much lower than for many other countries (Tracey et al 2004). Secondly, Australia is out of the flight path of most migratory birds known to carry bird flu (DAFF, 2006). Those birds which do migrate here mainly aggregate along the coast or at specific inland wetlands (Tracey et al., 2004) and are unlikely to come into contact with commercial poultry.
While Australian authorities continue to maintain active surveillance for evidence of bird flu in wild birds in northern Australia (DAFF, 2006), the most effective strategy remains minimising opportunities for contact between wild birds and commercial poultry (Olsen et al 2006; Tracey 2004).
Human-to-human
There have been at least 258 confirmed cases of bird flu infection in humans since 1997, 153 of which have been fatal (WHO, 2006a). The majority of these fatalities have occurred in Southeast Asia (Indonesia, Thailand, Vietnam), with others occurring in China, Turkey, and in parts of the Middle East and Africa (WHO, 2006b).
While some strains of avian influenza are highly pathogenic, it important to remember that bird flu is a bird disease. The virus displays a number of characteristics that make it difficult to pass from birds to humans (Kuiken et al., 2006). Considering the billions of direct interactions between chickens and humans during and since the recent outbreaks – particularly in developing countries – the risk of humans becoming infected with bird flu remains extremely low (EFSA, 2006; Thomas, 2005).
The majority of cases of human infection have occurred following close and intensive contact with infected live, dying or dead chickens and usually involved high risk practices such as hand slaughtering, de-feathering, butchering and preparing infected birds for consumption (EFSA, 2006). Bodily fluids of infected birds are considered to be the main sources of viral infection in humans. No risks of human infection have been identified in relation to either travel within infected countries or consumption of poultry products (EFSA, 2006).
Food consumption
There is no epidemiological evidence to suggest that bird flu can be transmitted to humans through correctly prepared food (EFSA, 2006) despite experiments that have shown the virus can survive in the edible products of infected chickens, ducks and geese (EFSA, 2006). The major points of entry for human infection are considered to be the upper respiratory tract or the conjunctival mucosae, and currently there is insufficient evidence to suggest that the virus can replicate in the gastrointestinal tract as a result of consuming infected food (EFSA, 2006).
In addition to this, conventional cooking processes have been shown to inactivate bird flu virus (EFSA, 2006). Conventional cooking, in respect of poultry meat, means temperatures of 70°C or greater in all parts of the food (FAO, 2005).
As with other forms of poultry products, there is no epidemiological evidence to suggest that people have been, or can be infected with bird flu through the consumption of eggs or egg products (EFSA, 2006; FAO, 2005; WHO, 2006b).
In the event of a clinical outbreak of bird flu this would be readily identified on farm by signs of ill health in the birds and reduced egg production. Eggs from these birds would be identified and isolated. In the event that a small number of eggs were released for sale to retail outlets Quality Assurance systems on Australian commercial layer farms, such as Egg Corp Assured, would enable the immediate traceability of the destination of such eggs.
There are a number of factors that would make it highly unlikely for humans to become infected as a result of consuming eggs from infected birds. The acute course of bird flu infection results in a sudden egg drop and rapid mortality in laying chickens, minimising the risk that infected eggs enter the food chain (EFSA, 2006). As mentioned earlier, there is also no evidence that bird flu can infect humans via the gastrointestinal tract.
There is inconsistent data about the transmission of bird flue virus avian influenza to eggs. While experimental trials have demonstrated that the bird flu virus can be passed to the shell or inside of eggs of infected birds, field studies suggest that this mode of transmission does not occur for all forms of the virus (ESFA, 2006). Eggs that are dirty or have cracked shells represent the greatest risk factor for the transmission of the virus from an infected bird to an egg. Correct handling and processing techniques, however, will eliminate this risk (ESFA, 2006).
Cooking food well has been shown to completely inactivate the virus (EFSA, 2006). Well cooked eggs means that neither yolk nor egg white is runny. In relation to commercial food preparation, there are specific methodologies for the pasteurisation of liquid egg products, which also inactivates the virus (FAO, 2005).
As the virus is known to exist in the faeces of infected birds it has been suggested that egg containers and other equipment that come into contact with infected bird faeces may potentially carry the virus (Capua & Alexander, 2006). While standard food safety protocols should ensure that eggs shells and egg cartons are clean of faeces, hand washing and proper food handling will eliminate the risk of this kind of contamination.
