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Current state of knowledge on Highly Pathogenic Avian Influenza

2.4 Transmission of Virus to Farms and the Risks Associated with Densely-Populated Poultry Areas

Once an avian influenza virus is introduced to a new location, infection may be extinguished unless it gains entry to a farm and is able to spread to other farms and/or markets.

HPAI epidemics in Europe, Canada and Southeast Asia have demonstrated the potential risks and major detrimental effects of HPAI in areas with a high density of poultry (referred to as ‘densely populated poultry areas’ or DPPA [Marangon et al,2004]). These areas are highly susceptible to HPAI due to the fact that a breakdown in a single, large farm can directly and indirectly infect or affect neighbouring farms. In many of these areas, there is considerable movement of vehicles and people from farm to farm (Capua and Alexander, 2004) leading to conditions that facilitate spread of a virus once established. Control measures implemented by veterinary authorities in areas of high poultry density can also contribute to the spread of virus to neighbouring farms, presumably through contaminated dust particles disturbed during the culling process or possibly through inadvertent carriage of virus by investigators checking farms for excess mortality or other evidence of infection (Power, 2005). Airborne spread of virus over short distances has probably occurred, especially from heavily infected farms (Brugh and Johnson, 1986; Power 2005). Transmission by flies and vermin is also possible, given the fact that virus has been isolated from blow flies in Japan (Sawabe et al, 2006) and that these viruses can multiply in a range of mammalian species, including mice, without prior adaptation.

In areas with high poultry density, a ‘stamping out’ policy involving the culling of poultry on infected farms, neighbouring farms, contact premises, or in a zone of a certain diameter around an infected farm, can lead to the destruction of millions of poultry, as was seen in the Netherlands (some 30 million head of poultry culled or died) (Stegeman et al, 2004) and Canada (17 million head) (Bowes et al, 2004) following outbreaks of HPAI in 2003 and 2004.

Many of the areas in Asia affected by HPAI also have high poultry densities, but these are often scattered across many households rather than being concentrated in large flocks.

sing well designed and properly implemented preventive measures, poultry reared under intensive conditions can remain free from infection even when farms are concentrated into relatively small areas. This has been demonstrated in HKSAR using a combination of enhanced biosecurity and vaccination since 2003.

However, in some production systems, notably those that allow poultry to forage freely, biosecurity cannot be enhanced significantly without changing the nature of the farming system. These farms remain at risk in areas where the virus is active. This is exacerbated if there is uncontrolled movement of traders and middlemen between farms and markets, leading to potential fomite transfer of viruses. In these systems, there is also considerable potential for wild birds to make contact with scavenging poultry such as grazing ducks, especially in wetlands that attract aquatic birds. Developing appropriate preventive measures for this type of farming represents a major challenge for animal health authorities, but is essential if these flocks are to remain free from infection.

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