The Veterinary record
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The Veterinary record · Aug 2005
ReviewOrigin and evolution of highly pathogenic H5N1 avian influenza in Asia.
Outbreaks of highly pathogenic avian influenza caused by H5N1 viruses were reported almost simultaneously in eight neighbouring Asian countries between December 2003 and January 2004, with a ninth reporting in August 2004, suggesting that the viruses had spread recently and rapidly. However, they had been detected widely in the region in domestic waterfowl and terrestrial poultry for several years before this, and the absence of widespread disease in the region before 2003, apart from localised outbreaks in the Hong Kong Special Autonomous Region (SAR), is perplexing. Possible explanations include limited virus excretion by domestic waterfowl infected with H5N1, the confusion of avian influenza with other serious endemic diseases, the unsanctioned use of vaccines, and the under-reporting of disease as a result of limited surveillance. ⋯ The rising incidence and widespread reporting of disease in 2003/04 can probably be attributed to the increasing spread of the viruses from existing reservoirs of infection in domestic waterfowl and live bird markets leading to greater environmental contamination. When countries in the region started to report disease in December 2003, others were alerted to the risk and disease surveillance and reporting improved. The H5N1 viruses have reportedly been eliminated from three of the nine countries that reported disease in 2003/04, but they could be extremely difficult to eradicate from the remaining countries, owing to the existence of populations and, possibly, production and marketing sectors, in which apparently normal birds harbour the viruses.