Epidemiology and ecology of highly pathogenic avian influenza with particular emphasis on South East Asia.

Highly pathogenic avian influenza (HPAI) has been recognised as a serious viral disease of poultry since 1878. The number of recorded outbreaks of HPAI has increased globally in the past 10 years culminating in 2004 with the unprecedented outbreaks of H5N1 HPAI involving at least nine countries in East and South-East Asia. Apart from the geographical extent of these outbreaks and apparent rapid spread, this epidemic has a number of unique features, among which is the role that asymptomatic domestic waterfowl and more particularly free-ranging ducks play in the transmission of highly pathogenic H5N1. Field epidemiological studies have been conducted by the Food and Agriculture Organization and several collaborative centres to explore the factors that could have led to a change from infection to the emergence of widespread disease in 2003-2004 and 2005. Domestic waterfowl, specific farming practices and agro-ecological environments have been identified to play a key role in the occurrence, maintenance and spread of HPAI. Although there are some questions that remain unanswered regarding the origins of the 2004 outbreaks, the current understanding of the ecology and epidemiology of the disease should now lead to the development of adapted targeted surveillance studies and control strategies.