Abstract
The highly pathogenic avian influenza (HPAI) H5N1 virus that emerged in southern China in the mid-1990s has in recent years evolved into the first HPAI panzootic. In many countries where the virus was detected, the virus was successfully controlled, whereas other countries face periodic reoccurrence despite significant control efforts. A central question is to understand the factors favoring the continuing reoccurrence of the virus. The abundance of domestic ducks, in particular free-grazing ducks feeding in intensive rice cropping areas, has been identified as one such risk factor based on separate studies carried out in Thailand and Vietnam. In addition, recent extensive progress was made in the spatial prediction of rice cropping intensity obtained through satellite imagery processing. This article analyses the statistical association between the recorded HPAI H5N1 virus presence and a set of five key environmental variables comprising elevation, human population, chicken numbers, duck numbers, and rice cropping intensity for three synchronous epidemic waves in Thailand and Vietnam. A consistent pattern emerges suggesting risk to be associated with duck abundance, human population, and rice cropping intensity in contrast to a relatively low association with chicken numbers. A statistical risk model based on the second epidemic wave data in Thailand is found to maintain its predictive power when extrapolated to Vietnam, which supports its application to other countries with similar agro-ecological conditions such as Laos or Cambodia. The model's potential application to mapping HPAI H5N1 disease risk in Indonesia is discussed.
Highlights
Pathogenic avian influenza (HPAI) H5N1 virus emerged in Southern China in the mid-1990s [1], but the first large-scale epizootic took place in the winter of 2003/2004 in East and Southeast Asia [2]
The spatial pattern of wave II in Thailand is explored in further detail in Gilbert et al [10] and Tiensin et al [19], and the temporal and spatial patterns of waves I–III in Vietnam are examined in Pfeiffer et al [14]
The variables showing the highest level of consistency in their relationship with highly pathogenic avian influenza (HPAI) H5N1 virus presence are human population, rice cropping intensity, and, to a somewhat lesser extent, duck numbers, all of which were found to be significantly associated with virus presence in both countries
Summary
Pathogenic avian influenza (HPAI) H5N1 virus emerged in Southern China in the mid-1990s [1], but the first large-scale epizootic took place in the winter of 2003/2004 in East and Southeast Asia [2]. It was demonstrated that the risk of HPAI H5N1 virus presence was associated with free-ranging duck numbers in Thailand [10] and the local abundance of both duck and geese in Vietnam [14], in addition to other risk factors such as chicken numbers, human population, and topographical features. Both studies found evidences of a relationship between HPAI H5N1 virus presence and rice production. The satellite image-based algorithms permit the production of maps of cropping intensity, cropping calendar (planting and harvesting dates), and irrigation practices at moderate spatial resolution (250–500 m) and in near-real-time fashion
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