Travelling waves in the occurrence of dengue haemorrhagic fever in Thailand

Dengue fever is a mosquito- borne virus that infects 50 - 100 million people each year(1). Of these infections, 200,000 - 500,000 occur as the severe, life- threatening form of the disease, dengue haemorrhagic fever ( DHF)(2). Large, unanticipated epidemics of DHF often overwhelm health systems(3). An understanding of the spatial - temporal pattern of DHF incidence would aid the allocation of resources to combat these epidemics. Here we examine the spatial - temporal dynamics of DHF incidence in a data set describing 850,000 infections occurring in 72 provinces of Thailand during the period 1983 to 1997. We use the method of empirical mode decomposition(4) to show the existence of a spatial - temporal travelling wave in the incidence of DHF. We observe this wave in a three- year periodic component of variance, which is thought to reflect host pathogen population dynamics(5,6). The wave emanates from Bangkok, the largest city in Thailand, moving radially at a speed of 148 km per month. This finding provides an important starting point for detecting and characterizing the key processes that contribute to the spatial - temporal dynamics of DHF in Thailand.