Dengue hemorrhagic fever (DHF) is a viral disease which has spread throughout Southeast Asia over the past 25 years. Ae. aegypti is the main vector of the disease. The location of DHF outbreaks, the number of infected individuals, and the diffusion of the disease are related to several socio-economic, environmental and host factors. A number of writers have discussed the relationships between DHF outbreaks and rainfall in different parts of Southeast Asia. A review of the literature suggests that there are positive correlations between the two variables in Burma, Thailand and the Philippines, all of which have one wet season and prolonged “dry periods”, but that the relationships in countries closer to the equator, such as Peninsular Malaysia and Singapore, where there are two wet seasons, are unclear. In two case studies of Selangor and Johore in Peninsular Malaysia for the period 1973–1977, the relationships between DHF cases and moisture surpluses and deficits, the latter derived from Thornthwaite's method for calculating the water balance, are investigated on a monthly basis. It is shown that there is an increase in DHF cases following the March to May wet season and that the size of the increase is positively related to the size of the moisture surplus. The importance of moisture deficits is also underscored. There is, however, an apparent lack of association between DHF cases and rainfall during the second wet season, September–November, of each year. It is suggested that relationships between DHF and rainfall should be sought for a sequence of years, and that investigations must be based on a more sophisticated measure of moisture availability than raw monthly rainfall data. Topics for further research are outlined.
