Etiology of interepidemic periods of mosquito-borne disease

Dengue viruses and malaria protozoa are of increasing global concern in pub lic health. The diseases caused by these pathogens often show regular seaso nal patterns in incidence because of the sensitivity of their mosquito vect ors to climate. Between years in endemic areas, however, there can be furth er significant variation in case numbers for which public health systems ar e generally unprepared. There is an acute need for reliable predictions of within-year and between-year epidemic events. The prerequisite for developi ng any system of early warning is a detailed understanding of the factors i nvolved in epidemic genesis. In this report we discuss the potential causes of the interepidemic periods in dengue hemorrhagic fever in Bangkok and of Plasmodium falciparum malaria in a highland area of western Kenya. The alt ernative causes are distinguished by a retrospective analysis of two unique and contemporaneous 33-year time series of epidemiological and associated meteorological data recorded at these two sites. We conclude that intrinsic population dynamics offer the most parsimonious explanation for the observ ed interepidemic periods of disease in these locations.