Relationship between number of human Japanese encephalitis cases and summer meteorological conditions in Nagasaki, Japan.

Analysis of epidemiological and meteorological data for Nagasaki Prefecture, southwest Japan, for the period 1950-1979, showed a correlation between size of epidemics of human Japanese encephalitis (JE) and weather factors. It was demonstrated that during the period 1950-1969 the epidemic size (ES) had two types of correlation: ES correlated inversely with total summer precipitation, and ES correlated directly with mean summer temperature. The fact that in the 1970s there were fewer cases of JE cannot be attributed to changes in the weather factors. Large epidemics occurred in summers with low precipitation and high temperatures, while the epidemic size was small when there was heavy precipitation and temperatures were low. The two weather factors jointly explain 0.53 of the observed variance in annual JE ES. Furthermore, they strongly correlate with each other, and total summer precipitation alone is sufficient to explain about one-half of the total variance. How these factors influence the number of JE cases is discussed in relation to the population dynamics of the vector mosquito, Culex tritaeniorhynchus.