Correlation of rainfall with upsurge of malaria in Rajasthan

There has been a recent increase in malaria cases in Rajasthan. To determine whether environmental factors such as rainfall explain the upsurge we performed a study correlating the amount and duration of rainfall with annual malaria incidence. Data of annual incidence of malaria and Plasmodium falciparum malaria, annual and monthly rainfall and cultivated land area in Rajasthan were acquired from government sources from 1980 to 1994. The mean annual malaria incidence was 277 +/- 90 cases/100,000 population and falciparum malaria 70 +/- 43 cases/100,000 population per year. There was rising trend in total malaria and specially falciparum malaria incidence (r = 0.50, p = 0.057) over these years. The mean annual rainfall in the state was 541 +/- 144 mm/year and the mean cultivated land area was 15.5 +/- 1.2 million hectares/year with a significant interrelationship (r = 0.66, p = 0.008). The overall malaria incidence showed a moderate correlation (r = 0.48, p = 0.069) with annual rainfall while the incidence of falciparum malaria showed a significant correlation (r = 0.61, p = 0.016). Rainfall in the months of June, July and August did not show any significant correlation with annual malaria incidence. Rainfall in the month of September showed significant correlation with the incidence of overall malaria (r = 0.62, p = 0.015) as well as falciparum malaria (r = 0.71, p = 0.003). This relationship remained significant after adjustment for years of study and cultivated land-area (standardised regression coefficients: total malaria (b = 0.72, p = 0.060), falciparum malaria (b = 0.64, p = 0.059). There is a positive correlation of rainfall with incidence of malaria. Prolonged rains as suggested by rainfall extending to the month of September are associated with significantly higher incidence of total as well as falciparum malaria.