Highland malaria in Uganda: prospective analysis of an epidemic associatedwith El Nino

Malaria epidemics in African highlands cause serious morbidity and mortalit y and are being reported more frequently. Weather is likely to play an impo rtant role in initiating epidemics but limited analysis of the association between weather conditions and epidemic transmission parameters has been un dertaken. We measured entomological variables before and during an epidemic of malaria (which began in February 1998) in a highland region of south-we stern Uganda and analysed temporal variation in weather data against malari a incidence (estimated from clinic records), mosquito density and entomolog ical inoculation rates (EIR). Indoor resting density of Anopheles gambiae s .l. was positively correlated with malaria incidence (r = 0.68, P<0.05) des pite extremely low vector densities. EIR totalled only 0.41 infectious bite s per person during the entire 8-month study period. Rainfall during and fo llowing the El Nino event in 1997 was much higher than normal, and rainfall anomaly (difference from the mean) was positively correlated with vector d ensity 1 month later (r = 0.55, P < 0.05). Heavier than normal rainfall ass ociated with El Nino may have initiated the epidemic; the relationship betw een temperature and transmission parameters remains to be defined. The resu lts from this study indicate that, in this highland population, epidemic ma laria may occur at extremely low inoculation rates.