Correlation of Culex Tarsalis Population Indices with the Incidence of St. Louis Encephalitis and Western Equine Encephalomyelitis in California *

Mosquito population indices from California for the period 1953-1973 were analyzed to determine their association with activity of St. Louis encephalitis (SLE) and western equine encephalomyelitis (WEE) viruses. Culex tarsalis female populations, as measured by New Jersey light trap indices (LTI), correlated positively with the incidence rates of encephalitis in humans, and were a reliable means of forecasting the years of highest incidence. The critical level of C. tarsalis in urban areas below which no human cases of SLE and WEE were detected was an LTI of 0.1. Critical urban levels of C. tarsalis associated with significant human SLE or WEE incidence ranged between LTIs of 6.4 (for rural mosquito abatement districts [MADs] with large resident human populations). Peaks in annual incidence of SLE and WEE in humans occurred during years when seasonal average C. tarsalis female populations in urban areas reached a LTI of 21. Peaks in weekly incidence of SLE and WEE were associated, respectively, with weekly LTIs of 21 and 81 in urban traps. Isolation rates of SLE virus from mosquito pools and transmission of the virus to enzootic hosts were highest when urban LTIs were between 10 and 19.9 and between 5 and 9.9, respectively. The WEE viral isolations and enzootic transmission rates were highest when LTIs in urban areas were 1-4.9.