Temporal Variation in the Susceptibility of Culex tritaeniorhynchus (Diptera: Culicidae) to Japanese Encephalitis Virus in an Endemic Area of Tamil Nadu, South India

The study area, Cuddalore, is one of the endemic districts for Japanese encephalitis (JE) in southern India and there is a strong seasonality in JE case incidence, as well as JE virus (JEV) infection in the principal vector Culex (Culex) tritaeniorhynchus Giles. In a longitudinal 3-year study (July 2003 to June 2006), we determined the susceptibility of wild-caught female Cx. tritaeniorhynchus for JEV infection over several seasons from several villages. The susceptibility varied in all four seasons with the lowest value (4.82 geometric mean [GM]) in hot and wet seasons and highest (13.22 GM) in cool and wet seasons. Infection rate was significant between seasons (7.08-11.85 GM) and years (4.82-13.22 GM). Although the vector was abundant throughout the year, with an average per man-hour density ranging from 58 to 652, the JEV infection rates showed no correlation with vector abundance during different seasons in the index villages. The temporal and spatial changes in the competency of the vector appeared to influence the JEV infection rate in vector, which may at least partially explain the seasonality in JEV human cases in the study area.