Density of Culex vishnui and appearance of JE antibody in sentinel chicks and wild birds in relation to Japanese encephalitis cases.

In the District of Burdwan, a rural area of West Bengal State, India, Japanese encephalitis (JE) virus is endemic. In one village a longitudinal survey was conducted in order to find out whether associations could be established between the density of the vector mosquito Culex vishnui in two types of resting places, the incidence of infections in sentinel chicks exposed at monthly intervals, the prevalence of antibodies in wild birds and the occurrence of clinical infections in the human population. The experiment lasted from August 1981 till August 1982. Meteorologically a summer season (March-June), a rainy season (July-October) and a winter season (November-February) are distinguished. In the sentinel chicks infections were observed in all three seasons; in wild birds antibodies were prevalent throughout the year; these observations suggest perennial transmission of the virus in its maintenance cycle. Human infections were observed periodically with, in August 1982, a pronounced peak; this may point to fluctuations in the level of circulation of the virus in its maintenance cycle and spillover to the human population at times of peak circulation. The peaks may be related to the influx of young non-immune birds and newborn mammals into the animal population in summer. Further studies, including virus isolation attempts from mosquitoes and nestling birds, are required to prove this hypothesis.