A long-term study on vector abundance & seasonal prevalence in relation to the occurrence of Japanese encephalitis in Gorakhpur district, Uttar Pradesh

Abstract

Background & objectives: Japanese encephalitis (JE) virus is an important cause of viral encephalitis in Gorakhpur district, UP. The area has been experiencing outbreaks of JE since 1978. No in-depth longitudinal studies have been carried out on the mosquito species, particularly JE vectors prevailing in the area. Entomological studies were carried out in the district in order to determine the species composition, relative abundance and seasonal prevalence of mosquitoes in relation to the incidence of JE.

Methods: Three JE affected villages were selected as study sites. Weekly visits were made to each village and mosquito collections were made in and around the pig enclosures and cattle sheds with the help of mouth aspirators, aided by flash lights at dusk.

Results: The overall mosquito population showed a bimodal pattern with short and tall peaks during March and September respectively. Based on the elevated density and infection with JE virus, Culex tritaeniorhynchus has been considered responsible for causing epidemics in the area. Cx. pseudovishnui, Cx. whitmorei, Cx. gelidus, Cx. epidesmus, Anopheles subpictus, An. peditaeniatus and Mansonia uniformis are suspected to have played some role in the epidemiology of JE in the region. JE cases were reported between August and November with the peak in October when the vectors population, particularly Cx. tritaeniorhynchus was on the decline.

Interpretation & conclusion: The results of the study indicate that Cx. tritaeniorhynchus, the most likely vector of JE together with other known vector species remained more active during the period of paddy cultivation. Integrated antilarval measures before the beginning of paddy irrigation may check the breeding of JE vectors in the paddy fields. It may prove beneficial in reducing the vector population during the JE transmission season.