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

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.

Experimental studies on the vector potential of certain Culex species to West Nile virus.

Experimental studies were carried out to determine the vector potential of four species of mosquitoes to West Nile (WN) virus, viz. Culex tritaeniorhynchus, Cx. vishnui, Cx. bitaeniorhynchus and Cx. univittatus. All the four species of mosquitoes successfully transmitted and supported the growth of WN virus. The study indicated that the four species of mosquitoes could act as potential vectors of WN virus in nature.

Studies on the mosquito vectors of Japanese encephalitis virus in Mandya District, Karnataka, India.

Entomological investigations were carried out in areas affected by Japanese encephalitis (JE) in Mandya District, Karnataka, India, from 1983 to 1988, to determine species composition and the density of mosquito vectors, in relation to the incidence of JE cases. JE cases occurred in two spells in a year, one during April-June (summer epidemic) and another during October-December (winter epidemic). There was very high incidence of JE cases in extensively irrigated areas and a low incidence in some of the taluks with less or no irrigation systems. Among culicines, Culex tritaeniorhynchus was the most predominant species (20.54%), followed by Cx. fuscocephala (16.94), Cx. vishnui (16.48%), Cx. gelidus (10.70%) and other species. The overall mosquito population showed two peaks in a year, one during the March-April, and another during September, usually preceding the human epidemics. Relative abundance of certain species varied in different years.

Susceptibility status of five species of Japanese encephalitis vectors to insecticides from Kolar district, Karnataka.

Susceptibility tests were conducted on the adults of five species of mosquito vectors of Japanese encephalitis (JE) viz., Culex tritaeniorhynchus, C. vishnui, C. pseudovishnui, C. gelidus and C. fuscocephala from Kolar district, Karnataka during 1990-91 against organo chloride compounds (DDT 4%, dieldrin 4%), organo phosphate compounds (malathion 5%, fenitrothion 1.0%), carbamate (propoxur 0.1%) and pyrethroid (deltamethrin--OMS 0.025%). All the five species were found susceptible to malathion. C. tritaeniorhynchus was resistant to DDT, dieldrin and fenitrothion; C. vishnui was also susceptible to fenitrothion and propoxur; C. pseudovishnui was resistant to dieldrin but it was susceptible to fenitrothion and propoxur. C. gelidus and C. fuscocephala were susceptible to dieldrin, fenitrothion and propoxur. Deltamethrin did not exhibit a good adulticidal effect except for C. pseudovishnui and C. fuscocephala. However, verifications are required to determine the susceptibility status of C. vishnui, C. pseudovishnui, C. gelidus and C. fuscocephala against DDT, C. tritaeniorhynchus against propoxur and C. vishnui against dieldrin.

Monitoring Japanese encephalitis virus activity using domestic sentinel pigs in Mandya district, Karnataka state (India).

Domestic pigs were used as sentinels in Mandya district (Karnataka state) in India during 1984 to 1986 to monitor the activity of Japanese encephalitis (JE) virus in nature. Out of 213 sentinel pigs studied in 11 localities, 44 pigs placed at 5 localities showed seroconversion to Japanese encephalitis virus. A pig mosquito cycle, operating throughout the year appears to be one of the maintenance mechanisms of the virus in the study area in addition to the bird-mosquito cycle.