Impact of season on filarial vector density and infection in Raipur City of Chhattisgarh, India

Bionomics and transmission dynamics of vector population are profoundly influenced by local climatic conditions. The study monitored variations in density, infection and infectivity rates of Culex quinquefasciatus with respect to season in randomly selected localities of Raipur City of Chhattisgarh state. The indoor resting density of Cx. quinquefasciatus, Wuchereria bancrofti infection and infectivity rates were monitored in Raipur City at regular monthly intervals for over a period of 12 months and the impact of meteorological conditions on transmission indices were assessed. The frequency distribution of different stages of filarial larvae in naturally infected vector population, host efficiency and transmission intensity index of the vector population was also worked out. The mean vector density of Cx. quinquefasciatus during the study period was 55.22 while the mean vector infection and infectivity rates recorded in Raipur were 4.05 and 0.25% respectively. A gradual increase in Culex infection rate was recorded from June onwards with a maximum (22.14%) in August and minimum (2.38%) in February. No vector with filarial infection was detected during December when the vector density was high. Analysis of frequency distribution of different stages of larvae revealed dominance of microfilarial stage with a mean larval intensity of 5.37. The highest infectivity rate was observed during June (1.15%) and the lowest was in March (0.41%). Both the infection and infectivity rates were at their low during winter season although the density of Cx. quinquefasciatus was at its maximum. Highest mean host efficiency of 0.44 was recorded in February. The annual transmission intensity index (TII) was 32.72. The highest TII was recorded during January and February months. The climate appeared to have profound impact on vector density, infection and infectivity rates. The vector infection and infectivity rates were high in rainy season followed by summer and winter seasons. The highest host efficiency was observed in winter followed by summer and rainy seasons. An apparent negative correlation was observed between vector infection rate and vector density. Lower temperatures (23-25 degree) with low rainfall favoured progression of mf to L3 in Cx. quinquefasciatus in the study region.