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

Background & objectives: Bionomics and transmission dynamics of vector population are profoundlyinfluenced by local climatic conditions. The study monitored variations in density, infection andinfectivity rates of Culex quinquefasciatus with respect to season in randomly selected localities ofRaipur City of Chhattisgarh state.Methods: The indoor resting density of Cx. quinquefasciatus, Wuchereria bancrofti infection andinfectivity rates were monitored in Raipur City at regular monthly intervals for over a period of 12months and the impact of meteorological conditions on transmission indices were assessed. Thefrequency 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.Results: The mean vector density of Cx. quinquefasciatus during the study period was 55.22 whilethe 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 wasdetected during December when the vector density was high. Analysis of frequency distribution ofdifferent stages of larvae revealed dominance of microfilarial stage with a mean larval intensity of5.37. 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 althoughthe density of Cx. quinquefasciatus was at its maximum. Highest mean host efficiency of 0.44 wasrecorded in February. The annual transmission intensity index (TII) was 32.72. The highest TIIwas recorded during January and February months.Interpretation & conclusion: The climate appeared to have profound impact on vector density,infection and infectivity rates. The vector infection and infectivity rates were high in rainy seasonfollowed by summer and winter seasons. The highest host efficiency was observed in winter followedby summer and rainy seasons. An apparent negative correlation was observed between vectorinfection rate and vector density. Lower temperatures (23–25oC) with low rainfall favouredprogression of mf to L3 in Cx. quinquefasciatus in the study region