Non-involvement of nulliparous females in the transmission of bancroftian filariasis

Estimation of permissible levels of transmission of bancroftian filariasis based on some entomological and parasitological results of a 5-year vector control programme

The impact of a 5-year vector control programme on transmission of bancroftian filariasis and microfilaraemia levels in the human population was examined in Pondicherry, South India. The programme reduced and maintained the transmission of Wuchereria bancrofti at appreciably low levels in many localities of Pondicherry town, but its impact on overall microfilaraemia prevalence was only limited. However, prevalence of infection was reduced drastically in children born after establishment of the programme and only 18% of the number expected without vector control were found infected. No new infections were found in localities with less transmission intensity. It is proposed that a Transmission Intensity Index of 0.50 and Annual Transmission Potential of less than the 96-105 range are permissible levels of transmission, and below these levels no new infections may occur. The required duration of vector control to eliminate Culex-borne filariasis foci is estimated to be 11-12 years, slightly longer than that required to eradicate anopheline-borne filariasis.

Risk of infection of Wuchereria bancrofti to humans by Culex quinquefasciatus in Pondicherry and its relationship with microfilaria prevalence

The risk of infection (RI) with Wuchereria bancrofti by vector mosquitoes has been conventionally estimated from the biting population of all age classes. This estimate of RI had no correlation with microfilaria (mf) prevalence and intensity of infection in humans. A new method for estimating RI from a resting population of Culex quinquefasciatus is proposed based on a longitudinal study in seventeen sites in Pondicherry, India. Unlike previous estimates, RI calculated by the present method considering the density of parous mosquitoes collected per man-hour and the mean number of infective stage larvae per parous mosquito significantly correlated with mf prevalence in humans during both early (r = 0.559; P = 0.038) and late in the control operation (r = 0.572; P = 0.033). RI varied significantly between the early and late in the control operation and different sites. On average, RI was reduced significantly during the control operation. Further epidemiological implications are discussed.