Transmission dynamics of diurnally subperiodic lymphatic filariasis transmitted by Ochlerotatus (Finlaya) niveus in the Andaman & Nicobar Islands.

BACKGROUND & OBJECTIVE In India diurnally subperiodic filariasis is prevalent only in the Nicobar district of Andaman and Nicobar Islands, with significant health problem. Sample surveys indicated that this form of filariasis is restricted to a small region of Nancowry group of islands with Ochlerotatus niveus as the vector. We therefore carried out a comprehensive study to assess the transmission dynamics of LF in Teressa island in Nicobar district in view of its control and evaluation of interventions. METHODS Entomological studies were carried out for a period of twelve months covering all the seasons in the year, by means of man landing catches in Teressa Island, an endemic island for this form of filariasis. Parameters viz., the annual biting rate (ABR), annual infective biting rate (AIBR), annual transmission index (ATI), risk of infection index (RII) and annual transmission potential (ATP), which reflect the dynamics and intensity of transmission of filariasis, were estimated. Host efficiency of Oc. niveus was also assessed. RESULTS The number of vectors biting a person in a year was estimated to be 21851, of which 107 were harbouring infective parasite. Risk of infection intensity was 0.02332. Every person in this study area was at the risk of receiving an estimated number of 22 infective stage larvae per year. The host efficiency index of Oc. niveus indicated that over 40 per cent of the microfilariae ingested were able to develop into infective stages. The ATP was 169 with evidence of year round transmission. The pattern of monthly transmission potential suggested that the intensity of transmission was high during summer months. INTERPRETATION & CONCLUSION Perennial transmission of subperiodic Wuchereria bancrofti in the typical forest ecosystem was evident in Teressa Island with transmission parameters suggesting that summer is a high risk season for transmission. Personal protection measure is the method of choice to protect from the risk of infection, because of day biting, exophilic and exophagic behaviour of the vector and larvae are not amenable to larvicidal measures. An alternative method to control the transmission would be to use selective or periodic mass chemotherapy to reduce the parasite load in this community.