A new realistic index of experimental transmission efficiency for Bancroftian filariasis.

An index for experimental transmission of filariasis infection was first introduced by Kartman and subsequently modified by Wharton who defined it as 'the number of infective larvae per mosquito when the microfilaria count is 1.0 mm-3 of human blood'. Since mosquitoes may not imbibe the expected number of microfilariae from the donor's blood during feeding because of various interference phenomena and/or mechanisms, this paper presents the index mosquito meal (Index MM), based on the actual number of viable microfilariae that are observed in the mosquito gut immediately after feeding. The Index MM is defined as 'the number of infective larvae per mosquito when the microfilaria count is 1.0 mm-3 of mosquito blood meal'. The Index MM proved to be a more valid indicator of transmission potential than the Wharton Index when applied to the data of two controlled field studies where groups of laboratory-reared Culex pipiens molestus, the primary vector of filariasis in Egypt, were fed on volunteers having different microfilaraemic densities.