The transmission dynamics of bancroftian filariasis:: the distribution of the infective larvae of Wuchereria bancrofti in Culex quinquefasciatus and Anopheles gambiae and its effect on parasite escape from the vector

The anatomical distribution of the infective larvae (L3) of Wuchereria bancrofti in Culex quinquefasciatus and Anopheles gambiae, and its influence on L3 escape, was evaluated by exposing the vectors to human individuals infected with W. bancrofti. After the extrinsic incubation period of W. bancrofti, a random sample of the infected mosquitoes was dissected to determine the distribution of infective larvae in the body of the mosquitoes and the proportion of mosquitoes that were infected. The remaining mosquitoes were exposed to an experimental definitive host (mouse skin). The infective larvae on and in the host tissues were counted. The engorged mosquitoes were dissected to determine the proportion infected and the distribution of L3 in mosquitoes after exposure. The results show that 53.4% of L3 escaped from Cx. quinquefasciatus to enter the experimental host whereas only 4% of L3 escaped from An. gambiae. Analysis of the results indicates that the number of L3 in the vector and the proportion of L3 in the head and mouthparts influence the number of L3 escaping from the vector to enter the definitive host and consequently the number that gain access to host tissues. The implication of these findings on the transference of the parasite from the vector to the definitive host is discussed.