Studies to determine possible mosquito vectors of Wuchereria bancrofti (nocturnally-periodic strain) in the United States were continued. A total of 1314 dissections was made of mosquitoes from 14 species.
Seventy-eight per cent of Culex pipiens examined 9½ days or longer after infection contained infective larvae, and an additional 13 per cent contained larvae in late stages of development. A total infectibility rate of 91 per cent was obtained with this species.
Thirty-three per cent of Psorophora discolor in late dissections contained infective larvae, and an additional 34 per cent contained larvae in late stages of development. The infectibility rate of this species was 67 per cent.
Occasional development of the larvae to advanced or infective stages was observed in the following species: Culex salinarius, 3 per cent; Anopheles crucians, 2 per cent; Aedes thibaulti, 3 per cent.
No development beyond the first stage was observed in Anopheles quadrimaculatus, Anopheles maculipennis freeborni, and Psorophora ciliata.
Studies were not completed on Aedes canadensis, Culex erraticus, Mansonia perturbans, and Psorophora cyanescens. However, on the basis of 50 or more late dissections of each species, none of these was a good intermediate host with the exception of Culex erraticus which permitted development of the larvae to late and infective stages in 14 of 49 specimens.
The few dissections obtained of Psorophora ferox and P. howardii were negative.
It is concluded that Culex pipiens and Psorophora discolor are capable of serving as vectors of Wuchereria bancrofti should other conditions prevail for the spread of the parasite. Incomplete studies on Culex erraticus indicate that this species also might be involved as a transmitter although to a lesser extent. C. salinarius, Anopheles crucians, and Aedes thibaulti might serve as occasional vectors, although their low infectibility rates preclude their playing a major rôle in the spread of the disease. Anopheles quadrimaculatus, A. maculipennis reeborni, and Psorophora ciliata are incapable of transmitting infection. Finally, although studies are not completed, it is apparent that Aedes canadensis, Mansonia perturbans, and Psorophora cyanescens could not serve as vectors of Wuchereria bancrofti.