Experiments to determine possible mosquito vectors of W. bancrofti in the continental United States were inaugurated in Puerto Rico and later transferred to the continental United States.
Mosquito larvae collected in the field were raised to adults. The latter were permitted to feed upon volunteers infected with the periodic strain of W. bancrofti. The mosquitoes were dissected at various intervals after feeding and the development of the microfilariae was followed. A total of 1,801 dissections was made of mosquitoes from 15 species.
All of 17 Culex quinquefasciatus dissected 9½ days or more after feeding showed larvae which had developed to the infective stage.
Sixty per cent of the Psorophora confinnis collected in the continental United States contained infective larvae and an additional 20 per cent had well advanced larvae 9½ days after feeding. A potential infectibility rate of 80 per cent was obtained with this species.
Anopheles albimanus carried infective stage larvae in 35 per cent and well advanced larvae in 32 per cent of the specimens examined, giving an infectibility rate of 67 per cent.
Complete development of the larvae was observed occasionally in certain species which had the following infectibility rates: Psorophora confinnis (collected in Puerto Rico) 12 per cent; Culex nigripalpus, 7 per cent; Aedes aegypti, 5 per cent; A. triseriatus, 3 per cent.
No development beyond the first larval stage was observed in Aedes sollicitans, A. taeniorhynchus, A. vexans, and Anopheles punctipennis.
Inconclusive evidence has been obtained for Anopheles quadrimaculatus, Culex erraticus, C. salinarius, Psorophora ciliata and P. discolor. Data secured thus far indicate that none of these species is a good host, with the possible exception of P. discolor which permitted development to the infective stage in one specimen with well advanced stages in five other specimens.
Development to the infective stage was attained in most mosquitoes within two weeks and followed a pattern previously described by other investigators. Head or proboscis infections developed in over half of the specimens showing infective larvae.
It is concluded that C. quinquefasciatus, P. confinnis, and A. albimanus are capable of serving as excellent vectors of W. bancrofti should the proper conditions prevail for the spread of filariasis. C. nigripalpus, A. aegypti, and A. triseriatus might serve as vectors although their low infectibility rates preclude their playing an important role in the spread of the disease. A. sollicitans, A. taeniorhynchus, A. vexans, and A. punctipennis are apparently incapable of transmitting the infection.