Factors Influencing Infection of the Mosquito with Dirofilaria immitis (Leidy, 1856).

1. 1. A quantitative evaluation of the development of Dirofilaria immitis in Aedes aegypti, Culex quinquefasciatus, and Anopheles quadrimaculatus was established. The types of evidence used in this work consisted of (1) the host reaction from day to day, and (2) the parasite reaction from day to day. A third category of analysis was the concept of host efficiency defined as a ratio between the theoretical number of microfilariae ingested by a group of mosquitoes and the total number of developing larvae found in them during a specified period. A similar ratio involving only infective larvae was suggested as basis for the supplementary concept of infective potential. 2. 2. As one of the measures of a genetic basis underlying susceptibility of the mosquito to a filarial parasite, taxonomically related mosquito species were compared as hosts for D. immitis. Infection of the strains of Aedes aegypti and A. albopictus used here showed that the host efficiency of albopictus was twenty times better, and the infective potential was fifty times greater than the corresponding ratios for aegypti. Infection of F1 and F2 hybrid females from a cross between aegypti females and albopictus males showed no segregation of susceptibility in either generation and resulted in complete dominance of the aegypti parent female characteristics. Infection of Culex pipiens and C. quinquefasciatus showed that both the host efficiency and infective potential of quinquefasciatus were two and one half times greater than those forpipiens. Infection of F2 reciprocal hybrids of quinquefasciatus and pipiens showed some physiological segregation in that host efficiencies and infective potentials were close to those of the quinquefasciatus parent, whereas the growth of the larvae most resembled that seen in pipiens. Infection of Anopheles quadrimaculatus and A. freeborni indicated almost identical physiological reactions to D. immitis and comparable development of the parasite. 3. 3. As another measure of the genetics of susceptibility, several geographical strains of Aedes aegypti were infected with D. immitis and the resulting infections quantitatively evaluated. A. aegypti from the United States, S. Africa, and Anglo-Egyptian Sudan were shown to give similar host and parasite reactions, whereas a Hawaiian strain was shown to be more susceptible and a Fiji strain to be more refractory to the dog filaria. 4. 4. Individual susceptibility of Aedes aegypti to infection with filarial parasites was tested by feeding females on D. immitis and Foleyella brachyoptera either simultaneously or at intervals. Each species of filaria reached its particular site of development in the mosquito with no apparent mutual antagonism and their subsequent development was typical for the species. 5. 5. Selective breeding of mosquitoes for differential susceptibility to D. immitis was conducted to evaluate further genetic and helminthic interactions in the intermediate host. With Aedes aegypti, artificial selection resulted in the establishment of susceptible and refractory strains for D. immitis. Selective breeding of a Culex hybrid for differential susceptibility to D. immitis failed to establish either susceptible or refractory strains. The percentage infection of the selected strains was below that of both the parents and controls. 6. 6. The ability of D. immitis microfilariae to survive passage through the midgut of several species of culicine and anopheline mosquitoes was studied. It was found that all the microfilariae in midguts between 24 and 48 hours are killed and digested in Culex quinquefasciatus, C. pipiens, and their reciprocal hybrids, whereas some microfilariae may remain alive in the midguts of Aedes aegypti and A. albopictus for over 72 hours and in Anopheles quadrimaculatus and A. freeborni for at least 48 hours. 7. 7. The active migration of D. immitis microfilariae from the mosquito's midgut to its Malpighian tubules was found to be mechanically inhibited by the rapid formation of blood clots in the midgut of theAedes species used in these experiments, whereas no such barrier to migration was noted in the Anopheles spp. By twenty-four hours an average of about 91% of the parasites in A. quadrimaculatus andA. freeborni were in the tubules whereas those in A. aegypti and A. albopictus tubules averaged 45 per cent. The addition of an anticoagulant to in vitro suspensions of microfilariae fed to the Aedes spp. caused an increase in the rate of parasite migration to the tubules comparable to that seen in the Anopheles spp. 8. 8. Variations in number of microfilariae ingested by individual mosquitoes were studied by allowing Aedes aegypti to feed on an infected dog and upon in vitro suspensions of D. immitis microfilariae. Dog-fed females showed wide individual variations in numbers of ingested microfilariae, whereas suspension-fed females showed significantly less individual variation. 9. 9. Variations in the number of ingested microfilariae were shown to be affected by periodicity of the microfilariae in the dog. The present work indicated that A. aegypti ingested a maximum mean number of D. immitis microfilariae between 6 P.M. and.10 P.M. and a minimum between 6 A.M. and 10 A.M.