Susceptibility and Resistance of Avian and Mosquito Hosts to Strains of Plasmodium relictum Isolated from Pigeons*

Abstract

SYNOPSIS. Two strains (1B and 1P) of Plasmodium relictum isolated from pigeons could be easily passed serially in pigeons by transfer of infected blood. What appeared at first to be a qualitative difference between the infectiousness of gametocytes of strain 1P in pigeons and of gametocytes of the strain after being adapted to canaries (1P1) was found to be only a quantitative difference in their infectivity for mosquitoes. Earlier work had indicated that gametocytes of strain 1P were noninfectious for mosquitoes. The experiments reported here show that Culex tarsalis is highly susceptible to gametocytes of both strains 1B and1P and that Culex pipiens is slightly susceptible to gametocytes of strain 1P.The susceptibility of 72 pigeons (belonging to 24 varieties, plus mongrels and 3 species hybrids) to sporozoites of strain 1P1 was tested; only 5 transient infections lasting no more than two days and 3 subpatent infections were obtained. Similar results were obtained with sporozoites of strain 1B.The two strains are similar in both mosquito and avian hosts except that 1B is more highly infectious for C. pipiens and C. tarsalis than 1P. There is evidence that one or both of these strains may be Plasmodium relictum matutinum, Huff(7). A proposed hypothesis indicates that (1) various species of mosquitoes have different susceptibilities, (2) various species of avian hosts are infectious to different degrees, and (3) that the probability of a mosquito of a given species being infected from a given species of parasite may be expressed by the degree of overlap between the normal curves of susceptibility of the mosquito and infectiousness of the gametocytes in the pertinent avian host. The susceptibilities of pigeons to sporozoites of the two strains of parasites and of mosquitoes to gametocytes produced in pigeons are so low that it can be questioned whether these strains could persist in nature without the presence of other avian hosts on the one hand possessing greater susceptibility than the pigeon to sporozoites and on the other hand contributing more to the infectiousness of the gametocytes than the pigeon does.