PLASMODIUM-GALLINACEUM - A REFRACTORY MECHANISM OF OOKINETE KILLING IN THE MOSQUITO, ANOPHELES-GAMBIAE

We have identified a mechanism for refractoriness to a bird malaria, P lasmodium gallinaceum, in the African vector of human malaria, Anophel es gambiae. Oocysts fail to develop in the refractory mosquitoes as a result of ookinete death which occurs within 27 hr of midgut invasion, Ultrastructural studies showed that parasite death occurs while the o okinete lies free in the midgut epithelial cell cytosol, usually surro unded by an organelle-free zone that consists of finely fibrillar mate rial. The mechanism of parasite killing does not involve a previously described refractory mechanism of parasite encapsulation. We selected genetic lines which are refractory and susceptible to midgut infection . Genetic crossing of the lines suggests that the refractory trait is inherited as a single dominant genetic locus. Other loci probably infl uence oocyst number in susceptible mosquitoes. Intracellular ookinete killing appears to involve a previously unrecognized host defense mech anism against malaria parasites that involves direct destruction of th e invading organism. (C) 1995 Academic Press, Inc.