Interactions between the malaria parasite, its vector and its mammalian host

Malaria is transmitted by Anopheles mosquitoes. We have studied 3 key steps during development of the malaria parasite. A better understanding of these steps could lead to new approaches for control of malaria.; Development of the parasite in the mosquito starts when the mosquito ingests malaria gametocytes from an infected host. After fertilization of gametocytes within the mosquito midgut, the resulting zygote transforms into a motile ookinete and migrates through the midgut epithelium. Mosquitoes frequently respond to parasites by means of a rejection mechanism, melanization. This process requires the activity of a phenoloxidase enzyme (PO) with 2 key enzyme activities. We examined the activity of PO in the midgut epithelium, hemolymph plasma and hemocytes. We characterized differences in their ability to oxidize a wide spectrum of substrates and differences in their sensitivity to specific inhibitors.; The final stages of malaria development in the mosquito lead to formation of sporozoites, which specifically invade mosquito salivary glands, thus suggesting receptor-mediated recognition of the salivary gland by sporozoites. We developed an in vitro binding assay using isolated mosquito organs and recombinant CS protein, which is the major surface protein of the sporozoite. We showed that CS protein bound specifically to the distal lateral and median lobes of salivary glands and not to other mosquito organs. These results were confirmed in vivo. Inhibition of CS binding was achieved with a peptide encompassing region I of the CS protein.; Finally, the mosquito transmits malaria when it injects salivary gland sporozoites into a host. It is not clear whether infected mosquitoes inject sporozoites directly into the blood or by initial deposition into avascular tissue of the skin. We investigated this by allowing P. yoelii-infected mosquitoes to probe upon a restricted portion of the ear of mice. This fed-upon portion of the ear was resected at various times post-feeding to determine whether subsequent infection of the host by sporozoites was blocked. Our data show clearly that most Plasmodium sporozoites are injected into avascular tissue rather than directly into the blood.