LIFE HISTORY. Plasmodium ovale has developmental cycles in the human host and in the vector mosquito. Following introduction of sporozoites via the bite of infected mosquitoes, these forms rapidly invade the liver, where, within a single parenchymal cell, the parasite matures in approximately 9 days.

Eventually, many hundreds of merozoites are produced. Upon release, these merozoites invade reticulocytes and initiate the erythrocytic cycle. The development of some of the parasites in the liver cells is delayed or suspended as hypnozoites, occasionally for many months. Following a developmental cycle in the erythrocyte that lasts, on average, 49 h, from 8 to 20 merozoites are released to reinvade other erythrocytes. As with other species of Plasmodium that infect humans, some of the merozoites that invade erythrocytes develop into two forms of gametocytes. The developmental time to maturity of gametocytes is the same as that of the asexual stage, approximately 49 h.

During feeding, mosquitoes take up both microgametocytes and macrogametocytes. Within the gut of the mosquito, exflagellation of the microgametocyte occurs, resulting in the formation of up to eight microgametes. Following fertilization of the macrogamete, a mobile ookinete is formed that penetrates the peritropic membrane surrounding the blood meal and travels to the outer wall of the midgut of the mosquito. There, under the basal membrane, the oocyst develops. After a period of several weeks, depending on the temperature, hundreds of sporozoites are produced within each oocyst. The oocyst ruptures, and sporozoites are released into the hemocoel of the mosquito. Circulation carries the sporozoites to the salivary glands, which the sporozoites invade and where they become concentrated in the acinal cells. During feeding, sporozoites are introduced into the salivary duct and are injected into the venules of the bitten human, initiating the cycle again.