In natural hosts, protozoal sporozoites injected by the fly vector enter the circulation and undergo schizogony in cells such as Kupffer cell macrophages. These small schizonts form merozoites that either infect blood cells and undergo gametogeny, form additional small schizonts in hepatic cells, or form megaloschizonts in many tissues. 8 Leucocytozoon simondi megaloschizonts are detectable as early as 4 to 6 days after infection. Schizonts and megaloschizonts eventually rupture to release merozoites that infect blood cells and form gametocytes. Leucocytozoon gametocytes may occur in elongate forms that distort the size and shape of infected blood cells or in round forms that leave infected blood cells undistorted.
Gametocytes ingested by the fly vector undergo sporogony to continue the cycle.
A specific arthropod vector for the protozoan described here has not been identified, but probably exists and presumably has not been totally excluded from the screened aviaries that housed the infected birds. Identification of the protozoal parasite described here as a Leukocytozoon or some other species remains to be accomplished. Antibodies useful for tissue immunostains and specific for Leukocytozoon tissue stages are not routinely available. Gametocyte-infected cells have never been identified in blood smears from dead or surviving birds in affected aviaries. There are at least three possible explanations for the failure to identify the gametocyte form of this protozoal pathogen.