Species of the genus Leucocytozoan are parasitic protozoa enclosed in the larger phylum Apicomplexa. They use blackflies as a vector and definitive host, and numerous bird species as intermediate hosts.
Sporozoites in the salivary glands of the blackflies are transmitted to the bird when the blackfly takes a bloodmeal. The sporozoites migrate to the liver, where they infect hepatocytes and develop into trophozoites which develop into schizonts, which divide into merozoites after a few days. The merozoites can infect erthrocytes, leukocytes, macrophages, or endothelial cells. Infected macrophage and endothelial cells develop into megaloschizonts, which divide into cytomeres, which multiply again, mature into schizonts, and finally divide into merozoites. Infected erythrocytes and leukocytes develop into gametocytes, which are taken up by the insect vector when it takes a blood meal. They mature in the insect midgut and fuse to form an ookinete, which matures into an oocyst. This produces around 100 sporozoites which migrate to the salivary glands of the blackfly.
These parasites have relatively low virulence, and generally do not kill their host. Anaemia and enlargement of the liver and spleen are common signs of infection, but the majority of infected birds show no signs of infection.