Synonyms: Plasmodium praecox Grassi and Feletti, 1890. (Bray, 1957 considered the correct specific name to be praecox, but he continued to use relictum.)
Disease: Pigeon malaria.
Hosts: Pigeon, mourning dove, pintail, cinnamon teal, falcated duck, black swan, and various passerine and other wild birds. This species was first described from the English sparrow. Experimental infections have been studied in the canary, duck, chicken and other birds.
Location: Erythrocytes. The exo-erythrocytic stages are in endothelial cells.
Geographic Distribution: Worldwide.
Prevalence: This species is common in wild passerine birds. It occurs occasionally in domestic pigeons, having been found in them by Sergent and Sergent (1904) in Algeria, Coatney (1938) in Nebraska, Herman et al. (1954), Mathey (1952) and Graue (cited by Levi, 1957) in California, Becker, Hollander and Pattillo (1956) in Iowa, Pelaez et ah (1951) in Mexico, Cassamagnaghi (1950) in Uruguay, Haiba (1946, 1948) in Egypt, and Rousselot (1943) in the French Sudan.
Morphology: The gametocytes and schizonts are round or irregular. The host cell nucleus is displaced and often expelled by the larger forms. The pigment granules of the gametocytes are relatively fine and dot-like. The schizonts produce 8 to 32 merozoites, the number depending on the particular strain.
Life Cycle: The life cycle of this species has been studied extensively (Bray, 1957). The exoerythrocytic stages occur in the endothelial cells. The asexual cycle has been reported to take from 12 to 36 hours in different strains; some have a very high and others a low degree of synchronicity (Hewitt, 1940). Many species of mosquitoes can act as vectors. Huff (1954) listed 12 of Culex, 4 of Anopheles, 3 of Aedes and 2 of Culiseta, and remarked that 93% of the species which had been tested had been found susceptible.
Pathogenesis: P. relictum is highly pathogenic for the pigeon but less so for the mourning dove and canary. Affected squabs become weak and anemic, with enlarged and heavily pigmented spleens and livers. Pigment may also be deposited in the fat. Hill (1942) showed that anemia is the principal cause of death.