Plasmodium ovale was the last of the malaria parasites of humans to be described. The pronounced stippling of the infected erythrocyte and its tertian periodicity led early investigators to consider it a variant form of Plasmodium vivax. In 1900, Craig (32) described a malaria parasite in the blood of American soldiers returning from the Philippines that had peculiar morphological characteristics and a tertian fever pattern. It is possible that he was describing infections with P. ovale. Macfie and Ingram in 1917 (64) described a parasite in the blood of a child in the Gold Coast that may also have been P. ovale. Subsequently, Stephens (90) observed in the blood of an East African patient some erythrocytes that were oval and with fimbriated edges. In 1922, he published a full description of the forms in the blood and named the parasite P. ovale in recognition of the oval shape of some of the infected erythrocytes.
Some investigators were slow to recognize P. ovale as a distinct species (40). However, subsequent detailed studies confirmed the validity of the species (48, 49, 50, 51, 52, 86, 91). Following the establishment of the Donaldson strain of the parasite for use in malaria therapy for the treatment of patients with neurosyphilis, additional detailed studies on the morphology and periodicity of the parasite were made. The Donaldson strain of P. ovale was isolated from a returning serviceman who had acquired the infection in the western Pacific, probably the Philippines (53, 57, 59, 96). Plasmodium ovale is seldom seen except in sub-Saharan Africa and on some islands of the western Pacific. The movement of human populations poses the possibility of its presence and establishment in other tropical regions where susceptible vectors may be present. Reported here is a summary of the biology, morphology, diagnosis, and experimental vectors of the parasite.