Plasmodium ovale generally produces a malaria that because of its pronounced stippling of infected red blood cells and tertian paroxysms, was once thought to be a variant of Plasmodium vivax. Humans are considered the only natural hosts for P. ovale. The organism produces is a relapsing infection in that, much like P. vivax, secondary infections can be generated from latent parasites in the liver. These relapsing infections are often asymptomatic.
Disease / Pathogenesis
Plasmodium ovale is one of the least common of the Plasmodium causing malaria. Plasmodium ovale causes paroxysms that are tertian (paroxysms that typically recur every 48 hours or every third day, reckoning the day of the paroxysm as the first). Symptoms are related to these paroxysms due to destruction of red blood cells, production of toxic waste products, and possible hypoxic effects on affected organs. Symptoms include shaking chills, fever and generalized diaphoresis followed by the resolution of the fever. In small cases there may be involvement of the brain (cerebral malaria) in which there is disorientation, progressing delirium, coma and possible death.
Location in the Host
Plasmodium ovale is an infection of the red blood cells and/or the reticuloendothelial system by the parasite. This Plasmodium tends to infect young red blood cells and therefore the infected cells will be enlarged (1.25-1.5 times larger).
Plasmodium ovale is most common in tropical areas of western Africa. Is also found in Papua New Guinea, the Philippines, and Indonesia. The incidence of this malaria is significantly less then the more common P. vivax and P. falciparum. 3% of all malaria disease diagnosed are Plasmodium ovale.
Plasmodium ovale infects young red blood cells and therefore the red blood cells will show a detectable enlargement to them as with P. vivax. Parox
Morphology & Diagnosis
Plasmodium ovale infected red blood cells will characteristically have ragged edges (fimbriated). Because red blood cells infected are young they can be slightly enlarged (up to 1 1/4x) in size or normal in size. The infected red blood cells, much like those of Plasmodium vivax, can have Schuffner’s dots visible in them when stained with Giemsa stain.
Trophozoites in the red blood cells tend to be compact while ring forms are described as having a sturdy cytoplasm and a prominent chromatin dot.
Schizonts will have between 6-14 merozoites present whereas Plasmodium vivax will have between 12-24 merozoites.