Disease. ovale malaria
Geographic Distribution. Worldwide, malaria usually restricted to tropical and subtropical areas and altitudes below 1,500 m. P. ovale is relatively unusual outside of Africa.
Infection rate. WHO estimates that yearly 300-500 million cases of malaria occur and more than 1 million people die of malaria. P. ovale, where it is found, comprises <1% of isolates.
Life cycle. A malaria-infected female Anopheles mosquito inoculates sporozoites into the human host during a blood meal. Sporozoites infect liver cells and mature into schizonts, which rupture and release merozoites (exo-erythrocytic schizogony). In P. vivax and P. ovale a dormant stage (hypnozoites) can persist in the liver for weeks, or even years. The merozoites infect red blood cells. The ring stage trophozoites mature into schizonts, which rupture releasing merozoites (erythrocytic schizogony). Some parasites differentiate into sexual erythrocytic stages (gametocytes). The gametocytes are ingested by an Anopheles mosquito during a blood meal. The microgametes penetrate the macrogametes generating zygotes in the mosquito's stomach. The zygotes become ookinetes and invade the midgut wall where they develop into oocysts. The oocysts grow, rupture, and release sporozoites, which make their way to the mosquito's salivary glands (sporogonic cycle).
Morphology. Ring: sturdy cytoplasm; large chromatin. Trophozoite: compact with large chromatin. Schizont: mature = 6 to 14. Gametocyte: round to oval; compact; may almost fill RBC.
Pathogenesis and clinical symptoms. The disease is caused by the direct effects of red cell invasion and destruction by the asexual parasite and the host's reaction. The symptoms of uncomplicated malaria can be rather non-specific. The most frequent symptoms include fever and chills, which can be accompanied by headache, myalgias, arthralgias, weakness, vomiting, and diarrhea. Other clinical features include splenomegaly, anemia, and thrombocytopenia.
Diagnosis. Microscopic identification is the method most frequently used to demonstrate an active infection. Molecular diagnostic techniques can complement microscopy. Antibody test can detect past (not necessarily active) infections.
Prevention. Personal protection against mosquito bites is the first line of defence against malaria. In addition, travellers should take chemoprophylaxis where appropriate.
Plasmodium ovale trophozoite in a red blood cell with fimbriated edges.
Tai Soon Yong