Plasmodium ovale malaria acquired in Viet-Nam.

Four cases of Plasmodium ovale malaria are reported among US servicemen stationed in Viet-Nam between January 1966 and March 1969. Taken together with other cases cited by the authors, these provide strong evidence of the existence (sometimes disputed) of this Plasmodium in continental South-East Asia.None of the men had served in any other area of endemic malaria and their travel and medical histories suggest that all 4 infections were acquired by mosquito transmission. They constitute only 0.066% of the 6036 malaria cases reported among servicemen returning from Viet-Nam during this period and represent only 0.11% of the blood films from 3686 individuals examined at the US National Malaria Repository during the same period.Serological testing for malaria antibodies with the indirect fluorescent technique corroborated the diagnosis of P. ovale in 1 case. Speciation was not possible in the other 3 cases since titres to P. vivax and P. ovale antigens were identical. Only 1 of the patients reported previous experience with vivax malaria.Most of the parasites seen in thin blood films were developing trophozoites and immature schizonts; ring forms and gametocytes were rare; mature schizonts were not found. The morphology of the parasites was typical of P. ovale, with more than 50% of the infected cells showing fimbriations, an oval shape or both.