Hereditary ovalocytosis and reduced susceptibility to malaria in Papua New Guinea

Ovalocytosis, an hereditary condition in which most erythrocytes are oval in shape, is a polymorphism that occurs in up to 20% or more of the population in Papua New Guinea and Malaysia. Due to the geographical correlation of the trait with endemic malaria, the possibility of a selective advantage in resistance to malaria has been raised. In a study of 202 individuals with ⩾50% oval red cells matched by age, sex and village of residence with controls having ⩽30% oval cells, ovalocytic subjects had blood films negative for Plasmodium vivax (P = 0·009), for P. falciparum (P = 0·044), and for all species of malaria parasites (P = 0·013), more often than controls. Among individuals parasitaemic at any time there were no clear differences in density of parasitaemia. However, in children 2 to 4 years old, parasite densities of both species were lower in ovalocytic subjects than in controls (0·01 < P < 0·025). The differential susceptibility to malaria infection suggested by this study has implications for the evaluation of interventions, including possible future vaccine field trials, in populations where high-frequency ovalocytosis is present.