High and low spleen rates distinguish two populations of women living under the same malaria endemic conditions in Madang, Papua New Guinea

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In a malariometric survey of 594 non-pregnant women living in 17 villages in Madang Province, Papua New Guinea, village groups were identified with average spleen rates of <35%, 40–49% and ⩾50%, with no difference in parasite rates between groups. Most villages where a non-Austronesian language was spoken had high spleen rates, but the association was not consistent. Some women were not indigenous to the study area and to control for the effects of migration on spleen rates, women were grouped according to their area of origin. This analysis, which cuts across village groups, identified 2 subpopulations, one with a high, and the other with a low spleen rate (relative risk 2·23; P < 0·0001). Persistent splenomegaly was observed more frequently in the high spleen rate population ( P < 0·025) which also showed a significantly increased spleen size ( P < 0·01). The existence of 2 subpopulations living under the same conditions of malaria endemicity, but with different splenic responses to malaria, suggests an altered host immune response to malaria in the high spleen rate group. The absence of intermarriage between these two subpopulations indicates genetic differences distinguish them.