Haematological surveys were carried out in 3 schools in 2 areas where Schistosoma mansoni is endemic in Machakos District, Kenya, before and after a treatment campaign using praziquantel. Earlier clinical impressions of differences in the levels of anaemia between the 2 areas were not confirmed. Although individual haemoglobin levels and haematocrits often fell below international norms, significant anaemia with abnormal red blood cell morphology was rare (<5%), but varied between schools. Altitude could have accounted for some of these differences, but other factors, including diet and parasitism, were involved. Anaemia was associated with splenomegaly and, to a lesser extent, hepatosplenomegaly. Epidemic malaria (mainly Plasmodium falciparum) appeared to be the main cause of parasite-induced anaemia. There was no significant association with the scarce hookworm infections (mainly Necator americanus); nor did the much commoner S. mansoni cause severe anaemia at the community level, but haemoglobin levels dropped as its intensity increased. Treatment with praziquantel eliminated this trend except in a few subjects with splenomegaly alone (probably due to malaria) or with schistosomal hepatosplenic disease. Possible pathogenic mechanisms are reviewed, including the consumption of red blood cells by adult schistosomes as a possible cause of anaemia.

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