High rate of mixed and subpatent malarial infections in Southwest Nigeria

The rate of malarial parasitemia in children and adults was assessed by mic roscopy and the polymerase chain reaction in a holoendemic area in Nigeria, A high rate of subpatent Plasmodium falciparum parasitemia (19.6%) was fou nd. Plasmodium malariae and P. ovale infections were common in a rural area (26.1% and 14.8%) but were observed sporadically in individuals from an ur ban area. Simultaneous infections with P. falciparum, P. malariae, and P. o vale were frequent in the rural area (11.7% triple infections). The rate of triple infections was higher than expected from the prevalences of each sp ecies (P < 0.00001). Spleen enlargement was associated with mixed infection s of P. falciparum and P. malariae (odds ratio [OR] = 5.9, 95% confidence i nterval [CI] 3.0-11.7) and less frequently observed in individuals without detectable parasitemia (OR = 0.06, 95% CI = 0.01-0.3). Spleen enlargement a nd titers of antibodies to schizonts were positively correlated with parasi te densities, The results also suggest that in some individuals a long-last ing subpatent parasitemia might occur.