The importance of sensitive detection of malaria parasites in the human and insect hosts in epidemiological studies, as shown by the analysis of field samples from Guinea Bissau.

A method based on the polymerase chain reaction (PCR) for highly sensitive detection and identification of human malaria parasites was applied to blood and mosquito samples obtained from a village in Guinea Bissau. The prevalence of parasites in the human population was shown to be greatly underestimated by microscopical examination. In particular, a high incidence of Plasmodium malariae and P. ovale parasites was revealed only by the PCR assay. Preliminary evidence was obtained to show that the distribution of P. malariae infections within the village was non-random. This was supported by analysis of the parasite species infecting the mosquito vector. The implication of these results for the design and interpretation of epidemiological surveys is discussed.