Prospective risk of morbidity in relation to multiplicity of infection with Plasmodium falciparum in Sao Tome

The prospective risk of acute morbidity was analysed in relation to multipl icity of Plasmodium falciparum infection in 491 individuals in a peri-urban community in Sao Tome. In an initial cross-sectional survey, 40.5% of indi viduals were recorded by microscopy as infected with P. falciparum, and by PCR 60.5%, with the maximum prevalence in children aged 5-10 years. PCR-RFL P typing of the msp-2 gene of P. falciparum found a mean of 2.4 parasite ge notypes per infected person, with little age dependence in this multiplicit y and a total of 43 different msp-2 alleles identified. None of these were unique for Sao Tome. Study participants were encouraged to report to a proj ect worker whenever they suffered a febrile illness. During the 3 months fo llowing the parasitological survey the recorded incidence rates decreased w ith increasing baseline msp-2 multiplicity. both for P. falciparum-positive episodes and for fever without parasitaemia. While this is consistent with suggestions that multiple P. falciparum infections may protect against sup er-infecting parasites, confounding by patterns of health service usage is an alternative explanation, The incidence of clinical malaria episodes was only a little higher in children than in adults. This weak age-dependence i n clinical immunity might be a consequence of a cohort effect resulting fro m resurgence of the disease after the breakdown of malaria control programs in the 1980s. (C) 2001 Elsevier Science B.V. All rights reserved.