Prospective risk of morbidity in relation to multiplicity of infection with Plasmodium falciparum in São Tomé

The prospective risk of acute morbidity was analysed in relation to multiplicity of Plasmodium falciparum infection in 491 individuals in a peri-urban community in São Tomé. In an initial cross-sectional survey, 40.5% of individuals 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-RFLP typing of the msp-2 gene of P. falciparum found a mean of 2.4 parasite genotypes per infected person, with little age dependence in this multiplicity and a total of 43 different msp-2 alleles identified. None of these were unique for São Tomé. Study participants were encouraged to report to a project worker whenever they suffered a febrile illness. During the 3 months following the parasitological survey the recorded incidence rates decreased with 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 super-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 in clinical immunity might be a consequence of a cohort effect resulting from resurgence of the disease after the breakdown of malaria control programs in the 1980s.