Characteristics of Plasmodium falciparum parasites that survive the lengthy dry season in eastern Sudan where malaria transmission is markedly seasonal.

We have examined 83 inhabitants of Asar village in eastern Sudan, where malaria transmission lasts approximately 2-3 months each year, for the presence of Plasmodium falciparum during the prolonged dry season. All patients were treated with a standard dose of chloroquine following the first diagnosis, then examined by microscopy and the polymerase chain reaction (PCR) every two weeks for the first two months and subsequently once each month for the next 15 months throughout the dry season until the following transmission season. The PCR primers used amplified polymorphic regions of the merozoite surface protein-1 (MSP-1), MSP-2, and glutamate-rich protein genes. Results show that subpatent and asymptomatic parasitemias persisted in some patients for several months throughout the dry season, often as genetically complex infections. Different genotypes could coexist together in a single infection and the proportions of each could fluctuate dramatically during this period. However, in some individuals, single genotypes appeared to persist for several months. Reappearance of clinical symptoms among patients with chronic infections was often associated with appearance of new alleles, indicating reinfections with parasites of novel genotypes.