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

Abstract

Abstract. 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.In the poor savannah area of eastern Sudan, the malaria transmission season is short and confined to approximately three rainy months of the year. In this region, the epidemiology of malaria is consistent with that of a hypoendemic area, where transmission occurs only following rains. A brief expansion of the mosquito population at this time leads to malaria outbreaks in October and November each year. However, during the long dry season from January to June, apart from a few clinical reports, very little is known about parasite reservoirs, or whether mosquitoes continue to transmit the parasite. A previous entomologic study suggested that the main mosquito vector, Anopheles arabiensis, retracts to scanty breeding sites where it can survive the dry season with continuous feeding activity but an incomplete reproductive cycle. 1 Nothing is known about how the long dry season influences the dynamics and the structure of the parasite populations in this region.During the past seven years, we have carried out regular surveys of the Plasmodium falciparum population in October-November in Asar village in eastern Sudan. 2,3 The principal findings have been that 1) a considerable amount of allelic polymorphism exists among the genes of parasites infecting patients at this time of the year, 2) no two patients are infected with identical parasite clones, and 3) most patients are infected with a mixture of parasite clones. A great diversity was found even among parasites that caused the very early malaria cases in the transmission season. 3 Such a level of diversity suggests that the parasites that appear at this time of the year may belong to a large parasite reservoir existing prior to the beginning of the rainy season. A recent study in a neighboring village has shown that sub-pa...

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