Molecular Epidemiology of Chronic Plasmodium falciparum Infections in an eastern Sudanese Village

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Amel Awad Hamad, Karar
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Chronic Plasmodium falciparum infections in Daraweesh village (eastern Sudan), an area of seasonal and unstable malaria transmission, were monitored and genetically characterized to study the influence of persistent infections on the immunology and epidemiology of low endemicity malaria. During October to December malaria season of 1996, 12% out of a population of 420 had slide confirmed and treated P. falciparum infection. In a cross sectional survey carried out at the end of the transmission season in December 1996, additional 6 individuals were found to harbour microscopically negative but polymerase chain reaction (PCR) positive Plasmodium falciparum infection. On the 1st of January 1997 a cohort of 43 individuals aged 9-53 recruited from these malaria infected individuals agreed to donate fortnightly blood samples for the next nine months. The first six months encompass the long dry season when transmission falls to essentially undetectable levels. Each blood sample was tested fortnightly for the presence of persistent malaria infection by microscopy and PCR sSlide-positive samples were genotyped using PCR assays that detect allelic polymorphism at the MSP-l, MSP-2 and GLURP marker gene loci. Of the 43 individuals 16 (37.2%) were found to maintain chronic P. falciparum infections detected by both microscopy and PCR, these infections were continuously genetically characterized for each individual from the initial infection right through 9 months encluding the dry season. The PCR genotyping data of the 16 individuals with chronic infections was categorized into three groups. Some of those individuals were found to carry out one parasite clone from the initial infection onwards. Some were found to harbour asymptomatic ally either multiple parasite clones of the same type or of different types in the initial infection that cleared after commencing standard dose of drug treatment to become chronically infected with only one drug resistant parasite clone. The third group showed mixtures of the same and/ or different parasite clone(s) that remained from the initial multiple infections appearing and disappearing in the longitudinal follow up with no change of drug. There was no significant relation of fever or fever complaints to chronic infection or to age or sex, although the initial parasite dose seemed to have a slight trend to be correlated to chronic infections. With respect to new infections in the following transmission season, no significant difference was observed between those who had chronic or cleared infections in the previous season. This indicates that chronic infections were not protective against super infection, at the following seasons out breaks. Some individuals got infected with the same parasite type but of different clone only identified by the sequence analysis of their per products. The implications of such persistent dry season infections for the immunology and epidemiology of malaria in the Sahel are considered.
Molecular Epidemiology of Chronic Plasmodium falciparum Infections in an eastern Sudanese Village