الملخص الإنجليزي
Following marked reduction in malaria cases and deaths in the recent years, policies and research interests have now shifted toward elimination and eradication of the disease by 2050. Among the various challenges that face the prospect of malaria elimination/ eradication is the frequent carriage of the parasite as asymptomatic infection at levels not detectable by conventional methods. Thus asymptomatic parasitaemia is a hidden source of infection with great transmission potential. This thesis examined some biological characteristics of asymptomatic parasitaemia and factors affecting their transmission potential (gametocyte production) in seasonal transmission areas. The present work investigated the infection parameters; parasite and gametocyte densities, multiplicity of infection and carriage of drug resistance genotypes among a cohort of 36 asymptomatic carriers, who were initially enrolled with microscopic confirmed P. falciparum during the transmission season, treated and then followed up monthly for 12 consecutive months, including 9 months of transmission-free and absence of therapy. The impact of season, parasite density and within host multiplicity of infection on gametocyte density was evaluated using generalized linear mixed model that consider longitudinal design and allows for extreme variability in gametocyte density between the transmission and the dry season. In addition, the relative abundance of wild-type compared to the mutant genotype of drug resistance genes and its association with gametocytes was examined. Also, the effect of therapy on gametocytogenesis was studied on a cohort of uncomplicated malaria (n=106) treated with chloroquine (CD) or sulfadoxinel pyrimethamine (SP) and then examined on weekly bases; day 0, 7, 14 and 21. qPCR of 18S rRNA was used to quantify parasite density, while gametocyte stage specific genes qRT-PCR assays were used to quantify early (pfgel) and late (pfs25 and pfs230p) gametocytes stages. qPCR allelic discrimination was used to identify alleles of drug resistance genes pfort and pfmdr-1, and qPCR relative quantification was used to estimate relative density of sensitive compared to resistant alleles. Multiplicity of infection was estimated by genotyping of two polymorphic genes pfg377 and msp-2. Among the asymptomatic cohort, low levels of P. falciparum parasitaemia (monthly average < 50 parasites/ul) and gametocytaemia (monthly average of 2-20 gametocytes/ul) persisted for up to 9 months in absence of new infection, during the dry and transmission-free season. However, both gametocyte density and gametocyte parasite ratio increased significantly following the start of annual rains (surrogate for mosquitoes abundance), prior to the appearance of clinical malaria, demonstrating an impact of season on transmission of asymptomatic parasitaemia. Gametocyte production was associated with infections with multiplicity 2 parasites clones). Relative wild-genotype densities of both pfcrt 72-76 and pfmdr-1 86 showed a progressive increase among asymptomatic carriers in absence of therapy during the dry season. Following treatment of uncomplicated P. falciparum malaria with CQ or SP, there was an increase in early gametocyte/parasite ratio on day14 post-treatment among infections with adequate parasitological response despite a > 90% reduction in parasitaemia on D7 indicating enhanced gametocytogenesis following drug
administration. In addition, late gametocyte peaked on day 7 and 14 following treatment suggesting release of sequestrated early gametocytes.
The present results provided clear evidence for season as independent modulator of gametocyte density among asymptomatic P. falciparum carriers, supporting the hypothesis that malaria parasites might enhance gametocytogenesis when the opportunity of transmission occurs. In addition, it quantitatively proved the hypothesis of fitness cost of drug resistance mutations in absence of therapy. The present results also provided some answers to questions regarding the effect of anti malarial drugs on gametocytogenesis. These information are of significant implications for malaria treatment, control and elimination, particularly in seasonal transmission areas
