A Comparison of Different Parasitaemia Levels on Malaria Transmission Post-asexual Drug Treatment

Abstract

Malaria remains a public health concern despite remarkable progress towards its control. The current anti-malarial drug treatment, artemisinin-based combination therapy (ACT) is effective in clearing asexual stages and immature sexual stages, gametocytes, of Plasmodium falciparum but has limited effects on mature gametocytes which are responsible for human to mosquito transmission. Asymptomatic infections, generally of low parasitaemia, are common in endemic areas and yet their contribution to malaria transmission is not fully known. Moreover, the association between different parasitaemia levels and gametocyte carriage is not yet fully understood. Here, we determine gametocyte carriage of individuals with low parasitaemia and/or asymptomatic infections, kinetics of gametocyte appearance and clearance post-asexual anti-malarial drug treatment. Asexual parasitaemia in asymptomatic individuals was determined by quantitative polymerase chain reaction (PCR) targeting 18s ribosomal DNA in a Malaria Parasite Clearance study conducted along the Kenyan coast, an area of low to moderate transmission, between November 2013 and February 2014. A total of 90 healthy adults aged between 18-50 years were recruited and randomized to receive the following anti-malarial drugs: 1) atovaquone-proguanil and artesunate (for 3 days); 2) artesunate (for 7 days); and 3) sulphadoxine-pyrimethamine, artesunate and primaquine (1 dose/ 3 days/ 1 dose); and monitored for a period of 84 days. Samples from this study, covering 20 time points, were then used to determine gametocyte carriage as detected by reverse transcriptase polymerase chain reaction (RT-PCR) targeting the Pfs25 messenger RNA. Analysis of gametocyte kinetics was confined to subjects with more than two time points positive for asexual parasites (N=44 individuals) during the study period. Gametocyte prevalence was 25% (11/44) at enrollment. Individuals with gametocytaemia at enrollment presented with no carriage by day 9 post-treatment irrespective of the drug administered. Gametocyte carriage acquisition during follow up (up to 84 days) was 81.3% (26/44) for participants negative at enrollment irrespective of treatment (32/44). There was a significant difference between gametocyte proportions shortly after treatment (7 days post-treatment, 45.5% (20/44)) and at follow up (84 days post-treatment, 77.3% (34/44), p= 0.0022). After stratifying for different parasite levels in individuals presenting with parasites between 100-50,000 parasites/μl (moderate/microscopic parasitaemia), these parasite densities were positively associated with the prevalence of gametocyte carriage (p=0.0004) whilst a negative association was observed in individuals

presenting with much higher densities (>50,000 parasites/μl). On the other hand, low level asexual parasitaemia (≤100 parasites/μl) was more likely to be positive for sexual stages than those above 100 parasites/μl (p= 0.1557). Low level asexual parasitaemia, here defined as ≤100 parasites/μl, is more likely to result in gametocyte carriage post-treatment in our setting, an area of changing malaria epidemiology. Even after anti-malarial drug treatment, there is an increased likelihood to develop gametocytes for those not presenting with sexual parasitaemia at baseline.