The epidemiology of malaria and challenges to elimination in a low transmission setting in southern Zambia
Searle, Kelly M.
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Background: Recently, malaria has become a major global health priority. As a result there has been renewed interest in malaria control, elimination, and eradication. Zambia is one of the Elimination 8 countries and one of the President’s Malaria Initiative focus countries. Southern Province, Zambia has maintained a parasite prevalence of <10% since 2012, and the National Malaria Control Center made a goal of creating 5 malaria free zones in the province. As areas approach elimination, better understanding of the changing epidemiology of malaria transmission should be used to inform and determine how and where to target specific interventions. Additionally, challenges to elimination need to be evaluated to understand the risk for importation and resurgence of transmission. Methods: The study was conducted in the rural catchment area of Macha Hospital, Choma District, Southern Province, Zambia. First, spatial and temporal trends in passively and actively detected malaria infections were determined. Second, the genetic diversity and complexity of the parasite populations infecting individuals identified through passive and active surveillance was evaluated and compared. Third, a reactive screen-and-treat strategy was evaluated and coverage cascades were developed to inform and improve the intervention. Fourth, the impact of population movement was evaluated using GPS data loggers, in which movement patterns were characterized and quantified, and the amount of time spent in high and low malaria risk was determined. Results: A fractured spatial pattern was detected for both passively and actively detected infected individuals, and temporally stable, space-time clusters were detected, suggesting the presence of ecologically receptive areas. Phylogenetic analysis showed evidence of two distinct parasite populations from infected individuals identified through passive and active surveillance, with genetic diversity decreasing in actively detected infected individuals but not in passively detected cases. In the initial stages of a reactive screen-and-treat strategy, challenges such as poor follow-up and coverage, difficulties in maintaining sufficient RDTs, and poor sensitivity of the RDTs impeded the success of the program and a reactive focal drug administration may be more efficient. Most time was spent in the participant’s household compound, with time spent in high malaria risk areas was dependent on whether or not the house was located in a high malaria risk area. Seasonal movement patterns were observed, with greater long-distance movements during the dry season. Conclusions: Temporally stable ecologically receptive areas remain in malaria elimination settings but the chronically infected population may not be contributing to local transmission. Reactive focal drug administration within index case households may be a more efficient at identifying and treating infected individuals than a reactive test and treat strategy. Population movement patterns have the potential to increase the risk of importation at the end of the rainy season when clinical malaria cases peak; however, the risk of malaria importation is likely to be low throughout the remainder of the dry season.