Abstract
BackgroundThe probability of contracting malaria in a given individual is determined not only by the individual's characteristics, but also the ecological factors that characterize the level of human-vector contact in the population. Examination of the relationship between "individual" and "supra-individual" variables over time is important for understanding the local malaria epidemiology. This is essential for planning effective intervention strategies specifically for each location.MethodsA retrospective cohort study was conducted, which followed a community-cohort of about 3,500 residents in seven hamlets along the Thai-Myanmar border between 1999 and 2006. Potential malaria determinants measured at different levels (temporal variables, individual variables, and hamlet variables) were incorporated into multilevel models to estimate their effects on an individual's risk of malaria attack.ResultsThe monthly minimum temperature was significantly associated with the seasonal variation of malaria risk. An individual risk of malaria attack decreased by about 50% during the period that active surveillance was conducted; an additional 15% and 25% reduction of Plasmodium falciparum and Plasmodium vivax incidence, respectively, was observed after the use of artesunate-mefloquine combination therapy (ACT) for treatment of P. falciparum. Male children (age < 16 years old) were at highest risk of both P. falciparum and P. vivax attack. An increase in the hamlet's incidence of P. falciparum and P. vivax by 1 per 100 persons in a previous month resulted in 1.14 and 1.34 times increase in the risk of P. falciparum and P. vivax, respectively, among individuals in a particular hamlet.ConclusionIn a small area with low malaria transmission intensity, the variation in mosquito abundance is relatively similar across the residential areas; incidence of malaria between hamlets, which reflects the community level of human infectious reservoirs, is an important predictor for the malaria risk among individuals within these hamlets. Therefore, local malaria control strategies should focus on interventions that aim to reduce the gametocyte carriage in the population, such as early detection and treatment programmes and the use of ACT for P. falciparum.
Highlights
The probability of contracting malaria in a given individual is determined by the individual's characteristics, and the ecological factors that characterize the level of human-vector contact in the population
The malaria risk factors at community-level, such as vegetation levels, immunity levels, and control programmes, have been widely studied in malaria research, these studies are generally interested in the aggregated outcomes, such as the overall incidence within specified clusters or areas; their results can be confounded by the variability among individuals within clusters and may not reflect the actual risk in an individual [3,4]
While the relations between individual and ecological determinants are of particular interest for understanding malaria epidemiology, it is important to simultaneously incorporate factors defined at multiple levels in epidemiological analysis
Summary
The probability of contracting malaria in a given individual is determined by the individual's characteristics, and the ecological factors that characterize the level of human-vector contact in the population. Examination of the relationship between "individual" and "supra-individual" variables over time is important for understanding the local malaria epidemiology This is essential for planning effective intervention strategies for each location. In high malaria transmission areas, individual immune status, leading to asymptomatic infection in adults, and competent and efficient malaria vectors play an important role in malaria transmission and malaria clinical episodes [1,6]. Environmental conditions, such as altitude, rainfall, humidity, and temperature, which affect the vector density, could have a substantial impact on malaria transmission in these hyper-endemic areas [1]. Studies that examine the effect of the changes in density of human reservoir at community level on the individual risk of malaria have not been established in malaria low-transmission areas
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