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The Plasmodium falciparum parasite rate (PfPR) is a commonly reported index of …

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- Standardizing estimates of the Plasmodium falciparum parasite rate

The intensity of malaria transmission by its anopheline vectors varies enormously [1], and this affects most quantitative aspects of malaria epidemiology [2]. A commonly used index of malaria transmission intensity is the Plasmodium falciparum parasite rate (PfPR), the proportion of the population found to carry asexual blood-stage parasites. In mathematical models, PfPR is related to the entomological inoculation rate (EIR), the number of bites on a person by sporozoite positive vectors, at the steady state [3,4]. The notion of a steady state has limited application for PfPR, however, because PfPR follows a well-established pattern as a function of age and transmission intensity; PfPR rises during infancy and early childhood [5,6], settles to a plateau in older children, and declines in adolescents and adults as malaria immunity develops [7-10]. This pattern has been known for decades [11,12], but there are no established standards for reporting PfPR, so thousands of studies have reported crude PfPR, without stratifying by age [13]. As a result, the different age-ranges over which studies have reported PfPR make it difficult to compare prevalence estimates at different times and places. Assemblies of PfPR data aimed at describing the current global distribution of malaria endemicity [13], therefore, need a mechanism to standardize these data to a consistent age-grouping in order to be meaningfully summarized.

Mathematical models with slowly acquired immunity qualitatively reproduce the empirical relationships between age and PfPR [14-17]. The models suggest that EIR (or the force of infection) determines both the rate that PfPR rises in children and the level of the plateau, so either measure would provide a reliable index of transmission intensity. The PfPR in children is correlated with EIR [4], and the PfPR in children aged 2–10 has provided a basis for the classical categorical measures of malaria transmission: hypoendemic (<10%), mesoendemic (10–50%), and hyperendemic (50–75%) [18]. Holoendemicity refers to PfPR >75% in those less than 12 months old [18].

The variable way of reporting PfPR also limits the number of studies where PfPR can be used to index transmission intensity, which poses a particular problem for recent approaches aimed at using empirical PfPR data to model the spatial distributions of malaria transmission intensity at regional [19-21] or potentially at global scales [22]. For example, maps of transmission intensity in Africa are often based on studies of PfPR that include only children [23,21,25]. The evidence base for mapping malaria, the geographical extent and coverage of places where PfPR has been measured directly, could all be greatly expanded if it were possible to standardize crude PfPR in studies that include adults. The consistent pattern apparent in age-stratified PfPR data suggests that it is amenable to mathematical or statistical modeling. A set of candidate algorithms was derived and evaluated in an attempt to provide a single evidence-based method for converting crude estimates of PfPR to a standard age range for improving future global comparisons of malaria risk using existing extensive PfPR data [13].

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