Spatial Effects on the Multiplicity of Plasmodium falciparum Infections

PLoS One. 2016 Oct 6;11(10):e0164054. doi: 10.1371/journal.pone.0164054. eCollection 2016.

Abstract

As malaria is being pushed back on many frontiers and global case numbers are declining, accurate measurement and prediction of transmission becomes increasingly difficult. Low transmission settings are characterised by high levels of spatial heterogeneity, which stands in stark contrast to the widely used assumption of spatially homogeneous transmission used in mathematical transmission models for malaria. In the present study an individual-based mathematical malaria transmission model that incorporates multiple parasite clones, variable human exposure and duration of infection, limited mosquito flight distance and most importantly geographically heterogeneous human and mosquito population densities was used to illustrate the differences between homogeneous and heterogeneous transmission assumptions when aiming to predict surrogate indicators of transmission intensity such as population parasite prevalence or multiplicity of infection (MOI). In traditionally highly malaria endemic regions where most of the population harbours malaria parasites, humans are often infected with multiple parasite clones. However, studies have shown also in areas with low overall parasite prevalence, infection with multiple parasite clones is a common occurrence. Mathematical models assuming homogeneous transmission between humans and mosquitoes cannot explain these observations. Heterogeneity of transmission can arise from many factors including acquired immunity, body size and occupational exposure. In this study, we show that spatial heterogeneity has a profound effect on predictions of MOI and parasite prevalence. We illustrate, that models assuming homogeneous transmission underestimate average MOI in low transmission settings when compared to field data and that spatially heterogeneous models predict stable transmission at much lower overall parasite prevalence. Therefore it is very important that models used to guide malaria surveillance and control strategies in low transmission and elimination settings take into account the spatial features of the specific target area, including human and mosquito vector distribution.

MeSH terms

  • Alleles
  • Animals
  • Antigens, Protozoan / genetics
  • Culicidae / physiology
  • Humans
  • Insect Vectors / physiology
  • Malaria, Falciparum / transmission*
  • Models, Statistical
  • Plasmodium falciparum / genetics
  • Plasmodium falciparum / physiology*
  • Protozoan Proteins / genetics
  • Spatial Analysis*

Substances

  • Antigens, Protozoan
  • Protozoan Proteins
  • merozoite surface protein 2, Plasmodium

Grants and funding

SK, AB and IM acknowledge support from the Victorian State Government Operational Infrastructure Support and Australian Government NHMRC IRIISS. SK was supported by an NHMRC (Australia) Early Career Fellowship (GNT 1052760). MTW was supported by an MRC (UK) Research Fellowship. The authors thank all researchers and study participants involved in the PNG studies. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.