Abstract
BackgroundIn Cambodia, malaria transmission is low and most cases occur in forested areas. Sero-epidemiological techniques can be used to identify both areas of ongoing transmission and high-risk groups to be targeted by control interventions. This study utilizes repeated cross-sectional data to assess the risk of being malaria sero-positive at two consecutive time points during the rainy season and investigates who is most likely to sero-convert over the transmission season.MethodsIn 2005, two cross-sectional surveys, one in the middle and the other at the end of the malaria transmission season, were carried out in two ecologically distinct regions in Cambodia. Parasitological and serological data were collected in four districts. Antibodies to Plasmodium falciparum Glutamate Rich Protein (GLURP) and Plasmodium vivax Merozoite Surface Protein-119 (MSP-119) were detected using Enzyme Linked Immunosorbent Assay (ELISA). The force of infection was estimated using a simple catalytic model fitted using maximum likelihood methods. Risks for sero-converting during the rainy season were analysed using the Classification and Regression Tree (CART) method.ResultsA total of 804 individuals participating in both surveys were analysed. The overall parasite prevalence was low (4.6% and 2.0% for P. falciparum and 7.9% and 6.0% for P. vivax in August and November respectively). P. falciparum force of infection was higher in the eastern region and increased between August and November, whilst P. vivax force of infection was higher in the western region and remained similar in both surveys. In the western region, malaria transmission changed very little across the season (for both species). CART analysis for P. falciparum in the east highlighted age, ethnicity, village of residence and forest work as important predictors for malaria exposure during the rainy season. Adults were more likely to increase their antibody responses to P. falciparum during the transmission season than children, whilst members of the Charay ethnic group demonstrated the largest increases.DiscussionIn areas of low transmission intensity, such as in Cambodia, the analysis of longitudinal serological data enables a sensitive evaluation of transmission dynamics. Consecutive serological surveys allow an insight into spatio-temporal patterns of malaria transmission. The use of CART enabled multiple interactions to be accounted for simultaneously and permitted risk factors for exposure to be clearly identified.
Highlights
In Cambodia, malaria transmission is low and most cases occur in forested areas
In this study the force of infection was estimated in different ecological settings of Cambodia and Classification and Regression Tree (CART) was used as a novel approach to analyse changes in antibody responses over time to help identify risk factors for malaria exposure during the rainy season
The results highlighted the strong heterogeneity of malaria transmission in time and space (at national as well as local level), suggested that adults were experiencing a higher force of infection than children and showed that CART can be a useful approach for analysing longitudinal serological data
Summary
In Cambodia, malaria transmission is low and most cases occur in forested areas. Seroepidemiological techniques can be used to identify both areas of ongoing transmission and high-risk groups to be targeted by control interventions. Serological indices have demonstrated their use as an informative additional measure [2] and population level sero-prevalence to specific malarial antigens can be used to estimate the force of infection the rate at which individuals become infected - in a given area [3]. These techniques have been exploited to document changes in transmission intensity [4], to identify ‘hotspots’ of malaria transmission [5,6] and, historically, to confirm elimination in Greece and Mauritius [7,8]. Whilst individual responses may fluctuate [9,10,11,12], previous studies suggest that population sero-prevalence remains similar if transmission intensity remains consistent over the years [13]
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