A multi-level spatial analysis of clinical malaria and subclinical Plasmodium infections in Pailin Province, Cambodia

Abstract

BackgroundThe malaria burden is decreasing throughout the Greater Mekong Subregion, however transmission persists in some areas. Human movement, subclinical infections and complicated transmission patterns contribute to the persistence of malaria. This research describes the micro-geographical epidemiology of both clinical malaria and subclinical Plasmodium infections in three villages in Western Cambodia.MethodsThree villages in Western Cambodia were selected for the study based on high reported Plasmodium falciparum incidence. A census was conducted at the beginning of the study, including demographic information and travel history. The total population was 1766. Cross-sectional surveys were conducted every three months from June 2013 to June 2014. Plasmodium infections were detected using an ultra-sensitive, high-volume, quantitative polymerase chain reaction (uPCR) technique. Clinical episodes were recorded by village health workers. The geographic coordinates (latitude and longitude) were collected for all houses and all participants were linked to their respective houses using a demographic surveillance system. Written informed consent was obtained from all participants.ResultsMost clinical episodes and subclinical infections occurred within a single study village. Clinical Plasmodium vivax episodes clustered spatially in each village but only lasted for a month. In one study village subclinical infections clustered in geographic proximity to clusters of clinical episodes. The largest risk factor for clinical P. falciparum episodes was living in a house where another clinical P. falciparum episode occurred (model adjusted odds ratio (AOR): 6.9; CI: 2.3–19. 8). Subclinical infections of both P. vivax and P. falciparum were associated with clinical episodes of the same species (AOR: 5.8; CI: 1.5–19.7 for P. falciparum and AOR: 14.6; CI: 8.6–25.2 for P. vivax) and self-reported overnight visits to forested areas (AOR = 3.8; CI: 1.8–7. 7 for P. falciparum and AOR = 2.9; CI: 1.7–4.8 for P. vivax).DiscussionSpatial clustering within the villages was transient, making the prediction of spatial clusters difficult. Interventions that are dependent on predicting spatial clusters (such as reactive case detection) would only have detected a small proportion of cases unless the entire village was screened within a limited time frame and with a highly sensitive diagnostic test. Subclinical infections may be acquired outside of the village (particularly in forested areas) and may play an important role in transmission.

Reactive Case Detection (RACD) and foci investigation strategies in malaria control and elimination: a review

“…Significant predictors of Plasmodium vivax infection, diagnosed by PCR, were fever (p = 0.058) and history of malaria infection (p < 0.001) [ 29 ]. In another study in Western Cambodia, the most important risk factor for clinical P. falciparum episodes was living in a house where another clinical P. falciparum episode occurred (adjusted odds ratio (AOR):6.9; 95% CI 2.3–19.8) [ 36 ]. Sub-clinical infections of both P. vivax and P. falciparum were associated with clinical episodes of the same species (AOR: 5.8; 95% CI 1.5–19.7 for P. falciparum and AOR: 14.6; 95% CI 8.6–25.2 for P. vivax ) and self-reported overnight visits to forested areas (AOR = 3.8; 95% CI 1.8–7.7 for P. falciparum and AOR = 2.9; 95% CI 1.7–4.8 for P. vivax ) [ 36 ].…”

“…In another study in Western Cambodia, the most important risk factor for clinical P. falciparum episodes was living in a house where another clinical P. falciparum episode occurred (adjusted odds ratio (AOR):6.9; 95% CI 2.3–19.8) [ 36 ]. Sub-clinical infections of both P. vivax and P. falciparum were associated with clinical episodes of the same species (AOR: 5.8; 95% CI 1.5–19.7 for P. falciparum and AOR: 14.6; 95% CI 8.6–25.2 for P. vivax ) and self-reported overnight visits to forested areas (AOR = 3.8; 95% CI 1.8–7.7 for P. falciparum and AOR = 2.9; 95% CI 1.7–4.8 for P. vivax ) [ 36 ]. In the Amazon Basin of Brazil, subjects in index and neighbouring households were significantly more likely to be parasitaemic than control household members, after adjusting for potential confounders, and together harboured > 90% of the P .…”

How can interventions that target forest-goers be tailored to accelerate malaria elimination in the Greater Mekong Subregion? A systematic review of the qualitative literature

“…The findings provide an overview of qualitative research on behaviours and perceptions that influence uptake of and adherence to malaria prevention and control interventions among forest-goers in the GMS. The review focused on this group because they are deemed the priority population for the region’s malaria elimination [13]: forest-goers are at particular risk of sub-clinical malaria infections [39] and report sub-optimal use of preventative interventions, particularly ITNs [14]. As a result of the greater infection outside villages, forest-goers are at particular risk of Plasmodium vivax.…”

Asymptomatic Natural Human Infections With the Simian Malaria Parasites Plasmodium cynomolgi and Plasmodium knowlesi

“…For example, Malaysia has done very well to eliminate nearly all human malaria but is left now with residual P. knowlesi in the monkey population as a source of human infections [6]. Submicroscopic malaria parasite infections [7] are important contributors to malaria endemicity in areas of generally low seasonal transmission [8, 9]. In the course of conducting malariometric surveys in western Cambodia, we discovered asymptomatic human infections with both P. knowlesi and P. cynomolgi and present here a molecular evaluation of these infections.…”