Submicroscopic Plasmodium prevalence in relation to malaria incidence in 20 villages in western Cambodia

Abstract

Background: Cambodia has seen a marked reduction in the incidence of Plasmodium falciparum over the past decade without a corresponding decline in Plasmodium vivax incidence. It is unknown to what extent local transmission is sustained by a chain of clinical and sub-clinical infections or by continued re-introduction via migration. Using an ultrasensitive molecular technique, 20 villages in western Cambodia were surveyed to detect the low season prevalence of P. falciparum and P. vivax and local treatment records were reviewed.

Methods: During March to May 2015 cross-sectional surveys were conducted in 20 villages in Battambang, western Cambodia. Demographic and epidemiological data and venous blood samples were collected from 50 randomly selected adult volunteers in each village. Blood was tested for Plasmodium infections by rapid diagnostic test (RDT), microscopy and high volume (0.5 ml packed red blood cell) quantitative polymerase chain reaction (uPCR). Positive samples were analysed by nested PCR to determine the Plasmodium species. Malaria case records were collected from the Provincial Health Department and village malaria workers to determine incidence and migration status.

Results: Among the 1000 participants, 91 (9.1%) were positive for any Plasmodium infection by uPCR, seven (0.7%) by microscopy, and two (0.2%) by RDT. uPCR P. vivax prevalence was 6.6%, P. falciparum 0.7%, and undetermined Plasmodium species 1.8%. Being male (adjusted OR 2.0; 95% CI 1.2-3.4); being a young adult <30 years (aOR 2.1; 95% CI 1.3-3.4); recent forest travel (aOR 2.8; 95% CI 1.6-4.8); and, a history of malaria (aOR 5.2; 95% CI 2.5-10.7) were independent risk factors for parasitaemia. Of the clinical malaria cases diagnosed by village malaria workers, 43.9% (297/634) and 38.4% (201/523) were among migrants in 2013 and in 2014, respectively. Plasmodium vivax prevalence determined by uPCR significantly correlated with vivax malaria incidences in both 2014 and 2015 (p = 0.001 and 0.002, respectively), whereas no relationship was observed in falciparum malaria (p = 0.36 and p = 0.59, respectively).

Discussion: There was heterogeneity in the malaria parasite reservoir between villages, and Plasmodium prevalence correlated with subsequent malaria incidence. The association was attributable chiefly to P. vivax infections, which were nine-fold more prevalent than P. falciparum infections. In the absence of a radical cure with 8-aminoquinolines, P. vivax transmission will continue even as P. falciparum prevalence declines. Migration was associated with over a third of incident cases of clinical malaria. Trial registration clinicaltrials.gov (NCT01872702). Registered 4 June 2013.

Keywords: Asymptomatic; Cambodia; Malaria; Plasmodium Falciparum; Plasmodium vivax; Sub-clinical; Sub-microscopic; uPCR.

Figures

Fig. 1

Plasmodium species determined by quantitative PCR in twenty villages in Battambang Province in 2015. Villages were selected for the survey according to their high malaria incidence in 2013–2014 from provincial malaria records; or, for villages with no village malaria worker, on the advice of local health centres which received referred malaria patients

Fig. 2

Parasite genome count by Plasmodium species, tympanic temperature ≥ 37.5 °C and method of detection

Fig. 3

Plasmodium species prevalence by uPCR in 2015 and incidence of malaria cases in 2014. aPlasmodium falciparum prevalence by uPCR and falciparum malaria and mixed malaria incidence 2014; bPlasmodium vivax prevalence by uPCR and vivax malaria incidence 2014; cPlasmodium vivax and undetermined Plasmodium species prevalence by uPCR and vivax malaria incidence 2014; dPlasmodium prevalence by uPCR and malaria incidence 2014. In each graph, X axis represents Plasmodium prevalence by uPCR during March–April 2015 and Y axis represents malaria cases per 1000 population in a 1-year period. API annual parasite incidence. API = (confirmed cases during 1 year/population under surveillance) ×1000. In 2014, *19 out of 20 villages malaria incidence rates were available from Battambang Provincial Health Department and are included in the analyses, except MKO

Fig. 4

Plasmodium species prevalence by uPCR and incidence of malaria cases in 2015. aPlasmodium falciparum prevalence by uPCR and falciparum or mixed malaria incidence 2015; bPlasmodium vivax prevalence by uPCR and vivax Malaria incidence 2015; cPlasmodium vivax and undetermined Plasmodium species prevalence by uPCR and vivax malaria incidence 2015; dPlasmodium prevalence by uPCR and malaria incidence 2015. In each graph, X axis represents Plasmodium prevalence by uPCR during March–April 2015 and Y axis represents malaria cases per 1000 population in a 1-year period, API annual parasite incidence. API = (confirmed cases during 1 year/population under surveillance) ×1000. In 2015, *19 out of 20 villages malaria incidence rates were available from Battambang Provincial Health Department and are included in the analyses, except for PEM

Fig. 5

Clinical malaria cases in the survey villages recorded by village malaria workers from 2013 to March 2015. Plasmodium falciparum mono-infections are red, mixed P. falciparum and P. vivax are orange, and P. vivax mono-infections are green