Background: The 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.
Methods: Three 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.
Results: Most 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).
Discussion: Spatial 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.