Transmission risk beyond the village: entomological and human factors contributing to residual malaria transmission in an area approaching malaria elimination on the Thailand-Myanmar border

Abstract

Background: A mixed methods study was conducted to look at the magnitude of residual malaria transmission (RMT) and factors contributing to low (< 1% prevalence), but sustained transmission in rural communities on the Thai-Myanmar border.

Methods: A cross-sectional behaviour and net survey, observational surveys and entomological collections in both villages and forested farm huts frequented by community members for subsistence farming practices were conducted.

Results: Community members frequently stayed overnight at subsistence farm huts or in the forest. Entomological collections showed higher biting rates of primary vectors in forested farm hut sites and in a more forested village setting compared to a village with clustered housing and better infrastructure. Despite high levels of outdoor biting, biting exposure occurred predominantly indoors, particularly for non-users of long-lasting insecticidal nets (LLINs). Risk of biting exposure was exacerbated by sub-optimal coverage of LLINs, particularly in subsistence farm huts and in the forest. Furthermore, early waking hours when people had left the safety of their nets coincided with peaks in biting in later morning hours.

Conclusions: Entomological and epidemiological findings suggest drivers and modulators of sustained infection prevalence in the area to be: higher mosquito abundance in forested areas where LLINs were used less frequently or could not be used; late sleeping and waking times coinciding with peak biting hours; feeding preferences of Anopheles taking them away from contact with LLIN and indoor residual spraying (IRS), e.g. exophagy and zoophagy; non-use of LLIN and use of damaged/torn LLIN; high population movement across the border and into forested areas thereby increasing risk of exposure, decreasing use of protection and limiting access to healthcare; and, Plasmodium vivax predominance resulting in relapse(s) of previous infection. The findings highlight gaps in current intervention coverage beyond the village setting.

Keywords: Forest malaria; Malaria; RMT; Vector behaviour; Vector control.

Fig. 1

Location and malaria prevalence level of the study site within the Greater Mekong Subregion

Fig. 2

Map of entomological collection sites across villages and forested farm huts. Map shows study villages (yellow stars) and farm hut entomology collection sites (red circles). The river marks the boundary between Thailand and Myanmar

Fig. 3

Transect walk routes. Map of Pha Man hamlet (left) and Suan Oi village (right) with sites of mosquito collections and route of transect walks

Fig. 4

Sleeping and waking times of community members. Box plots show median, interquartile range (IQR) and range of sleeping and waking times among respondents aged ≥ 18 years

Fig. 5

Zoophagy rates (%) of the primary and secondary vector species captured by cattle and outdoor human landing catch in the village study sites

Fig. 6

Hourly biting profiles of primary vector species captured by HLC. Plots show hourly biting rates of primary vector species caught by IHLC (blue line) and OHLC (black line) in three capture sites: Suan Oi village (left-hand column); Pha Man hamlet (middle) and Farm huts (right-hand column). Yellow star represents where a Plasmodium-positive sample was captured. Shaded grey area is time outside of median sleeping hours reported by adults in the cross-sectional survey

Fig. 7

Map showing abundance and species composition in each of the forested farm hut collection sites. Pie charts each represent the location of one of the six forested farm hut collection sites, denoted F1–F6. Total number of anophelines captured in each location is shown in brackets while pie charts show the breakdown by species. Orange line represents a 1 km buffer zone around the six sites. Green stars show each study village and green line is the international border between Thailand and Myanmar

Fig. 8

Activities conducted outside of the household (shaded squares) per hour of the night in Suan Oi (top) and Pha Man (bottom)

Fig. 9

Indoor and outdoor biting exposure per hour of the night in Pha Man from An. maculatus and An. minimus for users and non-users of LLINs. Stacked lines show: indoor biting exposure for non-users of ITN (white area), indoor biting exposure for users of ITN (grey area) and outdoor biting exposure (black area). Pie charts show: proportion of total nightly biting exposure occurring indoors and outdoors for users and non-users of LLINs