Re-imagining malaria: heterogeneity of human and mosquito behaviour in relation to residual malaria transmission in Cambodia

Abstract

BackgroundIn certain regions in Southeast Asia, where malaria is reduced to forested regions populated by ethnic minorities dependent on slash-and-burn agriculture, malaria vector populations have developed a propensity to feed early and outdoors, limiting the effectiveness of long-lasting insecticide-treated nets (LLIN) and indoor residual spraying (IRS). The interplay between heterogeneous human, as well as mosquito behaviour, radically challenges malaria control in such residual transmission contexts. This study examines human behavioural patterns in relation to the vector behaviour.MethodsThe anthropological research used a sequential mixed-methods study design in which quantitative survey research methods were used to complement findings from qualitative ethnographic research. The qualitative research existed of in-depth interviews and participant observation. For the entomological research, indoor and outdoor human landing collections were performed. All research was conducted in selected villages in Ratanakiri province, Cambodia.ResultsVariability in human behaviour resulted in variable exposure to outdoor and early biting vectors: (i) indigenous people were found to commute between farms in the forest, where malaria exposure is higher, and village homes; (ii) the indoor/outdoor biting distinction was less clear in forest housing often completely or partly open to the outside; (iii) reported sleeping times varied according to the context of economic activities, impacting on the proportion of infections that could be accounted for by early or nighttime biting; (iv) protection by LLINs may not be as high as self-reported survey data indicate, as observations showed around 40% (non-treated) market net use while (v) unprotected evening resting and deep forest activities impacted further on the suboptimal use of LLINs.ConclusionsThe heterogeneity of human behaviour and the variation of vector densities and biting behaviours may lead to a considerable proportion of exposure occurring during times that people are assumed to be protected by the distributed LLINs. Additional efforts in improving LLIN use during times when people are resting in the evening and during the night might still have an impact on further reducing malaria transmission in Cambodia.

Repeated mosquito net distributions, improved treatment, and trends in malaria cases in sentinel health facilities in Papua New Guinea

“…Age or gender-specific behaviour (e.g., evening activities, division of household chores) or other social or cultural determinants including location and quality of houses may result in different levels of exposure. Such factors have been well investigated in settings in which most of the transmission is limited to specific population groups (e.g., in Southeast Asia [21][22][23]). In highly diverse settings such as PNG, one challenge will be to identify risk factors that disproportionately affect particular population groups, and to translate such knowledge into targeted control action.…”

Effect of user preferences on ITN use: a review of literature and data

“…A study from Cambodia reported that respondents preferred soft, colorful market nets with small mesh over coarse, small, LLINs with larger mesh [36]. These findings were echoed in a willingness-to-pay study in Laos [37] and in Madagascar [38].…”

National Malaria Prevalence in Cambodia: Microscopy Versus Polymerase Chain Reaction Estimates

“…These data demonstrate how the malaria epidemiology can vary according to the ecological environment, the distribution of the vectors species, and human behavior, especially in unstable transmission areas. [22][23][24][25] This trend may be explained partly by the intense efforts at malaria control in Domain 2, where prompt malaria treatment is achieved through the widespread use of malaria RDTs and artemisinin-combined therapies in health facilities and at community level coupled with the increased coverage of LLITNs and indoor residual spraying that have led to a substantial fall in malaria transmission. This may have led to a slowing of the development of malaria immunity in adults who are still at a high risk of contracting malaria because of their work in the forests.…”

“…Strategies to protect high-risk individuals from contracting malaria need to be developed, taking into account that the two main forest-related vectors, Anopheles dirus and Anopheles minumus are outdoor biting mosquitoes. 22,32 Surprisingly, the PCR failed to detect 18 infections (0.23%) found positive by microscopy (five P. falciparum, 12 P. vivax, and one mixed P. falciparum/P. vivax).…”