- Open Access
Re-imagining malaria: heterogeneity of human and mosquito behaviour in relation to residual malaria transmission in Cambodia
© Gryseels et al.; licensee BioMed Central. 2015
Received: 30 September 2014
Accepted: 10 April 2015
Published: 24 April 2015
In 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.
The 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.
Variability 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.
The 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.
The effectiveness of existing malaria control measures has led to new radical challenges in settings where malaria has successfully been reduced. Residual transmission, defined as persisting transmission after full coverage of long-lasting insectical nets (LLIN) or indoor residual spraying (IRS) has been achieved, challenges malaria elimination [1,2]. Standard control programmes and measures seldom consider human heterogeneity such as local sociocultural variability, examples of which are mobility , the lower than expected uptake of preventive measures in certain vulnerable populations [4,5] and difficulties achieving optimal adherence to anti-malarial treatment [6-8]. Low malaria transmission in pre-elimination contexts might additionally cause a decreasing perception of risk, potentially leading to a lower use of malaria preventive measures [9,10]. In addition, vector behaviour has proven to be equally heterogeneous, as it can adapt to and thus challenge standard vector control tools [1,11,12]. After the scaling-up of LLINs in Southeast Asia, remaining vector populations developed a propensity to predominantly feed early and/or outdoors, as well as rest outdoors after feeding, fundamentally limiting the effectiveness of LLIN and IRS [13,14].
These challenges invoke memories of the Global Malaria Eradication Programme embarked upon in 1955 [2,15,16]. Despite significant local successes, in many regions malaria resurged after the abandonment of control programmes, hitting hard on populations that had lost much of their immunity against the disease . After the success of vector control with DDT, which transformed endemic areas into settings with a “manageable proportion” of infective cases– a situation comparable to the current low transmission context of Southeast Asia – interventions that successfully interrupted transmission were still lacking [15,16]. These failures strongly related to the application of a centrally defined and standardized plan to locally diverse epidemiological, entomological and socio-cultural contexts. The importance of this local heterogeneity, including social and cultural barriers and vector behaviour, which prevented success in the past, must not be forgotten as this is once again key to reaching the current goal of elimination [15-17].
Due to the changing malaria epidemiology, outdoor transmission is becoming an important focus for malaria control strategies today. In the Greater Mekong Region, malaria is now mainly reduced to forested regions, populated by ethnic minorities [4,9,11,18], where residual transmission is hypothesized to mainly occur by vectors that are active during early evening and morning hours when people are not sleeping in mosquito nets. In addition, the high diversity of potential vector species together with highly heterogeneous malaria vector behaviour both between and within vector species, complicates the malaria epidemiology [1,11]. The same vectors can behave differently in different villages in the same region, which is due to vector biology, ecology, human behaviour and the presence of vector control [12,13].
The evolving interplay of vector and human behaviour in Southeast Asia’s remaining transmission foci (i.e. forested areas) challenges elimination goals , echoing problems encountered during the first eradication campaign. Starting from the premise that, in order to target residual transmission in these low transmission settings, a clear understanding is needed of when and how humans and vectors meet, the focus of this research was on the sociocultural heterogeneity in Ratanakiri province, Cambodia, representing a transmission zone in a country otherwise well on its way towards a pre-elimination phase. This paper presents the results from an anthropological study and an entomological study, which were ancillary to an epidemiological trial investigating the effectiveness of the mass use of topical repellents in addition to the use of LLINs in controlling malaria infections.
Study site and population
Cambodia is inhabited by approximately 90% ethnic Khmer. However, there is a small ethnic minority population located mostly in the northeast, in Ratanakiri and Mondulkiri provinces. These populations are part of a larger cultural area, which extends from Laos in the north through the central highlands of Vietnam in the east and finally Ratanakiri and Mondulkiri in the south . The largest ethnic groups in Ratanakiri are the Jarai, the Tompuon and the Kreung, each with a distinct language and cultural system in terms of kinship and political organization . A characteristic shared by these ethnic groups is that they usually combine slash-and-burn agriculture with hunting, fishing and gathering various forest products. As distances between farms in the forest, rice fields and villages can be substantial, many families maintain residences at each location, and move from one place to another according to the agricultural cycle and the forest farm or rice field’s requirements [8,21].
Malaria transmission is perennial with two peaks, June-July and October-November, the rainy season lasting from May to October. At the end of the malaria season of 2012, the overall PCR prevalence in Ratanakiri, as recorded by the MalaResT study (cfr. Infra), was estimated at 4.9% . Species-specific areas with elevated risk of infection have been detected for all Plasmodium species. The clusters for falciparum, vivax and ovale malaria appear in the north of the province along the main river, while the cluster for Malariae is situated in the south of the province . The primary vectors in Ratanakiri are Anopheles minimus and Anopheles dirus, and many secondary vectors are present. These vectors are generally exophagic and exophilic, and their densities and behaviour vary extensively per village . Early biting proportion (EBP), which is calculated as the biting activity before 22.00 (i.e. assumed human sleeping time), was observed to be around 50% in Ratanakiri in 2005, the proportion of infectious bites before 22.00 was 29% .
This study took place in the framework of an intervention trial (MalaResT), which aimed to raise evidence on the effectiveness of the mass use of topical repellents in addition to LLINs to reduce malaria infections. In this intervention trial, 113 of the most endemic villages in Ratanakiri were randomly assigned to a control arm, in which every household received one LLIN per one person, or to an intervention arm, where in addition to LLINs topical repellents were distributed biweekly to every household one bottle per one person. For the epidemiological trial, sample size calculations were based on an expected outcome of 40% difference in malaria prevalence between intervention and control arm. During each of the four malariometric surveys organized in the trial, the aim was to collect blood samples of 65 randomly selected participants within each community. Blood samples were analysed using PCR detection in a mobile laboratory, allowing for a sensitive and rapid malaria diagnostic strategy in the field , alongside a small questionnaire on overnight stays at different locations in the month prior to the survey.
The research used a sequential mixed methods study design in which qualitative ethnographic research and quantitative survey research methods were used to complement the qualitative findings (in standard annotation [qual - > quan]) . Qualitative ethnographic data were collected in local communities to acquire an in-depth understanding of the study setting and population while a cross-sectional and a structured observation survey aimed at quantifying relevant variables from the qualitative study. In addition, a malariometric survey was performed, thus enabling to link previously determined relevant variables to malaria infection, and entomological surveys were performed in selected villages.
The exploratory and in-depth ethnographic research was done in 2012 in a selection of villages included in repellent study mentioned above, more specifically in the intervention villages (with repellents and LLINs) of Kachon Kraom, Lom and Sayos in respectively Voen Sai, Oyadao and Lumphat district, as well as during shorter visits in other communities around these villages, including some control villages (with only LLINs).
Participant observation and in-depth interviewing were carried out within the qualitative strand of the study. Participant observation consisted of observations and reiterated informal conversations and was especially used as an exploratory technique to detect unforeseen variables and to contrast stated opinions with actual behaviour, as it constitutes a respondent independent data collection tool. In 2012, 153 in-depth interviews were recorded and transcribed.
Multiple purposive sampling techniques were used, where informants were selected in relation to emerging preliminary results. In order to increase confidentiality with respondents and consequent reliability of the data, snowball sampling techniques - participants introducing us to other participants - were also used.
For the quantitative strand, two surveys were carried out. From August till November 2012, a cross-sectional survey was performed with a close-ended structured questionnaire based on relevant variables emerging from the ethnographic strand. It explored the following topics: mobility between farms, fields and villages, repellent use, bed net ownership and use, evening social activities, use of malaria preventive measures other than bed nets, perceived mosquito density and nuisance and malaria treatment.
From May until November in 2013, a structured observation survey was carried out. A first visit, in the evenings between 19.00 and 21.00 depending on the availability of the household, consisted of the observation of housing structures, people’s resting behaviour, bed net characteristics and topical repellent use of all household members. As actual bed net use at night could not be directly observed, bed nets that were suspended in the evenings before bedtime with at least two corners were considered ready for use. Holes in bed nets were observed but not systematically measured or counted. The next morning, a follow-up questionnaire was administered, exploring socio-economic status, seasonal sleeping spaces, perceived insect and mosquito protection, (alternative) use of nets, child care system, (alternative) repellent use, perceived mosquito nuisance, and previous malaria episodes was carried out with the household leader. Results regarding repellent use are not elicited in this paper.
For the cross-sectional survey, 900 individuals were randomly selected from the MalaResT study population. In total, 393 individuals from 56 intervention villages and 431 from 57 control villages were located and answered the structured questionnaire.
For the structured observation survey in 2013, ten intervention villages and four control villages were purposively selected based on the criteria of malaria incidence. In each village, half of all households were randomly selected from the population census. Although there was no prior information available on which household had a farm or not, based on exploratory qualitative research, it was assumed that the majority of households did have a farm and commuted between farm- and village house, favouring the farmhouse during the rainy season and the village house during the dry season. As such, each selected household was assigned either to a farm list (meaning this household had to be observed and interviewed on their farm or rice field, if they had any) or to a village list (the interviewer had to observe and interview this household in their village house, if they had one) to explore potential differences between locations. A total of 653 households were randomly selected for the farm lists, of which 472 households were eligible (meaning they had a farm house where they stayed overnight); and 260 of those households were reached. A total of 655 households were randomly selected for the village lists, of which 555 were eligible (meaning they had a house in the village where they stayed overnight); and 291 households were reached. The main reason for not reaching households was because the selected family was not staying overnight at the respective location within the timeframe of the survey. Finally, a total of 431 households from the intervention arm and 120 households from the control were observed and interviewed.
For both surveys, an additional non-response form – recording reasons for not reaching the household - was used to measure possible systematic self-selection bias. The possible correlation between malaria infection and overnight stays at different locations was additionally tested in a malariometric survey in October 2012, on a sample of 6,640 individuals selected randomly out of the population censuses of the 113 villages included in the MalaResT study. Out of the list of 6,640 selected individuals, 4,996 individuals were reached.
To estimate human exposure to malaria vectors, pooled data of different entomological surveys conducted in Ratanakiri province are presented here. Prior to the MalaResT project, indoor and outdoor human landing collections were carried out in two villages (Phi and Lom) during four surveys of five to eight days in July-August 2009, July-November 2010, and July-August 2011. Human landing collections lasted from 18.00 until 06.00. Three collection points were chosen in each village, with paired indoor and outdoor collections per collection point. Within the MalaResT study only outdoor human landing collections were carried out in two intervention (Koy, Chrung) and two control (Kreh, Klis) villages selected out of 113 villages included in the project based on their malaria incidence, their accessibility and the availability of mosquito collectors. In every village, eight entomological surveys of ten successive nights were organized every two months between April and October of 2012 and 2013. Human landing collections lasted from 17.00 until 22.00 and from 17.00 until 08.00. Seven collection points were chosen per village in front of houses across the village, making a collection effort of 70 man-nights per village per survey. The same collection points were maintained throughout both studies. A rotation of collectors was ensured. Mosquitoes were identified and processed according to procedures described in Durnez et al. . Mosquito collections were pooled per collection context and per collection hour for data visualisation. Results regarding changing mosquito behaviour in relation to repellent use are not elicited in this paper as they are subject of a separate paper.
Qualitative data collection and analysis were performed concurrently and data analysis was an iterative process. Preliminary data were intermittently analysed in the field, and preliminary results were then translated into the question guides for follow-up interviews. Initial results were continuously confirmed or refuted in the field, until saturation was reached. Data were analysed in NVivo 9 Qualitative Data Analysis software (QSR International Pty Ltd. Cardigan UK) by refining and categorizing themes grounded in the data.
Preliminary analysis of the qualitative data was used to build the standardized questionnaires for the quantitative survey. The quantitative data was entered in Epi Info 7. The dataset was analysed in SPSS (IBM SPSS Statistics 19). Frequency tables for the main outcome variables were produced. Odds ratios and p-values for the association between overnight stays at the plot hut or in the forest and malaria infection were calculated using logistic regression models with a random intercept to adjust for clustering at village level.
Mosquito collections were registered on standardized data collection forms, and data were entered in an Access database. Mosquito collections were pooled per collection context and per collection hour for data visualization. Boxplots indicating median biting times and 25 and 75 percentiles were constructed in R . Exophagy was calculated as the proportion of mosquitoes biting outdoors as follows: O18→06hrs/(I18→06hrs + O18→06hrs).
Human exposure to malaria vectors was estimated by the analysis of the following data: the weighting of the mean indoor and outdoor biting rates throughout the mosquito collection period by the proportion of humans that are typically indoors or outdoors at each time period, and this in the scenario of (i) no protective measures, and (ii) the observed use of protective measures. Each type of observed net being used was assigned a level of protection from exposure based on the levels of protection observed by Lines et al. in experimental hut trials of (i) untreated intact nets, (ii) untreated holed nets, (iii) intact permethrin-treated nets and (iv) holed permethrin-treated nets . The proportion of indoor and outdoor mosquito bites averted by the use of protective measures was then calculated.
The study protocol, including the anthropological, entomological and epidemiological parts, was approved by the National Ethics Committee for Health Research in Cambodia, the Ethics Committee of the University Hospital of Antwerp, and the Institutional Review Board of the Institute of Tropical Medicine of Antwerp. For the anthropological part, the interviewers followed the Code of Ethics of the American Anthropological Association (AAA). As proposed by the AAA, all interviewees were informed before the start of the interview about project goals, the topic and type of questions, the intended use of results for scientific publications as well as their right to reject being interviewed, to interrupt the conversation at any time, and to withdraw any given information during or after the interview. Anonymity was guaranteed and confidentiality of interviewees assured by assigning a unique code number to each informant. The interviewers sought oral consent from all interviewees. Oral consent was preferable, since the act of signing one’s name when providing certain information can be considered a potential reason for mistrust. Moreover, oral consent avoided the stress associated with potential illiteracy.
Regarding the entomological study, the mosquito collectors were informed about the objectives, process and procedures of the study and written informed consent was sought from them. Collector candidates were invited among the adult village population and if individuals wanted to withdraw they were allowed to do so at any time without negative consequences. Access to malaria diagnosis and treatment was guaranteed throughout the study. For the epidemiological study, survey participants or his/her parents or guardian provided informed written consent for individual participation.
Variance in sleeping behaviour and bednet use in relation to vector biting times
Vector biting times
Human sleeping places and times
Reported multiple residence system (Cross-sectional survey 2012)
Has a house on farm(s)
Sleeps at farm during malaria season
Has a bed net to use at farm
Brings back net back and forth from village
Keeps bed nets at farm
Has village house
Always sleeps in village during dry season
Always sleeps in village during rainy season
Association between overnight stays at plot hut or forest in past month and malaria infection
Malaria infection (all species)
Plot hut overnight stay last month
Forest overnight stay last month
Reported sleeping and waking times at the village and farm (Cross-sectional survey 2012)
Village (n = 755)
Farm (n = 470)
- Before 19.00
- Between 19.00 and 20.59
- At or after 21.00
- Before 06.00
- At or after 6.00
Use, state and perceived protection of different types of nets
Reported bed net coverage and use
Cross-sectional survey 2012 (n = 824)
Sleeps in LLIN
Owns market net
Sleeps in market net
Structured observation survey 2013 (n = 551)
Mosquitoes enter LLIN
Mosquitoes die when entering LLIN
Small insects enter LLIN
Small insects die when entering LLIN
Mosquitoes enter market net
Small insects enter market net
Which net do you use in the village? (n = 489)*
- have no BN in village
Which net do you use at the farm? (n = 416)*
- have no BN in farm
Children sleep without net while parents are still awake (n = 370)
Observed bed net coverage and use (Structured Observation Survey 2013)
Observation bed nets
HH observed not using nets
HH observed using LLIN(s)
HH observed using market nets
HH observed using intact LLIN(s)
HH observed using intact market nets
HH observed using torn LLINs
HH observed using torn market nets
HH observed having intact LLINs ready for use
HH observed having intact market nets ready for use
HH observed having torn LLINs ready for use
HH observed having torn market nets ready for use
Observation evening resting
HH where children were observed sleeping
- Without net
- With net
HH where adolescents were observed sleeping
- Without net
- With net
HH where adults were observed sleeping
- Without net
- With net
HH where elderly were observed sleeping
- Without net
- With net
HH where somebody was observed sleeping (all age categories combined)
- Without net
- With net
Variance in evening and outdoor activities in relation to vector biting places
Vector biting place
Between 2009 and 2011, the majority of malaria vectors were collected outdoors, which was 70% of all mosquito species collected. Slight differences in exophagy rates were observed between vector species (80% for An. barbirostris s.l., 70% for An.dirus s.l., 68% for An. maculatus s.l., and 66% for An. minimus s.l.). For An. dirus s.l. and An. maculatus s.l., the median outdoor biting time was one hour earlier as compared to the median indoor biting time (Figure 1A).
Open housing structures
Observed housing characteristics per location (Structured observation survey 2013)
Village (n = 291)
Farm (n = 260)
- plastic sheeting walls
- no walls or only partly walled
Permanently open windows
Social evening activities
Reported evening activities at farms (Cross-sectional survey 2012)
Evening activities at farms (n = 470)
Housework related activities (handicraft, cleaning, cooking, etc.)
Forest activities (gathering firewood, fishing, hunting, etc.)
Other (reading, bathing, having dinner)
Access to electricity in the village (Cross-sectional survey 2012)
Household’s access to power (n = 824)
Large village generator
Reported mosquito bite protective measures at different locations (Cross-sectional survey 2012)
Village (n = 773)
Farm (n = 759)
Deep forest (n = 550)
Smoke from fire
Smoke from cigarettes
Clothes with long sleeves/pants
Traditional methods (herbs, etc.)
Qualitative research showed that, more than adults, children often rest (and fall asleep) on the floor in the house or on bamboo beds or hammocks outside, and more so at forest farms than in the village (Figure 2). Such ‘resting behaviour’ is not considered the onset of the night’s sleep, and is mostly done without bed net protection, despite often occurring in ‘sleeping places’ and during prime vector biting time (Figures 1A and 2). It is only later in the evening, when all members of the household go to sleep and actual ‘night time’ sets in, that children are put inside the bed net with their parents. During the structured observation survey, at forest farms 55.0% of observed households had children taking a nap before actual sleeping time and doing so without bed nets in 73.4% of cases (for resting behaviour of other age categories, see Table 5). The majority of household leaders (65.9%) also report to let their children sleep unprotected while parents are still awake in the evening. When looking at all age categories combined, people were observed sleeping in the evenings in 53.4% of households, and doing so unprotected in 80.6% of cases (Table 5).
Deep forest economic activities
Reported outdoor deep forest economic activities (Cross-sectional survey 201
Performs deep forest activities
Collecting forest products (bamboo shoots, fruit, honey, firewood, etc.)
Stays overnight in the deep forest:
Qualitative research showed that people urinate and defecate in the forest surrounding the village or the farm, and even call this “going to the forest” (Figure 2). There are few, if any, latrines, toilets or other sanitary constructions in the villages, and none at the forest farms and rice fields. People report to be very bothered by mosquito bites while going to the toilet, as usually several parts of the body become exposed. As people also go to the toilet during evening, night and morning hours, this activity potentially constitutes a risk for malaria infection.
Exposure to malaria vector bites in relation to the use of vector control tools
After adjusting for the typical movement of people, and not taking into account the use of vector control tools, the greatest part of potential vector exposure still occurs indoors (73%) (Figure 3A). When taking into account the observed use of intact LLINs and market nets (assuming an 85% efficacy), holed market nets (assuming a 24% efficacy), and holed LLINs (assuming a 71% efficacy)  in the village only (entomological data from farms not available), the portion of indoor vector exposure still consisted of 47% of the total exposure (Figure 3B). Taking into account protection levels of LLINs and market nets being used and local sleeping times, in the study context, the observed use of all nets is estimated to decrease indoor exposure by 67%.
In low transmission settings, finding appropriate strategies to prevent residual malaria transmission, including transmission due to early and outdoor biting, is currently one of the major challenges in Southeast Asia [27,28]. A close look at the characteristics of the relationship between vector and human behaviour shows a complex interaction over time and place. The resulting heterogeneity is related to the presence of early and outdoor biting malaria vectors, the slash-and-burn farmers’ multiple residence system, locally used (partially) open housing structures, variance in labour and social activities, sleeping times according to the place of residence and season, and variance in bed net use depending on related user preferences.
Although heterogeneity of human and mosquito behaviour is of general importance to malaria control, and as such transcends contextual particularities, in-depth socio-cultural research is needed to explore the way local communities shape this heterogeneity. In the current study context, focusing on ethnic minorities socio-culturally different from the larger Khmer society in Cambodia, more transmission occurs at forest farms than in the villages, which has also been shown in similar Southeast Asian contexts where ethnic minorities rely on the forest for farming [9,18]. In addition, the indoor/outdoor distinction is less clear at forest farms as housing here is often completely or partly open and differs from corresponding village homes. As such, the malaria vectors, which in this context preferably bite outdoor, will not be entirely prevented from biting people inside these open houses, challenging the current conceptualization of outdoor transmission. Moreover, the fact that people commute between these different houses with different levels of exposure to indoor and outdoor mosquito biting, results in a constantly varying vulnerability to malaria. Reported sleeping times also vary according to the context. People at fields go to bed earlier than in the villages and earlier than expected as entomological early man biting proportions, (EBP) calculated in similar contexts in Southeast Asia, often take 22.00 as cut-off times for people going to sleep [13,27]. However, these calculations could be more accurate using locally researched sleeping times as this directly impacts on the proportion of infections that can be accounted for by early or nighttime biting.
In a previous study conducted in Vietnam, also approximately half (52%) of respondents were asleep by 19.00, only 24.5% were still awake after 20.00, and by 21.00 almost everybody (92%) was asleep. Here, sleeping times at farms, where most transmission occurred, were also consistently earlier than in villages and earlier than used for calculating EBP . The same applies to the social evening activities varying according to the place and time of the year. Outdoor evening activities during early vector biting hours occur less during the agricultural peak season (which coincides with the malaria season), especially at those residences in the more forested areas where farmers are tired after an exhausting day’s work and the lack of electricity limits social activities. Although malaria vectors start biting outdoors as early as 17.00 and that they continue to do so until 08.00, for the majority of people that sleep under bed nets, the window for outdoor and early transmission was shorter than expected. These earlier than expected sleeping times in settings with a high bed net coverage could be responsible for further driving the selection of early-biting mosquitoes. Conversely, the small percentage of people going to sleep under bed nets after 21.00 or the people that do not use any bed nets or only torn market nets, could disproportionately contribute to malaria transmission. Without LLINs, exposure does indeed still occur largely indoors.
One of the defining elements of residual transmission is based on the premise of total LLIN coverage . While survey results report high LLIN use, due to response bias and the often suboptimal operationalization of the concept ‘net use’ in questionnaires, actual LLIN use may not be as high as self-reported data from surveys indicate . This is also suggested by the direct observation of the amount of households where market nets were being used, which corresponded to the percentage of people reporting to own a market net and not to the reported percentage of market net use, which was about 20% lower. In addition, many of these non-treated market nets were holed, offering only very limited protection , which, in turn would result in lower than expected community-level protection of non-users .
In addition to 30% of households not using LLINs, various gaps in protection were identified during vector biting times, which related, among others, to children and adults resting outside or inside in sleeping spaces in the evening before their reported sleeping times and, at night, when using non-treated torn nets while sleeping in often open housing. Additional gaps in night protection, however, cannot be addressed with LLIN, such as outdoor economic forest activities and toilet practices. Moreover, entomological data confirms that without LLIN, and not considering the potential protective mass-effect of LLIN in a village, a considerable proportion of exposure does indeed occur indoors. Considering all these factors, the contribution of night transmission may still be underestimated.
It has been hypothesized that controlling malaria with LLINs has certain fundamental limitations in regions characterized by early and outdoor biting, thus improving coverage of LLINs alone might not achieve malaria elimination . It is clear that additional interventions aiming for personal protection during evening and night activities are essential. However, based on the current evidence, additional efforts in improving LLIN use during times when people are resting in the evening and during the night may still have an impact on further reducing malaria transmission in Cambodia.
Re-imagining malaria interventions by focusing not only on the heterogeneity in malaria transmission, but more specifically on the connection between heterogeneous human and vector behavior, is crucial when evaluating what works, what is still missing, and how to accelerate the progress in malaria control towards elimination.
We would like to sincerely thank the indigenous communities in Ratanakiri who supported our research. The entomological research was made possible by the hard work of the entomological fieldwork teams of CNM. Special thanks goes to the malaria staff (Mr. Mean, Mr. Hien, Dr. Vannara) of the provincial health department for supporting and assisting us during fieldwork. Thanks to Dr. Siv Sovannaroth and Dr. Char Meng Chuor for supporting our research at CNM.
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