Demystifying Essential Containers and Places for Aedes Mosquito Control in Thailand

Background: The strategy for prevention and control of Aedes-borne diseases relies on timely elimination of key breeding containers. There is thus a crucial need to identify key breeding containers to enhance vector control activities. The visual larval survey of wet containers has then been conducted as a routine mission of the Department of Disease Control (DDC). To facilitate this, DDC has deployed a mobile application, namely TanRabad SURVEY, since May 2016. As per an inspected place, TanRabad SURVEY supports the real-time collection of its larval survey data and processing of its larval indices and key breeding containers.

that are typically sit on different places. The classic breeding containers for Aedes mosquitoes are artificially flooded sites which can produce a large number of larvae or pupae even when rainfall is low [7]. In recent decades, the incidence of Aedesborne diseases (especially dengue) has grown dramatically around the world [3,8,9]. As for Thailand, over 50,000 people [10, 11,12,13] infected with Aedes-borne diseases are annually reported throughout the country. The strategy for prevention and control of Aedes-borne diseases relies on timely elimination of key breeding containers [14,15,16,17,18,19]. There is thus a crucial need to identify key breeding containers to enhance vector control activities [7,20].
To achieve targeted vector control, the visual larval survey of wet containers has been conducted as a routine mission of the Department of Disease Control (DDC) [11,21]. DDC has adopted the larval survey guideline from WHO [15,22,23,24] to suite Thai culture and tradition. Particularly, containers are grouped into 12 types (see Figure 1): water tanks, water drinking jars, vases, anti-ant bowls, plant saucers, lotus basins, plant leaves, pet bowls, drip tray of water dispensers, old tyres, other used containers and unused containers. Places the accommodate those containers are classified into 6 categories (see Figure 2): villages [1] , schools, temples, hospitals, hotels and factories. The visual larval survey in all districts at risk must be performed. A district at risk is defined when a number of patients in the past 4 weeks is greater than an average number of patients in such 4 weeks of the previous 5 years. As per a district at risk, at least one place in each particular place type must be randomly examined. A random of 40% of houses should be examined as per a village while all buildings are for other places. In each examination, a number of indoor and outdoor wet containers with and without larvae must be reported. Upon a survey completion for a particular place, its larval indices [25] are calculated along with key breeding containers. To facilitate the visual larval survey, a mobile application, namely TanRabad SURVEY [4,26,27], is developed. Essentially, as per an inspected place, TanRabad SURVEY supports the real-time collection of its larval survey data and processing of [1] Villages here are a collection of only houses. Accordingly, this study relied on the larval survey data spatially and temporally collected by public health officials via TanRabad SURVEY. The aim of this study was to identify (i) the overall and regional key breeding containers and places that serves as primary sources and accommodation of Aedes mosquito breeding; and (ii) the regional transmission potentiality of Aedes-borne diseases. These identifications enable DDC to develop and implement an effective and sustainable prevention and control program. With this targeted vector control, there would be a minimized use of chemicals that may be costly and have long-term health and environment impacts [20].

Study Area
Thailand is located at the centre of the Indochinese Peninsula and roughly between 5.61 • N to 20.45 • N in latitude and 97.37 • E to 105.62 • E in longitude [28]. Thailand is administratively divided into 77 provinces and 4 regions as shown in Figure 3, covers an area of 513,120 square kilometers, and has a population of over 70 million people. Thailand has three official seasons [29] -summer, rainy and winter. The summer season runs from mid-February through to mid-May, with April and May the hottest months of the year. The rainy season is from mid-May to mid-October. The winter season is between mid-October and mid-February. In this study, the analysis was performed with respect to both national and regional aspects.

Data Source
This study relied on the larval survey data spatially and temporally collected via TanRabad SURVEY and stored in TanRabad database. The larval survey data is reported by public health officials under DDC. Essentially, all inspected containers and places throughout Thailand from 2017 to 2019 was chosen for data analytics. The number of inspected containers and places from 2017 to 2019 are illustrated in Table 1 -2, respectively.

Larval Indices
Larval indices [25,30] are (i) House Index (HI) -a percentage of houses infested with larvae and/or pupae; (ii) Container Index (CI) -a percentage of water-holding containers infested with larvae or pupae; and (iii) Breteau Index (BI) -a number of positive containers per 100 houses inspected. According to DDC, HI, CI and BI are for the villages, while CI alone is applicable for other places. This is mainly due to the fact that each village basically covers many houses in a large spatial area and have various residences with different ages and careers. Conversely, the others are public and sharing places with less number of buildings in a small area and has specific residences. In TanRabad SURVEY, HI, CI and BI are formally defined as per Equations 1 -3.
where B is a set of buildings inspected as per a particular place, b a building in B, |B| a number of buildings inspected, |B | a number of buildings infested, C a collection of container categories, c a container category in C, α b,c and γ b,c a total number of indoor and outdoor wet containers with category c inspected for a building b, and α b,c and γ b,c a total number of indoor and outdoor wet containers with category c infested for a building b.

Statistical Data Analysis
A descriptive analysis was done to investigate types of breeding containers and places that were primary sources for Aedes mosquito breeding. For breeding containers, different measurements were calculated: (i) breeding potentiality (BT)the percentage of each container category to total containers [7,20]; (ii) breeding productivity (BP) -the percentage of each container category infested [20] to determine its larval productivity; (iii) breeding contribution (BC) -the percentage contribution of each container category to total infested containers [20]; and (iv) the breeding preference ratio (BPR) [7] of breeding contribution (BC) against breeding potentiality (BT). Formally, BT, BP, BC and BT are defined as per Equations 4 -7.
where B is a set of buildings inspected as per a particular place, b a building in B, c a particular container category, α b,c and γ b,c a total number of indoor and outdoor wet containers with category c infested for a building b, α b,c and γ b,c a total number of indoor and outdoor wet containers with category c inspected for a building b, α b and γ b a total number of indoor and outdoor wet containers infested for a building b, and α b and γ b a total number of indoor and outdoor wet containers inspected for a building b.
Similarly to breeding containers, places that accommodate those essential containers must be investigated to reflect the targeted vector control. DDC has classified

Place Types Larval Indices
Risk Levels Villages HI = 0 very low risk 0 < HI ≤ 10 low risk 10 < HI ≤ 50 moderate risk HI > 50 high risk Other CI = 0 low risk places 0 < CI ≤ 5 moderate risk CI > 5 high risk Table 3: Risk Levels of Aedes-borne Disease Transmission based on Larval Indices risk levels of places based on their larval indices as shown in Table 3 [11]. In particular, places are expected to have promising vector control and hence be (very) low risk places. Villages should thus have their HIs less than or equal to 10, while the CIs of other places should remain 0. To assess larval productivity of places, place productivity (PP), the percentage of each place category in moderate or high risk levels, is thus calculated and formally defined as per Equation 8.
where t is a particular place type, P t a set of inspected places with type t, p a place in P t and LI(p) a function to evaluate the category of an appropriate larval index as per a place p. It should be noted that HI is for villages while other places rely on CI.

Regional Key Breeding Containers and Places
The breeding preference ratio (BPR) and place productivity (PP) were computed to identify regional key breeding containers and places. These were done based on the combination of all larval survey data from 2017 to 2019 altogether. Figure 9 illustrates the breeding preference ratio (BPR) of containers classified by 4 regions (central, north, northeast and south) and 2 locations (indoor and outdoor). For the Central region, indoor unused containers (2.49) had the highest BPR, followed by old tyres (1.29) and water tanks (1.25). The Northern region had indoor old tyres (2.51) with the highest BPR, followed by unused containers   Figure 10 depicts the map of essential containers for both indoor and outdoor locations by regions. Figure 11 illustrates the place productivity (PP) classified by place types and 4 regions (central, north, northeast and south). Villages (53.25-58.88%) had the highest PP for the Central, Northern and Northeastern regions, followed by temples (38.06-56.93%). Conversely, in the Southern region, temples (64.62%) had the highest PP, followed by villages (59.08%). The next places for all regions were schools (26-54.85%), factories (21.67-34.59%), hotels (20.60-35.71%) and finally hospitals (6.51-22.25%) with the lowest PP.

Regional Transmission Potentiality of Aedes-borne Diseases
The place productivity (PP) was computed to determine the regional transmission potentiality of Aedes-borne diseases. Particularly, the more PP the region had, the more transmission potentiality of Aedes-borne disease was likely. This was done  Figure 12 illustrates the overall place productivity (PP) classified by regions. The Southern region (44.84%) had the highest PP, followed by the Northern region (42.37%), the Northeastern region (35.47%) and the Central region (30.42%). Similarly, the Southern region (47.15%) had the highest proportion of high risk places to moderate places, followed by the Northern region (44.51%), the Northeastern region (40.29%) and the Central region (37.51%).

Discussion
In Thailand, according to Thai culture [2] , water tanks were the most potential (frequently found) containers for both indoor and outdoor locations. Although water tanks were regularly used containers, they were found as the most contributed (frequently infested) containers. Most abundance containers were unused containers, other used containers, old tyres, anti-ant bowls, drip tray of water dispensers and pet bowls. These containers were considered as productive and contributed containers. Particularly, the productive and contributed unused containers, other used containers and old tyres were found at outdoor location. The productive and contributed anti-ant bowls and drip tray of water dispensers were sat on both indoor and outdoor locations while pet bowls were found at indoor location. Beside these artificial containers, plant leaves as natural containers were also recognized as productive and contributed containers (for indoor location). It was thus important to pay much attention to these artificial and natural containers at breeding location preferences. Essentially, the garbage disposal in the environment surrounding houses/buildings must be well managed. Old tyres must be transformed into useful items. As for places, villages and temples were the top two riskiest places that accommodated most of key breeding containers. Schools and factories were the next risk places, followed by hotels and hospitals. To achieve (very) low risk places, the prevention and control program must thus be regularly conducted for all places regardless of their types.
For regional aspects, different regions had dissimilar geographies, climates and livelihoods. They thus had different essential containers in term of the number and types of containers in concern. However, they had essential containers in common which were unused containers, other used containers, old tyres and drip tray of water dispensers. Particularly, unused containers and old tyres were found at both indoor and outdoor locations for all regions. The rest containers were at only outdoor location for the Central region, while other regions had them at both indoor and outdoor locations. Besides these common containers, water tanks were essential for the Central and South regions. Pet bowls and anti-ant bowls were significant for the Northeastern and Northern regions. Plant saucers were for the Northeastern and Southern regions. Essentially, the Northeastern region must be aware of 9 different containers types, followed by the Southern (7), Northern (6), and Central (5) regions. As for places, all regions had similar experiences in which villages and temples were the top two riskiest places, followed by schools, factories, hotels and hospitals. Moreover, all regions had suffered with moderate and high risk places above 30% of all places and hence high transmission potentiality of Aedes-born diseases. The Southern region was found with the most number of moderate and high risk places, followed by the Northern, Northeastern and Central regions. This was due to the fact that the Southern region had rainfall for 8 months while it took about 5 months for the other regions.
The empirical evidence had shown that the breeding productivity of most containers and places in each particular type was gradually decreased from time to time. This was mainly because TanRabad SURVEY made significant contributions: (i) all prevention and control actions toward those essential containers and places could promptly be directed to community participants via health education; and (ii) the strategic prevention and control program could effectively be developed and implemented which was driven by spatially and temporally collected larval survey data.