Improved spatial ecological sampling using open data and standardization: an example from malaria mosquito surveillance

Vector-borne disease control relies on efficient vector surveillance, mostly carried out using traps whose number and locations are often determined by expert opinion rather than a rigorous quantitative sampling design. In this work we propose a framework for ecological sampling design which in its preliminary stages can take into account environmental conditions obtained from open data (i.e. remote sensing and meteorological stations) not necessarily designed for ecological analysis. These environmental data are used to delimit the area into ecologically homogeneous strata. By employing Bayesian statistics within a model-based sampling design, the traps are deployed among the strata using a mixture of random and grid locations which allows balancing predictions and model-fitting accuracies. Sample sizes and the effect of ecological strata on sample sizes are estimated from previous mosquito sampling campaigns open data. Notably, we found that a configuration of 30 locations with four households each (120 samples) will have a similar accuracy in the predictions of mosquito abundance as 200 random samples. In addition, we show that random sampling independently from ecological strata, produces biased estimates of the mosquito abundance. Finally, we propose standardizing reporting of sampling designs to allow transparency and repetition/re-use in subsequent sampling campaigns.


Grand Popo
The study site in Benin is in the southwestern coastal part of the country. Elevation ranges from 0m to 70m above sea level. The average temperature is 28.9°C, average relative humidity is 76% with average annual rainfall of 190mm. The rainy season is characterized by abundant rains during April to July, and a lower amount of rain from September to October. The area is mostly urban and cultivated, and use of pesticide is common. Studies have been published on malaria incidence and bednet use [2][3][4], however there are no studies on mosquito species distribution or insecticide resistance.

Malindi
The study site contains the large town of Malindi with approximately 210,000 inhabitants. The climate is tropical, a cooler season from June to September, with daytime temperatures around 27-28 °C, is followed by a hotter and humid season from November to April, with daytime temperatures above 30 °C. Relative humidity ranges between 80-85%. Malindi is comprised of commercial and residential areas, agricultural and undeveloped areas, and hotels and stores along the coast. Tourism, retail, fishing, and trading are the major economic activities. This area is within Kenya's endemic malaria zone with all-year risk of malaria transmission [5]. The major malaria control intervention in Malindi is the use of pyrethroid treated bednets. Studies to detect insecticide resistance show suspected Anopheles resistance to pyrethroids [6,7].

Migori
Migori is located in western Kenya, about 50km from Lake Victoria and with elevation ranging from 1,200m to 1,500m above sea level. The average annual temperature is 21°C, and average relative humidity is 65% with average annual rainfall of 1,000 -1,800mm. The area experiences long rains from April to June and short rains from September to October. The land is mainly used for cultivation and grazing. There are some studies on malaria burden from the area [8, 9], but none on mosquito abundance or insecticide resistance even if indoor residual spraying is taking place.

Muleba
Muleba is in the Kagera region of northwest Tanzania on the western shore of Lake Victoria. The district lies at 1,100-1,600m above sea level. There are two rainy seasons: "long rains" in March -June (average monthly rainfall 300 mm) and "short rains" in October-December (average monthly rainfall 160 mm). Average annual temperature is 21°C (with minimum-maximum range of 15°C-28°C) and average relative humidity of 66%. The area is mainly rural and is used for agriculture. Malaria transmission occurs throughout the year and peaks after the rainy seasons. The predominant malaria vectors are Anopheles gambiae s.s. and An. arabiensis, in which pyrethroid resistance has been detected [10].

Obuasi
Obuasi is located in the southern part of the Ashanti region of Ghana about 64 km south-west of the regional capital Kumasi. The area has an undulating terrain with most of the hills rising above 500 meters above sea level and vegetation characteristic of the moist semi-deciduous forest type. The climate is semi-equatorial and characterised by two rainy seasons. The first season starts from March and ends in July and the second from September to November. The mean annual rainfall ranges between 125mm and 175 mm, while the mean average annual temperature is 25.5 °C and relative humidity 75% -80% in the wet season. Agricultural activities in the area include crop farming, livestock rearing, tree planting and fish farming. Mining and quarry forms the second largest industrial activity in the municipality and creates potential mosquito breeding sites all year round. Resistance to multiple insecticides in Obuasi has been documented in Anopheles gambiae and An. funestus mosquitoes [11-13].