Study area and population
To assess the impact of this campaign, two sites were selected. These are Dembo, located in the sprayed area and Moïssala, which was not treated at all, served as a control.
Moïssala (8°20'16''N 17°46'05''E) is the chief-town of the department of Barh Sara, Mandoul Region. It is located 75 km from Koumra, the capital of the region and more than 700 km south of N'Djamena, the capital of Chad. It has a population of approximately 33 098. The city of Moïssala is located on the banks of a permanent river, the Barh Sara, a tributary of the Chari River that originates in the Central African Republic. The river can be crossed with a ferry or a canoe. However, during the rainy season and depending on the level of the floods, the crossing with the ferry is not always guaranteed. The alternative is the road for a two-day trip to Dembo.
Dembo is the chief-town of the Dembo Sub-Prefecture in the department of Barh Sara. It is located 25 km east of Moïssala and about 27 km north of the border with the Central African Republic. It has a population of approximately 19 013. The city is bordered by a pond (Goumoud) which is watered in the rainy season and dried up in the dry season. During wintering, swamps form between Dembo and Moïssala, making traffic extremely difficult. Sometimes, when the rains are very heavy, the road is impassable throughout the season but this difficulty disappears more or less completely towards the end of the dry season.
The two cities have geographical and cultural similarities. The climate is tropical of the Sudano-Guinean type, characterized by the alternation of a long rainy season, from May to November, and a dry season, from December to April. The average annual rainfall exceeds 900 mm per year with a maximum of rainfall in August. The average annual temperatures range from 17°C in December/January to 40°C in March/April. Soils are generally sandy or clayey with a high humus content. In some places, soil impermeability causes swampy areas during the rainy season. The once heavily forested region has undergone intense clearing due to high population pressure.
The forest, which consists of very large trees (fig trees, ‘’caïlcédrats’’, tamarind trees, Karités, ‘’Nérés’’, etc.), vines and epiphytic plants, has suffered particularly from human activity, which has largely cleared it to replace it with food crops, but now only remains in the form of highly degraded galleries along watercourses.
The main crops are: sorghum, groundnuts, rice, cassava, cotton, beans, sesame and market gardening. The population is composed of Mbaye, Nar, Gor, Gor, Ngama, Daye and Peulhs ethnic groups. The activities practiced by the population are: traditional rainfed agriculture, fruit growing, animal husbandry, trade and fishing. Large and small livestock are parked at night in enclosures in the vicinity of houses.
Most of the houses are of traditional, rectangular and circular types, with dry clay or brick walls. Their roofs are made of metal sheet or thatch. The population is supplied with drinking water through traditional wells and boreholes equipped with human-powered pumps.
The main diseases of the inhabitants of Moïssala and Dembo are in order of importance from the point of view of the number of cases reported by health facilities: malaria, mainly Plasmodium falciparum, is endemic, malnutrition, measles, meningitis, etc.
Entomological data from the region are non-existent. Both cities use the same vector control methods based on the use of long-lasting insecticide-treated nets widely distributed to all households during the 2014 mass campaign.
The study took place from 07th to 24th December 2016 in four of the eight districts of the city of Moïssala and in four of the seven districts that make up the city of Dembo (Figure 1).
Mosquito population sampling was carried out by two methods of capture:
- Direct capture on human bait at night (indoor and outdoor);
- Intra-domestic spraying of pyrethrum-based insecticide (Red Can®) during the day.
Capture on human
Aggressive fauna on humans has been studied using the Le Goff et al. method (9). For this method, 3 parameters were determined:
- The aggressiveness rate (ma) which is the number of mosquito bites per person per night;
- The sporozoite index (SI) of the captured mosquitoes;
- The daily inoculation rate (h = ma x IS).
The catches were made by volunteers, all young men recruited in each city. These volunteers were previously trained after obtaining their consent. ‘’Men-night’’ are used both as bait and as captors. They were protected from malaria by Sulfadoxine-Pyrimethamine chemoprophylaxis. Catches were made for four consecutive nights in four neighbourhoods of each site, with one hut per neighbourhood. This corresponds to 16 catch sessions per site, or 32 ‘’men-night’’. The neighbourhoods were selected according to a peripheral-intermediate-central transect. For each capture point, the consent of the head of the household was obtained for the access of the catchers to their household. For each compound, two catch points are retained. One inside the hut (used as a bedroom) and the other outside. The captures are organized from 6pm to 6am by two teams of two capturers each. The first team operates from 6pm to midnight and the second from midnight to 6am.
Volunteers are rotated at each capture session (between teams, capture points and houses) to minimize biases due to their individual ability and attractiveness. Each capturer is equipped with a torch, a watch, hemolysis tubes and bags labelled by time slot and bearing the identification of the capture point. In a practical way, each volunteer, seated on a bench/stool/chair, captures with a hemolysis tube the mosquitoes placed on his bare legs up to his knees. The hemolysis tubes containing the mosquitoes are then placed in bags corresponding to catch time slots.
Capture of endophilic fauna after indoor residual spraying with insecticides
This method was used for the collection of female endophilic mosquitoes to assess the density of resting mosquitoes in bedrooms. It consists of spraying a pyrethrum solution into bedrooms to collect mosquitoes at rest. The insecticides used are commercial products based on ORO® brand Pyrethroids (permethrin: 0.25%; tetramethrin: 0.20%; d-ferothrin: 0.01%; piperonyl butoxide: 0.34%).
Before each spray, white bed sheets were spread to cover the entire floor of the room and the furniture, then an agent protected by a mask and glasses, after closing the door and windows, sprays the room for about fifteen seconds before leaving. After about ten minutes of waiting, the sheets are carefully removed and the lightning-fallen mosquitoes are recovered with the help of pliers in petri dishes bearing indications on the site, the district, the compound and room number. These spraying sessions take place between 6am and 11am.
Treatment of captured mosquitoes
Mosquitoes captured by the above methods are identified using a binocular magnifying glass according to the morphological genus and species criteria . Females of anopheles are then counted and classified according to the location, time and method of capture, repletion state of their abdomen (unfed, fed, half-gravid, gravid) and individually deposited in microtubes containing a desiccator (silicagel) for transport and storage. The microtubes are assembled in bags and stored at minus 20°C for further analysis at the laboratory of ‘’Laboratoire du Centre de Recherche Entomologique de Cotonou (Benin)’’. The determination of the circumsporozoite antigen of malaria parasites in mosquitoes was done by the Elisa technique [11, 12]. Molecular identification of An gambiae complex species was performed by PCR SINE 200 according to the protocol of Santolamazza et al .
All data recorded on the survey sheets were entered into Microsoft Excel software and transferred to version 20 of the IBM SPSS statistical analysis software. The daily aggressiveness rate (ma) was calculated from the number of bites received per man per night. m represents the number of mosquitoes per man and a, the daily frequency of bites performed by a female anopheles on a man. The sporozoite index corresponds to the presence in females of anopheles of the circumsporozoite antigen established by ELISA CSP research. This index is estimated in number of infected female anopheles out of the total number of anopheles analysed times one hundred. The daily entomological inoculation rate (h = mas) was calculated from the product of the daily aggressive density (ma) and the proportion of females with CSP antigen (s). The daily entomological inoculation rate allows the monthly and annual inoculation rate to be deducted. Anophelian density is the product of endophilic anophelian fauna and the number of rooms sprayed.