Impact of indoor residual spraying on malaria transmission in the Moïssala health district, Chad

Backgroud Malaria is a major public health and development problem in Africa. In Chad in 2016, with 720 765 confirmed cases and 1 686 deaths, malaria is the main cause of consultations, hospitalizations and deaths in health facilities. A longitudinal entomological study was carried out from 07 to 24 December 2016 in the Moïssala health district. The objective of the study was to assess the impact of malaria transmission one year after two cycles of indoor residual spraying of 80% bendiocarb wettable powder (Ficam WP 80 W). Two areas were for the town of Dembo, the was chosen and as Two sampling methods were used: collection of resting endophilic fauna and direct capture of aggressive mosquitoes from human subjects.


Introduction
Malaria is a major public health and development problem in Africa. In 2016, 216 million cases of malaria were reported in 91 countries, an increase of 5 million from the previous year and the number of deaths reached 445 000. Africa still accounts for 90% of the associated cases and deaths [1].
However, since its creation, the World Health Organization has always had malaria eradication on its agenda [2]. Efforts have been made to eradicate this scourge in many parts of the world [3]. However, in Africa, eradication is being delayed not only for economic reasons, but also because of insufficient knowledge of how the disease is transmitted in the continent's different ecological areas.
In Chad in 2016, with 720 765 confirmed cases and 1 686 deaths, malaria is the main cause of consultations (34%), hospitalizations (30%) and deaths (35%) in health facilities [4]. In addition to the loss of human lives, malaria is costly in terms of public health expenditures [5].
Divided into three geo-climatic zones that determine three epidemiological facies of malaria, northern Chad is desert with no local transmission [6]. The Sahelian climate center corresponds to an area with unstable malaria due to short seasonal transmission. The south with a Sudanian to Sudano-Guinean climate is characterized by stable malaria, with long seasonal transmission.
Most of the entomological studies conducted in Chad have concerned the Sudanian and Sahelian zone where malaria transmission is attributed to An gambiae sl, An funestus, An pharoensis and An ziemanni [7,8]. Yet, to date, no survey has assessed the dynamics of malaria transmission in the transition zone between the Sudanese and Guinean zones. However, the epidemiology of malaria depends largely on the biotope. In an effort to strengthen protection against mosquito bites in addition to longlasting insecticide-treated nets distributed by the Ministry of Public Health, MSF-France considered using Fendona® (a pyrethroid insecticide) for an indoor residual spraying campaign in the eastern zone of the Moïssala health district. The 6 recommendations of the NMCP following a study carried out in 2015 on mosquito susceptibility in the said health district (data presented in another article) finally led to the use of Ficam® (insecticides of the Carbamates class).
The objective of the study is to evaluate the impact of this spraying campaign on the dynamics of malaria transmission in two villages, one (Dembo) in the sprayed area and the other (Moïssala) in the control area. More specifically, this involves determining the aggressive density of mosquito populations and assessing the rate of plasmodial infection of vectors.

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

Sampling techniques
Mosquito population sampling was carried out by two methods of capture:

1.
Direct capture on human bait at night (indoor and outdoor);

2.
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:

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 [10].

Anopheles species present
Three anopheles species have been identified in Moïssala in the control area. These are An gambiae sl, An funestus and An rufipes, whereas in Dembo in the treated area, An gambiae sl and An funestus were collected (Table 1).

Molecular forms within the An. gambiae complex
Of the 386 individuals in An gambiae complex randomly selected and subjected to PCR molecular forms, An colluzzii is 96% (371/385) in majority compared to An gambiae 4% (14/385) in Moïssala. In Dembo, the only individual caught belongs to An coluzzii species.

Time rhythm and aggressiveness rate
In Moïssala, the aggressiveness of mosquitoes starts from 6pm and continues throughout the night both indoor and outdoor. There is a high rate of biting between 12 10 p.m. and 4 a.m., no matter where you are. Inside, the bite peak occurs around 3 a.m. while outside, a high rate is reached earlier, between 10 p.m. and 11 p.m., and a second one between midnight and 1 a.m.
During the four successive night captures, 303 anopheles were captured in Moïssala while in Dembo, the aggressiveness rate was zero for the same number of capture sessions. The 303 anopheles were captured during 16 capture sessions totalling 32 Men-night. The average aggressive anophelian density in Moïssala was 9 bites/man/night ( Figure 2).
Anopheles gambiae sl bites both inside and outside houses. However, 55% of the females were caught indoors, indicating the tendency of this species to endophagy.
On the other hand, Anopheles funestus is exclusively caught outdoors. The distribution of the number of anopheles caught on humans is summarized in Table 2 for each species (Table 2).

Sporozoite indices of aggressive fauna
The sporozoite antigen indices tested in An funestus and An rufipes were negative.
On the other hand, out of a total of 295 Anopheles gambiae sl randomly selected and observed at Elisa for infection, 22 were found to carry Plasmodium sporozoites, representing a sporozoite index of 7.45% ( Table 3).

The entomological inoculation rate
The average entomological inoculation rate of Anopheles gambiae sl in Moïssala during the study period was 0.67 infected bites per man per night, or 244.73 infected bites per man per year.

Time distribution of malaria transmission in Moïssala
The

Discussion
Effective malaria control is based on an integrated approach of several strategies, [14,15,16]. For vector control, prevention of malaria transmission through the 14 widespread use of insecticide-treated nets and indoor residual spraying is recommended [17]. But first, knowledge of the sensitivity of malaria vector anopheles to insecticides and their biting behaviour is required before measures can be deployed and post-deployment evaluation of these measures [18,19]. The objective of this study was to assess the impact of the indoor spraying campaign on malaria transmission in the eastern zone of the Moïssala health district after two cycles of bendiocarb spraying. This evaluation was carried out by measuring certain entomological parameters of malaria transmission using methods adapted to the study of vectors.
To better evaluate this campaign, these parameters should have been studied for a year. This would take into account the seasonality of transmission and vector density. For example, the results of our study did not allow us to conclude on the degree of effectiveness of IRS in the short, medium and long term. Moreover, our results also did not allow us to conclude whether the difference in the specific  [7] in Goulmoun, Diarra et al, [8] in Douguia in Chad and Saotoing et al, [20] in Maroua in Cameroon where An gambiae predominates with 84.5%, 87.5% and 81.79% respectively during night captures on human bait and in residual morning intradomiciliary fauna. These results could be explained by the similarity of these areas in geoclimatic and ecological terms. The aggressive anophelian density is 9 bites/man/night in Moïssala and zero in Dembo. Moïssala's results are similar to those obtained by Himeidan et al [21] in Sudan, Dossou yovo et al [22] in Bouaké, comparison of the treated area with the untreated area in this study confirms the results that indoor spraying leads to a decrease in vector density and aggressiveness, with a significant decrease in malaria transmission [24,25]. This statement, even if it is not shared by some authors [26,27], is confirmed during this survey and seems relevant for malaria prevention even in areas of permanent transmission. Concerning this parameter, transmission in the control area is about 0.67 infective bites per person per night. However, it is zero in the treated area.
This study corroborates those of Kerah and al [57], Doannio et al, [28] in Ivory coast and Coz and Brengues, [29] in Burkina Faso, who observed a rate of aggression of 55.8% respectively, significantly higher inside than outside the home. Malaria transmission in the Moïssala health district is due to 2 vectors: An gambiae sl which has been found infected on human bait inside (7.40%) and outside (7.09%) as well as in morning fauna (6.70%) and An rufipes. Out of 10 An rufipes captured in the morning fauna and analysed by Elisa CSP, 2 were positive, i.e. 20% of infected animals. This result does not make An rufipes a secondary vector [10,32] but an excellent one to consider in the transmission of malaria to Moïssala. Our results according to which An rufipes is captured at rest in the rooms and on human bait are similar to those observed in Dielmo in Senegal [33] and Dori in Burkina Faso [34]. However, investigations incriminating this species in transmission are rare [35]. Although An funestus is, with An arabiensis, a major vector of malaria in Chad, this species plays no role in the transmission of malaria to Moïssala.

Conclusion 17
The indoor residual spraying campaign in the eastern zone of the Moïssala health district has led to the collapse of the density and aggressiveness of malaria vectors.
However, its evaluation over a short period of time is not sufficient to assess the impact of malaria transmission in this stable and highly endemic malaria zone. The long-term effectiveness of the indoor residual spraying campaign on malaria vectors and transmission will only be assessed after several years of transmission. In addition, the use of mosquito nets and other control measures should be taken into account in this assessment. Although, this intervention was appreciated and won the support of the target population. However, the high cost of the campaign could undermine the sustainability of the strategy, especially since the population's financial income is suffering as the country goes through a period of economic crisis.  Figure 1 Location of study sites. Hourly infection rates