Despite the coverage of more than 80% of the structures by IRS and a drastic reduction of EIR, there is still significant residual malaria transmission in the four regions. In Ouémé region, despite two rounds of IRS in 2010 and the reduction of 90% of EIR, the 10% remaining representing the residual malaria transmission should be considered relatively high. Therefore, despite IRS in Adjohoun, Dangbo and Sèmè in Ouémé region, this residual transmission was estimated between 12.91 and 55.14
An. gambiae infected bites per year [
2]. Theoretically, in this region, despite IRS, everybody under protection of IRS was at risk of being in contact with 1 to 4
An. gambiae carrying
P. falciparum in the salivary glands per month. The same situation was noticed in Alibori and Donga where a residual transmission of around 2 infecting bites per month after 80% reduction of EIR occurred; this was also the same in Atacora only in 2011 [
4]. In view of the complaints from the stakeholders about the lack of evidence of IRS epidemiological impact and the absence of significant reduction in the number of malaria cases suggested by the routine health facility data although these areas have been under annual IRS for many years, the reduction of 80–90% of EIR is encouraging, but should be observed with caution: it may be insufficient to decrease epidemiological indicators given that the residual EIR in IRS communes was still higher than it was in some regions of stable malaria. However, EIR calculated for the communes under IRS might be overestimated. As a matter of fact, bendiocarb and pirimiphos methyl make mosquitoes fall from treated walls among which most of them blood fed. Since the houses of the study area were not surrounded by ant traps, such mosquitoes could be removed before PSC mosquito collections carried out by the technicians of CREC. Thus, the number of mosquitoes entering the houses and dead after blood meal might be underestimated. However, as EIR was calculated using mosquitoes collected using HLC, the number of mosquitoes which could escape to the ability of mosquito collectors would be low. Regarding the 2 graphs representing the variation of the malaria case incidence in the Atacora and Alibori-Donga regions being under IRS from 2011 to 2018 as mentioned by the health facilities, there is no reduction (Fig.
4). As a matter of fact, a retrospective investigation was carried out based on the indicators recorded by Atacora and Alibori-Donga health’s centers on the one hand, and malaria incidence calculated by the statisticians of the NMCP using the formula of Kalala et al. [
27] and Hellenbrand et al. [
28] during the period of IRS on the other hand. The goal of this investigation was to have an idea on the evolution of malaria incidence in the communes where communities were protected by IRS. In addition, a report of a project implemented from 2011 to 2015 in Benin, Cameroon, Kenya, India and Sudan on the impact of vector resistance on the efficacy of vector control tools (LLINs and IRS) showed mitigated results. In this study, the authors found very little correlation between the decrease of entomological parameters and both parasitological and clinical parameters [
14,
17]. Another study on the efficacy of four vector control interventions supported by the French Ministry of Cooperation in an area of high vector insecticide resistance in Benin did not show significant differences from one intervention to another [
18,
19]. This study concerned a cluster randomized controlled trial carried out in 28 villages southern Benin villages to investigate on the efficacy of various vector control interventions: LLIN targeted coverage to pregnant women and children under 6 years (TLLIN, reference group), LLIN universal coverage of all sleeping units (ULLIN), TLLIN plus full coverage of carbamate-IRS applied every 8 months (TLLIN + IRS), and ULLIN plus full coverage of carbamate-treated plastic sheeting (CTPS) (ULLIN + CTPS). The clinical incidence density of malaria was reduced neither in the children from the ULLIN group nor in those from TLLIN + IRS group nor the ULLIN + CTPS group compared with reference group (TLLIN) despite reduction of HBR and EIR. Relations between
P. falciparum incidence and EIR had already been observed by Beier et al. [
29] in a study in western Kenya showing that measurements of either the EIRs or HBRs can be used to predict corresponding attack rates in children. Some years later, the basic relationship between EIR and
P. falciparum prevalence accross different ecological zones showed convincingly that substantial reductions in malaria prevalence are likely to be achieved only when EIRs are reduced to levels of less 1 infective bite per person per year [
30]. This report showed that some sites with EIRs < 5 infected bites per year had levels of
P. falciparum prevalence exceeding 40% and when transmission exceeded 15 infected bites per year, there were no sites with prevalence rates < 50%.
To curtail residual malaria transmission, additional interventions able to target vectors escaping IRS should be prioritized. For instance, people who sleep indoors should use LLINs. Those who remain outdoors for various activities (cooking, praying, listening to the radio, eating, doing household chores) should use anti-mosquito ointment or smoke coils. Another alternative would be wearing garments impregnated with repellent insecticides which are available in some countries.
The evolution of malaria incidence in the communes where communities were protected by IRS presented from data collected in the health facilities should be more precise. That is why, the NMCP has decided to implement, from next year, a research study on epidemiological data on P. falciparum incidence and prevalence in collaboration with the parasitological research unit of CREC and the Faculty of Health Sciences of the University of Abomey-Calavi, Benin.