Evaluation of the residual efficacy of indoor residual spraying with bendiocarb (FICAM WP 80) in six health districts in Senegal

Malaria continues to be a major public health problem throughout the world and particularly in sub-Saharan Africa [1]. According to the latest World Health Organization (WHO) estimates there were 219 million cases of malaria in 2017 of which 92% in the African region [2]. The strategies against malaria involve rapid diagnosis and treatment, and stopping disease transmission through vector control, which aims to prevent, interrupt or at least reduce transmission [3]. Malaria vector control is based predominately on the use of residual insecticides through indoor residual spraying (IRS) and insecticide-treated nets (ITNs) [4]. The use and scaling-up of these methods can significantly decrease malaria morbidity and mortality [5]. The recent recorded results have, therefore, contributed to the consideration of malaria elimination as a feasible objective [6]. However, with the development of insecticide resistance, there are serious concerns about the maintenance of the effectiveness of these control measures [7].

In Senegal, malaria is still a public health problem despite the valuable results obtained during the last years. A significant regression was noted between 2009 and 2016 with an average parasite prevalence range of 1–3% [8]. However, this decline hides disparities with an uneven distribution of the disease in the country. This situation has led to the implementation of a more operational stratification, allowing the adaptation of the control tools to the specificity of the different epidemiological strata.

As part of the Millennium Development Goals (MDG), Senegal, like many other African countries, benefits from the President’s Malaria Initiative [9]. Since 2007 this has permitted the National Malaria Control Programme (NMCP) to implement IRS campaigns in several health districts with the main objective to interrupt or reduce malaria transmission in selected sites [10]. During the IRS campaigns, several insecticides approved by WHOPES were used. Since the beginning of the IRS programme in Senegal, different insecticide molecules and formulations were successively used, the ICON WP 10 from 2007 to 2009, the K-Othrine WG 250 from 2010 to 2011 [11]. Five years of use of pyrethroid insecticides in IRS health districts has targeted vector populations to high insecticide pressure, which resulted to the rise of insecticide resistance, as observed in all the treated areas. Therefore, pyrethroid-based formulations were replaced by bendiocarb (FICAM WP 80) from 2011 to 2014, and by the 300 CS formulation of Actellic [11].

As part of the IRS campaigns, the present study was carried out to monitor and evaluate the residual efficacy of the treatments with the FICAM WP 80 from 2011 to 2014 in the surveyed health districts in Senegal.

The study of the residual efficacy of IRS was carried out in the selected IRS health districts taking into account their specific characteristics. The district of Malem Hodar was treated with FICAM in 2011, 2012, 2013, and 2014. Overall, except for the first IRS campaign, the effectiveness of treatment decreased regularly and rapidly over time with spatial heterogeneity within the district reaching 9 months of residual efficacy in Makka Bella and Niahène during the IRS1 (2011) whatever the type of support. A residual effect was recorded on cement support in both Dianke Souf and Taiba at the beginning and the end of the survey. For the following IRS campaigns, the efficacy of the treatment did not exceed 3 months (Fig. 2).

In the district of Velingara, the Bendiocarb monitored for three successive IRS campaigns (2011, 2012, 2013), revealed that the cement supports were effective up to 6 months in the village of Bokonto. In Kael Bessel, Nemataba and Sinthian Koundara, the efficacy was observed at the beginning and the end of the survey in 2011 (Fig. 3). On mud walls, no efficacy was recorded in Bonkonto. In Kael Bessel and Nemataba, the treatment was effective at the end of the survey whereas in Sinthian Koundara, the effectiveness was observed at the beginning of the survey. During the second year of treatment, the effectiveness was observed only on the cement supports with an effective duration period that did not exceed 2 months (Sinthian Koundara) to 3 months (Bonkonto, Kael Bessel and Nemataba). During the third year of treatment, the effectiveness was observed only during at least 2 months in Bokonto, Kael Bessel, Nemataba and Sinthian Koundara for both the supports on cement and mud (Fig. 3).

In the district of Nioro, the Bendiocarb was used during two consecutive campaigns (2011 and 2012). In all the villages, the effectiveness of the product remained higher on the cement supports excepted in Thiamène Walo village where it did not exceed 3 months at the beginning of the survey in 2011 (Fig. 4). During the second year of treatment, the product was effective only in Thiamène Walo (both on cement and mud supports), in Ndrame Ndimb (on cement support) and in Bamba Diakhatou (on mud support) at the beginning of the survey. The effectiveness decreased gradually during all post-IRS period in both supports (Fig. 4).

From 2012 to 2014, the treatments were done in the district of Koungheul with Bendiocarb. During the first year of treatment, in Touba Aly Mbenda, Keur Serigne Diebel, Touba Koungheul and Pakala, the efficacy was observed up to 2 months on cement support and in Ida Mouride and Touba Koungheul in the mud-plastered supports during 2 months (Fig. 5). During the second year, the effectiveness was observed on cement supports during 2 months in all the villages excepted in Pakala where the treatment in mud support was effective at the beginning of the survey (Fig. 5). The supports on cement retained the residual efficacy of 80% for at least 1 and 2 months in Ida Mouride village during the third campaign, respectively, on mud and cement walls. For the other villages, the effectiveness was observed both on mud and cement walls one month after the treatments (Fig. 5).

During the first year of treatment, the cement and mud-plastered supports showed 9 months of effectiveness excepted in Village 1 in the district of Koumpentoum where the residual efficacy was observed up to 4 months on cement and mud supports (Fig. 6). During the second and third years of treatment, the efficacy in mud walls was 2 months in Altou Fass. In Koumare, Kouthiaba, Fass Gounass and Village 1, an efficacy up to 2 months in cement walls was reported in 2012 (Fig. 6). In the third year of treatment, the residual efficacy was observed up to 3 months on cement supports in Village 1 (Fig. 6).

In the district of Guinguineo, the first IRS campaign in 2011 showed a residual efficacy of the treatments with bendiocarb in cement and mud walls up to 9 to 10 months (Fig. 7). In 2012, no efficacy was observed excepted in Athiou (on cement support) and Ngathie Naoude (both on cement and mud supports) for, respectively, the eighth and second month after the treatments (Fig. 7).

The residual efficacy of the IRS with the Bendiocarb WP 80 formulation was assessed on different supports (mud and cement) in field conditions in six health districts in Senegal: Velingara and Koumpentoum (2011–2013), Malem Hodar (2011–2014), Guinguineo and Nioro (2011–2012), and Koungheul (2012–2014). During the first campaign the residual efficacy of IRS with bendiocarb lasted up to 6 months post-treatment in Velingara, Nioro and Koungheul health districts. Similar results have been obtained in Mozambique in 2004 [18]. However, IRS efficacy varied within some health districts as previously reported from Zimbabwe in 1991 [19], lasting for only 2–4 months in Village 1 and Thiamène Walo in 2011, located, respectively, in the health districts of Koumpentoum and Nioro. During the following IRS campaigns, the decay rate and the residual efficacy of IRS varied between wall materials, study sites and between IRS campaigns. At health district level, the insecticide decay rate was short (≤ 3 months) in Malem Hodar in 2012, 2013 and 2014, Velingara in 2013, Koumpentoum in 2012 and 2013, as well as in Koungheul in 2013 and 2014. Similar observations have been reported from The Philippines where the post-exposure mortality rates of Anopheles flavirostris varied between 75 and 100% up to 3 months post-treatment [20]. Moreover, the residual efficacy of sprays differed between wall structures whatever the surveyed health districts and years, as previously reported from Tanzania and Benin [21, 22]. The observed difference in the decay rate between cement and mud-made walls may be explained by the highest porosity of the mud compared to cement support. The porosity of mud favoured a rapid insecticide intake and its highest decay rate compared to cement structures. Such difference confirms that the entomological efficacy of IRS may be influenced by the structure support due to different intake and decay rates of the insecticide over the time [23].

A surprising increase and rebound in the mortality rate among the exposed mosquitoes was noticed in some health districts for certain years which could be caused by possible relapse of the insecticide in treated surfaces walls due to yet unknown factors. Further investigations are needed to unravel factors underlying such an observation and the mechanisms of the insecticide release several months post-treatment.

The difference observed within and between health districts and between years may be explained by possible operators biases due to lack of good spray competencies of certain sprayers which cause bad spray quality. Therefore, it is highly recommended to carry out refreshing training before any IRS campaign to ensure good spray quality and compliance with operators, communities and environmental safety.

The Bendiocarb WP 80 formulation was used to control pyrethroid-resistant vectors populations in six health districts as part of the NMCP insecticide resistance management plan. The study revealed that the residual efficacy of IRS with bendiocarb WP 80 formulation lasted in general up to 3 months and varied between localities, health districts and campaigns. If the quality of the IRS application is excluded as a possible explanation of the short efficacy duration, the results suggest that, to cover the rainy season, that lasts 5 to 6 months in the southern regions of the country, at least two rounds of spray are needed to cover the transmission period that occurs between late August and October. Such treatments should be carried out rather before the intensification of rains in July and August in order to cover the period of transmission.