More than one billion people were estimated to be infected with soil-transmitted helminths (i.e. Ascaris lumbricoides, hookworm and Trichuris trichiura) in 2010 [1]. The majority of soil-transmitted helminth infections occur in Asia, sub-Saharan Africa and Latin America. Indeed, more than 90 % of the global burden due to soil-transmitted helminthiasis, which account for 5.1 million disability-adjusted life years (DALYs), are concentrated in this part of the world [2]. Infections primarily occur in remote rural and deprived urban settings among poorest communities in tropical and sub-tropical countries. Inappropriate hygiene practices, lack of access to clean water, improved sanitation, and health facilities are the main risk factors associated to the persistence of these infections [3–7]. Preventive chemotherapy with a strong focus on school-aged children is one of the main pillars of the global strategy against soil-transmitted helminthiasis [8]. However, this strategy alone does not prevent people from reinfection unless clean water, improved sanitation and adequate hygiene behaviour are available, used and practiced [9, 10].

Approaches, combining geographical information systems (GIS), remote sensing and Bayesian geostatistics, have been developed and applied in the last decade to model and predict the risk of helminth infections at high spatial resolution [11–14]. These efforts and resulting risk maps facilitate spatial targeting of control efforts in the context of resources scarcity [15]. Côte d’Ivoire is endemic to multiple helminth infections [16–19]. A particularly rich vein of investigation have been conducted in the mountainous western part of the country, including spatially explicit risk profiling [20–22]. In the Man area, elevation and land cover were identified as important predictors explaining the geographic variation of hookworm infections [11]. Yet, there is a paucity of studies investigating risk factors that explain the spatial distribution of soil-transmitted helminths at a national scale.

We present findings from a geostatistical analysis of soil-transmitted helminth infection data that were obtained from the first national school-based survey in Côte d’Ivoire. The aims of this study were (i) to map and predict the spatial distribution of soil-transmitted helminth infections in the school-aged population within a Bayesian geostatistical framework; (ii) to estimate school-aged children population-adjusted risk; and (iii) to calculate annual need for preventive chemotherapy according to guidelines put forward by the World Health Organization (WHO).

This study presents soil-transmitted helminth infection prevalence data from the first national school-based survey in Côte d’Ivoire and provides model-based estimates of the infection risk at high spatial resolution. Our results show that, in Côte d’Ivoire, hookworm are the predominant soil-transmitted helminths with an estimated school-aged children population-adjusted infection risk of 14.1 %. Only very low prevalence rates were found for A. lumbricoides infection (1.5 %) and T. trichiura infection (1.1 %). The negative association of urbanisation with hookworm infection risk is consistent with previous studies that highlight that hookworm infections are particularly rampant in rural settings [14, 37].

The prevalence of soil-transmitted helminth infections estimated by our study were lower than expected, especially with regard to A. lumbricoides and T. trichiura [14]. So far, soil-transmitted helminthiasis risk analyses have been based on historical data at broad scale [37], or with data restricted to well-known endemic areas [11, 38], collected from various sources that employ different methods and techniques [14]. A comparison of issues related to the use of historical data vs the use of recent national survey data is provided in the additional information (see Additional files 3 and 4). Thus, our results highlight the importance of a rigorous sampling design, such as the lattice plus close pair design for geostatistical modelling employed in this study [24]. In addition, it should be noted that after the armed conflict ended by the post-electoral crisis [39], the official health programme to control schistosomiasis, soil-transmitted helminthiasis and lymphatic filariasis and non-governmental health organisations restarted and intensified their activities through preventive chemotherapy campaigns targeting school pupils. Furthermore, in the last decades, many places of sub-Saharan Africa are undergoing rapid urbanisation and this development may have a positive impact on the helminth burden thus significantly reducing helminth infection prevalences [40].

We estimated that 1.3 million school-aged children should be dewormed yearly within the frame of preventive chemotherapy programmes implemented at the district level. This corresponds roughly to a third of the number of treatments estimated by Karagiannis-Voules et al. [14]. Indeed, these authors estimated the number of treatments required for school-aged children to amount to 3.6 million. The prior models were based on historical data, which do not necessarily reflect the actual situation, in particular when recent data are sparse [41, 42].

The risk of soil-transmitted helminth infection was estimated below 20 % for most health districts. According to WHO guidelines, school-aged children living in areas with a prevalence lower than 20 %, treatment on case-by-case basis should be undertaken whereas large scale administration of anthelminthic drugs should be advocated in health districts with an estimated moderate risk between 20 and 50 %. Emphasis should also be placed on water, sanitation and hygiene promotion (WASH) to sustain all efforts put in place to control the transmission of soil-transmitted helminthiasis [5, 7, 43].

Our study has several limitations that are offered for consideration. First, we have used the Kato-Katz technique and examined duplicate thick smears from a single stool sample for the presence of eggs of soil-transmitted helminths. Although this technique is recommended by WHO [44], it lacks sensitivity, particularly in low intensity settings. Hence, repeated sampling is required to account for day-to-day variation of egg output [45]. Thus, in our study, light infections were likely missed and the prevalence of soil-transmitted helminths observed may have been underestimated, given that most infections were of light intensity [46]. Secondly, the present study was carried out during the dry season, which could lead to underestimate the prevalence of soil-transmitted helminth infections that occur in Côte d’Ivoire as demonstrated by the effect of rainfall CV on the spread of T. trichiura. Thirdly, although the survey covered the entire Côte d’Ivoire, only 92 locations were visited. Increasing the number of schools to be investigated may give a more accurate estimation of soil-transmitted helminth infection prevalence in Côte d’Ivoire.

To conclude, this study provides smooth maps of the soil-transmitted helminth infection risk among school-aged children in Côte d’Ivoire that reflect well the observed prevalence. In addition, we offer a comprehensive map for the soil-transmitted helminth risk at health district level that can assist health officers controlling the infection according to WHO recommendations. Given that WHO does not recommend large-scale administration of anthelminthic drugs in areas of low soil-transmitted helminth infection, it is essential that monitoring of soil-transmitted helminth infection is regularly done so that treatment strategies can be readily adapted. Finally, although mass drug administration remains the cornerstone to fight against these diseases, we advocate integrated approaches that in addition to preventive chemotherapy aim at improving access to water and sanitation and at changing hygiene behaviour through health education campaigns for the sustainable control and future elimination of these diseases. The latter interventions become even more important when prevalences are very low and individuals with infections need to be treated on a case-by-case basis.