Delineation of onchocerciasis operational transmission zones of Burkina Faso.

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Learn more: PMC Disclaimer | PMC Copyright Notice Int Health . 2026 Jun 1;18(Suppl 1):i81–i88. doi: 10.1093/inthealth/ihag010 Delineation of onchocerciasis operational transmission zones of Burkina Faso Lassane Koala 1, ✉ , Nikiema Sindimbasba Achille 2 , Compaoré Justin 3 , Bougouma Clarisse 4 , Ouedraogo Mathias 5 , Adjami Gilles Aimé 6 , Sanon Karim 7 , Sanfo Sounkalo Moussa 8 , Tirados Iniaki 9 , Bessell Richard Paul 10 , Kafando Claude Montant 11 , Traoré Soungalo 12 , Boakye Adjei Daniel 13 , Roch Kounbobr Dabiré 14 Author information Article notes Copyright and License information 1 Centre National de Recherche Scientifique et Technologique/Institut de Recherche en Sciences de la Santé, Département Biomédical et Santé Publique, Direction Régionale de l’Ouest, Bobo Dioulasso, Burkina Faso 2 Centre National de Recherche Scientifique et Technologique/Institut de Recherche en Sciences de la Santé, Département Biomédical et Santé Publique, Direction Régionale de l’Ouest, Bobo Dioulasso, Burkina Faso 3 Ministère de la Santé du Burkina Faso, Direction de la Santé et de la Santé, Programme National de lutte contre les maladies tropicales négligées, Unité de lutte contre l’onchocercose, Ouagadougou, Burkina Faso 4 Ministère de la Santé du Burkina Faso, Direction de la Santé et de la Santé, Programme National de lutte contre les maladies tropicales négligées, Unité de lutte contre l’onchocercose, Ouagadougou, Burkina Faso 5 Ministère de la Santé du Burkina Faso, Direction de la Santé et de la Santé, Programme National de lutte contre les maladies tropicales négligées, Unité de lutte contre l’onchocercose, Ouagadougou, Burkina Faso 6 Expanded Special Project for the Elimination of Neglected Tropical Diseases Laboratory, World Health Organization, African Regional Office, Ouagadougou, Burkina Faso 7 Expanded Special Project for the Elimination of Neglected Tropical Diseases Laboratory, World Health Organization, African Regional Office, Ouagadougou, Burkina Faso 8 Expanded Special Project for the Elimination of Neglected Tropical Diseases Laboratory, World Health Organization, African Regional Office, Ouagadougou, Burkina Faso 9 Liverpool School of Tropical Medicine, Pembrooke Place, Liverpool UK 10 Independent Consultant, Edinburgh, UK 11 Ministère de la Santé du Burkina Faso, Direction de la Santé et de la Santé, Programme National de lutte contre les maladies tropicales négligées, Unité de lutte contre l’onchocercose, Ouagadougou, Burkina Faso 12 Ministère de la Santé du Burkina Faso, Direction de la Santé et de la Santé, Programme National de lutte contre les maladies tropicales négligées, Unité de lutte contre l’onchocercose, Ouagadougou, Burkina Faso 13 The Ending Neglected Diseases (END) Fund, New York, NY, USA 14 Centre National de Recherche Scientifique et Technologique/Institut de Recherche en Sciences de la Santé, Département Biomédical et Santé Publique, Direction Régionale de l’Ouest, Bobo Dioulasso, Burkina Faso ✉ Corresponding author: Tel: (+226)70587694; E-mail: koalalassane@gmail.com Received 2025 Mar 15; Revised 2025 Aug 25; Accepted 2026 Jan 20; Collection date 2026 May. © The Author(s) 2026. Published by Oxford University Press on behalf of Royal Society of Tropical Medicine and Hygiene. This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial License ( https://creativecommons.org/licenses/by-nc/4.0/ ), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact reprints@oup.com for reprints and translation rights for reprints. All other permissions can be obtained through our RightsLink service via the Permissions link on the article page on our site-for further information please contact journals.permissions@oup.com PMC Copyright notice PMCID: PMC13223583  PMID: 41636717 Abstract Background Defining the transmission zones for onchocerciasis is a critical first step needed in an endemic country aiming to eliminate the transmission of onchocerciasis. The process focuses on defining the geographical boundaries of areas where onchocerciasis transmission could persist with minimal influence from neighbouring regions. The use of operational transmission zones (OTZs) allows for assessment of interruption of onchocerciasis transmission with greater certainty within endemic countries. Methods We used available environmental, epidemiological and entomological data on onchocerciasis in Burkina Faso from the inception of the Onchocerciasis Control Programme to the present day. This information, together with field assessments, allowed delineation of OTZs across the whole country. Results Six OTZs were identified: Comoé-Léraba, Dienkoa, Bas-Mouhoun, Volta, Boucle du Mouhoun and Gourma. Conclusions Delineation of the onchocerciasis OTZs of Burkina Faso will facilitate the implementation of future interventions and assessment of the progress towards disease elimination within the country and bordering regions. Keywords: elimination program, onchocerciasis, operational transmission zones, river basins Introduction As part of the process of eliminating onchocerciasis in endemic countries, the World Health Organization (WHO) recommends the delineation of transmission zones (TZs) to support the elimination process. 1–3 Additionally, onchocerciasis elimination mapping (OEM) activities are recommended in endemic countries engaged in onchocerciasis elimination activities. 3 OEM is a strategy that aims to identify the status of onchocerciasis transmission in areas of a country that have not previously been under drug distribution. This is carried out by a process of desk review, exclusion mapping and entomological and epidemiological assessments in areas that were considered hypo-endemic or of unknown endemicity and where ivermectin has never been distributed. 4 OEM requires the delineation of transmission zones for entomological and epidemiological activities. However, the WHO does not provide an operational definition of transmission zones. The WHO defines a transmission zone as ‘a geographical area where transmission of Onchocerca volvulus occurs by locally breeding vectors and which can be regarded as a natural ecological and epidemiological unit for interventions’. 2 This definition does not have clear operational implications, nor does it specify the geographic size of potential transmission areas. Another difficulty is that TZs can be ‘open’ or ‘closed’ depending on the possibility of human and/or blackfly migration that could lead to reintroduction of the O. volvulus parasite. 5 Lacking a standard validated methodology, neglected tropical disease (NTD) programs in endemic countries commonly define epidemiological units, such as endemic basins or foci, as TZs, while others use health districts. 6 Almost all methods previously used to delimit TZs, whether based on molecular analyses characterizing the genetic diversity of O. volvulus populations or administrative boundaries such as districts, acknowledge the need to integrate entomological, environmental, epidemiological and sociological parameters to fully understand the extent of TZs. 7 Recently, Boakye et al. 8 introduced the concept of operational transmission zones (OTZs), which allows the delineation of geographic areas of onchocerciasis transmission within an endemic country based on hydrological data such as river basins, blackfly species present, blackfly breeding sites distribution and ecological conditions. Burkina Faso, a former Onchocerciasis Control Programme 9 country, has made significant progress towards interruption of transmission of onchocerciasis. The National Program for the Control of Neglected Tropical Diseases (PNMTN, for its French acronym) has carried out entomological and epidemiological surveillance activities in endemic regions in the country. 10 The regions of Cascades (now Tannouyan) and Southwest (now Region of Djoro) remain endemic to onchocerciasis. The prevalence of the disease exceeded 5% in 70% of the villages assessed in these two regions in 2010 and 2011. 11 , 13 , 14 Furthermore, the infectivity rates of blackflies collected and analysed in these regions exceeded the 0.05% threshold defined by the WHO as the critical threshold for interrupting transmission. 2 , 10 , 12 Data from these activities allow us to assess the transmission in different geographic classifications, namely villages, rivers, health districts or administrative regions. However, due to the interactions and interdependence among various geographical classifications, it is difficult to attribute transmission at any given time to a single geographical area. Transmission dynamics—such as the interactions between environment, competent vectors and human hosts—are not consistently confined to one geographical unit. Transmission disparities can sometimes occur on a small scale. For example, certain sections of watercourses may accommodate blackfly breeding sites, while other parts remain free. This is the case with the Mouhoun River, which flows through the non-endemic regions of Hauts-Bassins (now Guiriko) and Boucle du Mouhoun (now Bankui), but also the endemic Southwest region (now Region of Djoro). 15 Similarly, an administrative region may sometimes contain both endemic and non-endemic districts, as observed in the Tannouyan region. 10 Therefore, it is essential to delineate geographical units in which endemic countries could practically implement monitoring interventions and regularly assess the progress towards disease elimination. In this study we applied a systematic approach to delineate the OTZs of onchocerciasis in Burkina Faso, with the aim of informing and supporting the country’s elimination efforts. Methods Context/initiation of the present study This study was conducted through collaboration between the PNMTN and partner institutions, including European Society for Clinical Nutrition and Metabolism/WHO-AFRO, the Institute for Research in Health Sciences Western Regional Directorate and The END Fund. Description of study sites Burkina Faso is a West African country where onchocerciasis remains endemic. In July 2025, the government of Burkina Faso implemented a new administrative division to address ‘security challenges and the affirmation of cultural identities’. This new organization has no impact on our study other than the renaming of the administrative regions. Indeed, the regional names have been replaced with indigenous names inspired by the country’s local languages. Thus the two regions of the country where onchocerciasis is still endemic, the Cascades region and the Southwest region, are now called the Tannouyan region and the Djoro region, respectively. Burkina Faso has a tropical climate with a long dry season from November to April and a rainy season from May to October. The country is subdivided into three bioclimatic zones according to their rainfall: the Sahelian zone in the north, with <600 mm of rainfall per year and a high temperature range (15°C–45°C); the Sudano-Sahelian zone in the centre, with rainfall ranging between 600 and 900 mm per year and average temperatures between 28°C and 30°C; and the Sudano-Guinean zone in the west, where precipitation ranges from 900 to 1200 mm per year, with average temperatures <28°C. 15 Geographically, Burkina Faso benefits from an extensive hydrographic network formed by four major river basins: the Mouhoun, Nakambé, Niger and Comoé. The Mouhoun (Black Volta) basin includes the Dienkoa, Bougouriba and Bambassou subbasins as its main tributaries. The Nakambé (White Volta) basin comprises of the Nazinon (Red Volta) and Sissili Rivers. The watercourses of the Volta Rivers join in Ghana to form Lake Volta before flowing into the Gulf of Guinea. The Niger basin includes the Oti, Pendjari, Sirba, and Mekrou Rivers as tributaries, while the Comoé basin, which covers about 7% of the country’s surface area, consists of the Comoé River and its main tributary, the Léraba River, and flows southward into the Gulf of Guinea via Côte d’Ivoire. 15 Except for the Niger basin, whose tributaries flow into the Sahelian zone, all of the country’s hydrographic basins lie within the Sudano-Sahelian and Sudanese bioclimatic zones, where onchocerciasis has been prevalent for decades. In contrast, the Sahelian region, characterized by an arid climate and sparse river systems, is unsuitable for blackfly breeding. This was further confirmed by Schmidt et al., 16 who used a Bayesian model–based geostatistical approach to predict a prevalence of <1% across nearly all areas of northern and central Burkina Faso. History of the onchocerciasis situation since OCP Burkina Faso, formerly Upper Volta, served as the centre of OCP activities due to both the hyperendemicity of onchocerciasis at the time and its strategic geographical location. 9 In 1966, well before OCP activities began, the population of Upper Volta was estimated at 4.5 million people, of whom approximately 400 000 were infected with onchocerciasis, with a blindness rate of approximately 10% (Figure 1 ). 11 , 17 Figure 1: Open in a new tab Map of onchocerciasis endemicity in Burkina Faso (former Haute Volta) in 1974. The figure presents the prevalence of the disease in the country before the OCP interventions. The Office de la Recherche Scientifique et Techniques Outre Mer (ORSTOM) and other national structures had already gathered data on the clinical aspects of the disease as well as its entomology, parasitology, epidemiology and socio-economic impact. 11 In 1974, the OCP launched activities to eliminate onchocerciasis as a public health problem in the Volta basin countries of West Africa, including Burkina Faso. 17 The OCP later expanded to cover 11 West African countries (Ghana, Cote d’Ivoire, Togo, Benin, Mali, Senegal, Burkina Faso, Guinea, Guinea Bissau, Sierra Leone and Niger). After several decades of control, the OCP succeeded in eliminating onchocerciasis as a public health problem in Burkina Faso and other participating countries. 1 , 11 Currently in Burkina Faso, only the Tannouyan and Djoro regions are under community-directed treatment with ivermectin (CDTI), following the resurgence of onchocerciasis in these regions in 2010–11. 11 However, recent data indicate residual transmission in several river basins within these regions. 11 , 13 In the Sudanian savannah, Simulium damnosum s.s. and Simulium sirbanum are the main onchocerciasis vectors during the dry and rainy seasons, respectively. 10 , 18 Approach Criteria for delineating OTZs The African Programme for Onchocerciasis Control 1 has defined a TZ as ‘a river basin, or a major section of a river basin, where onchocerciasis is endemic and where the river is the core of the endemic area, with communities with the highest prevalence of infection generally located close to the river and infection levels falling with increasing distance from the river until they become negligible or reach a neighbouring transmission zone’. The river basin is a central concept in the biological approach described by Boakye et al. 8 These authors define an OTZ as the geographic area within a country delineated to support programmatic decision-making based on hydrology (e.g. river basins), ecological conditions, breeding site information, vector species distribution and the prevalence of O. volvulus infection. It is important to note that an OTZ, even when defined within national boundaries, is not necessarily a closed transmission zone. OTZs may be subject to human and/or blackfly migration and can therefore be classified as either open or closed. In Burkina Faso, OTZs were delineated as follows: Exclusion of non-conducive regions. Areas unsuitable for onchocerciasis and vector development, primarily desert regions, were excluded. River basins as central units. OTZs were defined around endemic river basins with known S. damnosum breeding sites (Figure 2 ) using historical data from the OCP (1974–2002) and African Programme for Onchocerciasis Control (APOC; 1995–2015) accessed via the WHO website (WHO.iris), supplemented by Burkina Faso NTD program reports, published literature, environmental data and grey literature. Extent of the OTZ. Starting from the river basin, the OTZ was extended to include surrounding areas based on data on vector species, vector movement capacity (dispersal and migration), interaction between neighbouring breeding sites, disease endemicity, formal boundaries such as international borders and natural or anthropogenic boundaries such as forests, mountains, roads and fields. Geographical boundaries. Once the extent of an OTZ was defined, its geographic boundaries were set where interactions between flies from different breeding sites were estimated to be limited or non-existent. This approach was applied in areas where the exchange of local vector populations was considered limited. For example, the exchange of blackfly populations between the Bougouriba and the Comoé Rivers is unlikely due to significant distance and unfavourable geographical conditions for blackfly migration. 11 A single transmission zone can encompass multiple endemic basins. In certain cases, different basins may be interconnected through their sources or tributaries, potentially forming a single watercourse. The extent of the transmission zone in such cases is determined by the level of interdependence between basins and the migratory flows of both humans and vectors. Classification as ‘open’ or ‘closed’. OTZs were classified as open transmission zones when the endemic river basin extends into a neighbouring country or when blackfly reinvasion and/or human migration has been reported in the area. Burkina Faso is a landlocked country bordering six countries, all endemic to onchocerciasis except for Niger. Figure 2: Open in a new tab Distribution of S. damnosum capture points from the inception of the OCP to the PNMTN. The blue lines represent the main rivers of the country and each blackfly on the map represents an active Simulium breeding site. Results We considered only areas prone to onchocerciasis. Areas with few or no river basins were automatically deemed as not prone and excluded from the transmission zones. This applies to northern Burkina Faso, which is non-endemic due to its desert climate, with only temporary watercourses and limited water points. 16 For the same reasons, this area was not included in the OCP intervention areas. 19 The remainder of the country was divided in OTZs based on river basins, Simulium breeding sites (Figure 2 ), disease epidemiology and the potential for human migration and/or blackfly reinvasion. The OTZs were delineated around the following river basins: Nakambe basin (White Volta): flows through central Burkina Faso and has two main tributaries, the Nazinon (Red Volta) and Sissili. Onchocerciasis is no longer considered endemic along these basins. Niger basin and tributaries: located in the east of the country, this basin is also no longer endemic to onchocerciasis. Comoe basin and Leraba tributary: situated in the southwest of the country, these basins flow toward Côte d’Ivoire. Mouhoun basin (Black Volta): originating in the west and continuing southwest towards Ghana, the Mouhoun basin includes the Dienkoa tributary upstream and the Bougouriba and Bambassou tributaries downstream in the southwest. Based on the analysis of river basins and surrounding areas, we delineated six OTZs in the regions of Burkina Faso endemic for onchocerciasis: the Comoé-Léraba, Bas-Mouhoun, Boucle du Mouhoun, Dienkoa, Volta and Gourma OTZs (Figure 3 ). Figure 3: Open in a new tab Delineation of OTZs of onchocerciasis in Burkina Faso. The OTZs are outlined in light red lines. These are the OTZs of Comoé-Léraba, Dienkoa, Bas-Mouhoun, Volta, Boucle du Mouhoun and Gourma. The non-surrounded zone constitutes the non-endemic zone for onchocerciasis. Country boundaries are from GADM ( https://gadm.org/ ) and rivers from HydroRivers ( https://www.hydrosheds.org/products/hydrorivers ). Comoé-Léraba OTZ The Comoé watershed includes the Comoé River and its main tributary, the Léraba River, along which numerous S. damnosum breeding sites have been identified. 12 Both rivers continue their courses into Côte d’Ivoire where they join to form the Haute Comoé. Administratively, the Comoé watershed lies in the Tannouyan region of Burkina Faso, which is comprised of three health districts: Banfora and Mangodara, crossed by the Comoé, and Sindou, crossed by the Léraba. Several active blackfly breeding sites exist in this TZ, but they are primarily productive during the rainy season. 12 During the recrudescence of onchocerciasis in the Tannouyan region (former Cascades region) in 2010, prevalence rates were highest (between 5% and 71%) in frontline villages directly bordering the Comoé River, whereas all the villages along the Léraba River had a prevalence of <1%. 11 Historically, the OCP treated the Comoé and Léraba basin as a single entity, referred to as ‘Léraba-Comoé’. Consistent with this approach, we considered the regions bordering the Comoé and its main tributary the Léraba as a single OTZ in Burkina Faso. Dienkoa OTZ The Dienkoa corresponds to the upper course of the Mouhoun (Black Volta), with a watershed of approximately 800 km. The river originates about 20 km southeast of Orodara, near the village of Mondon. 15 It flows eastward, then northward, and after roughly 70 km joins the Plandi, together forming the Black Volta. Its main tributaries include, on the east bank, the Guénako, the Dia, the Koin and the Sinsoa, and on the west bank, several small temporary tributaries. The OCP entomological surveillance network established capture points at Guéna Pont (Dienkoa), Banfoulagué (Dia), Gossiamandara (Sinsoa), Koulignon (Guénako) and M’Bié (M’Bié-Guénako). 20 All these points remain productive, as evidenced by the regular collection of S. damnosum s.l. ( S. damnosum s.s. / S. sirbanum ) during entomological surveillance activities carried out by the PNMTN. 10 Based on these data, we delineated the area bordering Samandeni village, and defined by the Dienkoa River and its tributaries, as a single OTZ. Bas-Mouhoun OTZ The Bas-Mouhoun OTZ corresponds to the administrative region of Djoro in Burkina Faso, which includes the health districts of Diebougou, Dano, Gaoua and Batié. The area is drained by the lower Mouhoun River and its main tributaries, the Bougouriba and Bambassou Rivers. The Mouhoun River, the largest river basin in Burkina Faso, includes an upper course, a middle course forming a loop and a lower course, with numerous tributaries. This basin, formerly called the Black Volta, crosses several regions of the country and was hyperendemic for onchocerciasis. 21 Data from both the OCP and later the PNMTN revealed a marked variation in endemicity along the basin. The Bougouriba was of particular concern, with recurrent upsurges. 22 In 1974, the riverside villages of Bougouriba and Mouhoun recorded the highest endemicity levels for onchocerciasis in West Africa, with a prevalence of 83.7% and community microfilaria load of 98 mf/s. 22 Recent PNMTN data further indicate the persistence of transmission along these waterways despite ivermectin distribution for onchocerciasis and lymphatic filariasis. 10 , 23 The Bougouriba and Bambassou Rivers join the Mouhoun, which continues its course in Ghana. Based on these data, we delineated the area surrounding the lower Mouhoun basin and the Bougouriba and Bambassou basins as a single OTZ, hereafter referred to as the Bas-Mouhoun OTZ. Boucle du Mouhoun OTZ We considered the section of the Mouhoun forming a loop as a single OTZ called the Boucle du Mouhoun OTZ. Administratively, this zone spans both the Boucle du Mouhoun (now Bankui region) and Center-West (now Nando) regions. The area is crossed by the Mouhoun River and its tributaries, including the Sourou, Grand Balé and Petit Balé. The OTZ has several blackfly breeding sites and capture points that were historically monitored by the OCP and later by the PNMTN. Analysis of historical data indicates that the OCP monitored several blackfly breeding sites in this area on the Sourou and Boucle du Mouhoun, with major sites located near Leri, Sanekui and Niempourou. 24 This area was formerly endemic for onchocerciasis. However, epidemiological surveys carried out in 2014 revealed that all villages assessed in the area had microfilaria prevalence rates of <5%. 1 , 13 Volta OTZ Based on available epidemiological and entomological data, we defined a single OTZ encompassing the basins of the Nakambé (formerly White Volta), Nazinon (formerly Red Volta) and Sissili Rivers. 23 Collectively this area is referred to as the Volta OTZ. Historical data indicate that in 1974 each of these three basins was endemic for onchocerciasis, with a prevalence ranging from 60% to 80% among the communities living nearby (Figure 1 ). 11 More recent epidemiological assessments have revealed that the prevalence of onchocerciasis in villages bordering these basins has decreased to <5% in all the villages assessed. 13 Entomological indicators also confirmed this trend. 10 It is worth noting that both the Nazinon and Sissili are tributaries of the Nakambé. Gourma OTZ We grouped all the tributaries of the Niger basin within Burkina Faso into a single OTZ, designated as the Gourma OTZ. This basin includes several tributaries, including the Sirba, Gourobi, Tapoa-Mékrou and the Dyamongou Rivers, all of which were historically endemic for onchocerciasis. These tributaries are seasonal streams that flow only during the rainy season. Historical data show that the OCP program monitored the upper Niger tributaries in Burkina Faso. Most blackfly breeding sites in this area are artificial and created through human activities such as bridges and dams. Examples include the Sirba fishermen’s dam on the Sirba (Upper Volta) or the Gourbi; road riffles or retention dams such as the Niena (Boulsa) and Bilanga (Fada Gourma) riffles on the Sirba; the Tougouri, Yalogo and Liptougou dams on the Faga (a tributary of the Sirba); and the Boudieri (Kantchari) dams on the Diamongou and Tapoa (Diapaga). The most recent serological assessment, conducted in 2017, detected no positive cases among children ages 5–9 y who were the subject of the assessment. 9 Discussion We delineated six OTZs for onchocerciasis in Burkina Faso. These OTZs serve as evaluation units for assessing interruption of onchocerciasis transmission. The delineation was based on both historical OCP data and more recent epidemiological and entomological findings. In some cases, endemic basins and their main tributaries were grouped together into single OTZs. For example, in the Comoé OTZ, the Comoé and the Léraba were considered together because the Léraba is a tributary of the Comoé, despite the differences in endemicity between the two rivers. Over the past decade, the onchocerciasis situation in Léraba has remained satisfactory, with prevalence generally <5%, even in the absence of mass drug administration. 11 In contrast, the Comoé basin experienced a resurgence in 2010–11, and the infection persists despite ivermectin treatment since 2011. 11 , 12 We found no evidence suggesting reintroduction of onchocerciasis transmission from neighbouring OTZs such as Dienkoa or Bas-Mouhoun. Importantly, there has been no indication of ‘contamination’ of the Léraba by the endemic areas of the Comoé, despite regular population movement between the two regions. 9 However, the risk of re-emergence remains due to reinvading blackflies from outside Burkina Faso, particularly from Côte d’Ivoire (Bandama and Marahoué and their tributaries). 25 , 26 Taken together, these findings support treating the Comoé-Léraba as a practical geographical entity for evaluating transmission interruption. For the Dienkoa OTZ, we grouped the Dienkoa and other tributaries of the Haut-Mouhoun into a single OTZ, given the proximity of Simulium breeding sites and their likely interactions. The OCP historically monitored the Dienkoa, with Samandeni village serving as a reference endemic site. 20 This area was long recognized as a hotspot for persistent transmission, driven by factors such as human migration, irregular larviciding, the introduction of community-Based treatment with ivermectin (CBTI) and then CDTI with suboptimal geographic and therapeutic coverage and the early cessation of larvicide treatments to evaluate the effect of ivermectin treatment. 22 More recent evaluations indicate a favourable situation, with molecular analyses of blackflies showing infection rates well below the 0.05% entomological threshold for transmission interruption. 2 , 10 However, significant ecological changes have occurred: the construction of the Samandeni Dam increased the number of breeding sites, increasing biting rates. 28 Given these dynamics, grouping all of Haut-Mouhoun tributaries with breeding sites into a single OTZ is justified. In the Volta OTZ, we combined the Nakambé, Sissili and Nazinon basins because of hydrological connectivity and interactions among their breeding sites. These rivers converge in northern Ghana to form the White Volta, which flows into Lake Volta. Historically, OCP monitored three key capture points in this area, Loaba (Nakambé), Pont du Ziou (Nazinon) and Pont Sissili Leo (Sissili), each of which was among the 10 sentinel sites recommended by the OCP. 10 This OTZ is open to Ghana via the White Volta, Black Volta and Pru Rivers (Figure 2 ). The Gourma OTZ, by contrast, has historically shown seasonal and moderate transmission compared with other endemic basins. However, it remains an ‘open’ OTZ with a subregional dimension, as the Niger basin extends across Niger, Burkina Faso and Benin. OTZs Although the objective of this study was to delineate national OTZs, several of them—the Comoé-Léraba, Bas-Mouhoun, Volta and Gourma—should also be considered as OTZs given their shared international borders with Côte d’Ivoire, Ghana or Niger, and the risks posed by human migration and Simulium reinvasion. 25 , 26 Verification of the interruption of onchocerciasis transmission in these cross-border OTZs must therefore account for the epidemiological situation in neighbouring countries. For instance, recent entomological studies in the Bas-Mouhoun OTZ detected circulation of O. volvulus in blackflies collected in Bambassou and surrounding areas. 10 This suggested that the Bas-Mouhoun OTZ, which straddles Ghana and Côte d’Ivoire, requires reinforced surveillance, like the Comoé-Léraba OTZ. In contrast, with the elimination of onchocerciasis in Niger, the risk of reintroduction into the Gourma OTZ has become minimal. 28 These considerations highlight the importance of coordinated cross-border activities between Burkina Faso and its neighbours (Côte d’Ivoire, Ghana, Mali, Benin, Togo and Niger) to accelerate progress toward elimination of onchocerciasis. Finally, while the APOC 5 suggested defining TZs as all communities located within a 20-km radius of the river, this criterion was not applied in Burkina Faso. Given the size of the endemic basins, often extending tens to hundreds of kilometres, this approach would have resulted in numerous interdependent OTZs and limited practicality. Conclusions We delineated six OTZs in Burkina Faso to serve as the basis for verifying interruption of transmission and guiding decisions on stopping CDTI in areas under mass drug administration. These OTZs represent epidemiologically coherent geographic units where onchocerciasis transmission occurs with little influence from neighbouring regions. This framework provides a practical tool for national efforts and aligns with WHO recommendations for surveillance. By applying this approach, Burkina Faso can improve the efficiency of surveillance and accelerate progress toward national elimination goals. Acknowledgements The authors express gratitude to Anne Heggen, Charles Mackenzie and Dziedzoum de Souza, who revised the manuscript. Contributor Information Lassane Koala, Centre National de Recherche Scientifique et Technologique/Institut de Recherche en Sciences de la Santé, Département Biomédical et Santé Publique, Direction Régionale de l’Ouest, Bobo Dioulasso, Burkina Faso. Nikiema Sindimbasba Achille, Centre National de Recherche Scientifique et Technologique/Institut de Recherche en Sciences de la Santé, Département Biomédical et Santé Publique, Direction Régionale de l’Ouest, Bobo Dioulasso, Burkina Faso. Compaoré Justin, Ministère de la Santé du Burkina Faso, Direction de la Santé et de la Santé, Programme National de lutte contre les maladies tropicales négligées, Unité de lutte contre l’onchocercose, Ouagadougou, Burkina Faso. Bougouma Clarisse, Ministère de la Santé du Burkina Faso, Direction de la Santé et de la Santé, Programme National de lutte contre les maladies tropicales négligées, Unité de lutte contre l’onchocercose, Ouagadougou, Burkina Faso. Ouedraogo Mathias, Ministère de la Santé du Burkina Faso, Direction de la Santé et de la Santé, Programme National de lutte contre les maladies tropicales négligées, Unité de lutte contre l’onchocercose, Ouagadougou, Burkina Faso. Adjami Gilles Aimé, Expanded Special Project for the Elimination of Neglected Tropical Diseases Laboratory, World Health Organization, African Regional Office, Ouagadougou, Burkina Faso. Sanon Karim, Expanded Special Project for the Elimination of Neglected Tropical Diseases Laboratory, World Health Organization, African Regional Office, Ouagadougou, Burkina Faso. Sanfo Sounkalo Moussa, Expanded Special Project for the Elimination of Neglected Tropical Diseases Laboratory, World Health Organization, African Regional Office, Ouagadougou, Burkina Faso. Tirados Iniaki, Liverpool School of Tropical Medicine, Pembrooke Place, Liverpool UK. Bessell Richard Paul, Independent Consultant, Edinburgh, UK. Kafando Claude Montant, Ministère de la Santé du Burkina Faso, Direction de la Santé et de la Santé, Programme National de lutte contre les maladies tropicales négligées, Unité de lutte contre l’onchocercose, Ouagadougou, Burkina Faso. Traoré Soungalo, Ministère de la Santé du Burkina Faso, Direction de la Santé et de la Santé, Programme National de lutte contre les maladies tropicales négligées, Unité de lutte contre l’onchocercose, Ouagadougou, Burkina Faso. Boakye Adjei Daniel, The Ending Neglected Diseases (END) Fund, New York, NY, USA. Roch Kounbobr Dabiré, Centre National de Recherche Scientifique et Technologique/Institut de Recherche en Sciences de la Santé, Département Biomédical et Santé Publique, Direction Régionale de l’Ouest, Bobo Dioulasso, Burkina Faso. Authors’ contributions LK, CJ, AA, TS and BAD were responsible for the study design, study implementation and analysis and interpretation of data. LK wrote the original draft of the manuscript. All authors reviewed and edited the manuscript. All authors read and approved the final version of the manuscript. 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