Onchocerciasis transmission in Kafolo, located in the Côte d’Ivoire – Burkina Faso cross-border region of Tchologo

doi:10.21203/rs.3.rs-2996807/v1

Background: Onchocerciasis or river blindness is a parasitic endemicity caused by a filariasis called Onchocerca volvulus. This nematode worm is transmitted to humans due to bites by infested female black flies. In 2002, WHO closed the Onchocerciasis control programme in West Africa at the time when the disease transmission risk was constricted in all countries where Onchocerciasis was a cause of concern. The aim of this study is to evaluate Onchocerciasis transmission in the Kafolo area, located at the Ivorian-Burkinabe border in the region of Tchologo in Côte d’Ivoire, after the period of interruption of the national Onchocerciasis control programme activities, and following the observation of an increase in the disease prevalence in both sides of the border.

Methods: Standardized human landing catches of black flies based on a WHO/OCP technique were carried out in 2019 at the intervals of two (2) days per month. Caught black flies were dissected to look for potential onchocerca larvae. These larvae were then analyzed through the PCR technique 0 - 150 to identify their species.

Results: Annual Bitting Rates (ABR) were 25,355 and 20,235 bites/person/year at Kafolo-Amont and at Kafolo-Aval respectively, with an average parturity rate of 60.33% at the level of the first site and 55.23% at the level of second one. Adjusted infectivity rates amounted to 3.93 IF/1,000 pares and 4.91 IL/1,000 pares for Kafolo-Amont (Upstream) and to 4.03 IF/1,000 pares and 6.71 IL/1,000 pares for Kafolo- Aval (Downstream). Adjusted ABR reported 75 bites/person/year in each site. This rate exceeds almost 4 times the threshold defined by WHO for the disease control. the high transmission rate observed in this study was not expected and could constitute a potential source of contamination of adjacent river basins across the border in Burkina Faso.

Conclusion: There is active onchocerciasis transmission continues at Kafolo. Therefore, Onchocerciasis elimination is not yet achieved in this site. The subsequent recommendation is that entomological and epidemiological transmission prevalence data be updated in this area.

Transmission

Onchocerciasis

Black flies- Entomological indices

Elimination

Surveillance- Kafolo

Côte d’Ivoire

Onchocerciasis or river blindness is a parasitic endemicity caused by a filariasis called Onchocerca volvulus. This nematode worm is transmitted to humans due to bites by infested female black flies [1]. Its symptoms mainly occur in human skin, lymphatic cells and eyes and could cause extreme risk of blindness in most advanced stages. Human onchocerciasis, a vector transmitted disease, is endemic in 31 sub-Saharan African countries, 3 Latin American countries, and in Yemen. WHO declared the elimination of the parasite transmission in Colombia, Ecuador and Mexico [2]. In West Africa, its severe health and socio-economic impact, and the availability of data contributing to setting up a regional control programme have motivated governments of countries infected by the disease and the international community to set up the Onchocerciasis Control Programme (OCP). In this way, OCP was set up under WHO leadership from 1974 to 2002 and covered 11 West African countries, including Côte d’Ivoire.

As far as Côte d’Ivoire is concerned, due to the relative density of the hydrographic network, which is conducive to the settlement of pre-imaginal breeding sites of Simulium damnosum s.l., onchocerciasis existed in the major part of the country in two forms: a blinding onchocerciasis form with high prevalence of blindness and severe eye damage that persisted in northern savannah areas, and a less blinding form with very little incidence on blindness, despite the high parasitic loads discovered in forest areas [3-5]. OCP activities started in the northern part of the country in the savannah area. The pre-control data indicated that the majority of the indicator villages were hyperendemic for onchocerciasis. Prevalence of O. volvulus infection was more than 60% in the most of these villages. The blindness rate as a result of onchocerciasis exceeded 3% in many villages and reached 10% in some of the worst affected villages. The CMFL, which express the intensity of infection and is more sensitive indicator for endemicity, was more than 30 mf/s in many villages [6].

Vector control by aerial larvicide spraying was the unique strategy used to control onchocerciasis in this initial area. The positive gains made from this strategy, namely less than 5% infection prevalence in the communities and less than 100 Annual Transmission Potentials (ATP), led to gradual reduction of larvicide treatments in each river basin from 1994 to 1997 [7]. Residual epidemiological and entomological monitoring and evaluation activities were carried on by the Côte d’Ivoire National Onchocerciasis Control Programme (NOCP-CI) soon after OCP closure in 2002. The purpose of these activities was to achieve early detection of cases and control of the disease resurgence. However, the socio-political unrest that affected the country during the last quarter of year 2002 led the activities to come to a halt. So, for five years no single M&E activity has been carried out; for this reason, vector-related information has not been available in this northern part of the country. Since 2007, signs of potential resurgence have been visible in both sides of Burkina Faso and Côte d’Ivoire [8,9].

In Côte d’Ivoire, an epidemiological evaluation, conducted in 2010 in the Kafolo area at the border with Burkina Faso at the Comoé River basin, showed 19.4% prevalence with 0.56 CMFL. Evaluations conducted in 2011 in the village of Ninclinvogo on the Léraba River and in the village of Temisseré on the Comoé River revealed 12.8% prevalence with 0.86 CMFL and 25% prevalence with 1.64 CMFL respectively. These data have permitted ivermectin mass distribution in these villages since 2012.

In Burkina Faso, epidemiological surveys conducted in 28 villages and hamlets in the Cascades region revealed some prevalences exceeding pre-defined thresholds in 13 villages (Bolibana: 46.25% prevalence and 2.21 CMFL/ skin biopsy; Badara Karaboro: 70.97% and 5.17 CMFL/ skin biopsy on the Comoé River basin. As a matter of fact, like other river basins, the Comoé River basin was declared stabilized in 2002 . Following these observations, a study initiated in Burkina Faso was conducted to evaluate the actual level of onchocerciasis transmission at the Comoé River in the Cascades region. It resulted from this study that in 2012, transmission was relatively significant in some of the villages cited above [10].

Unfortunately, on the Ivorian side, such a survey had not been conducted so far. The Onchocerca volvulus vector infestation level in this area remains unknown. Following this evidence, research studies were conducted from January to December 2019, in an entire year, to evaluate onchocerciasis transmission level at Kafolo, at the level of the middle Comoé River in Côte d’Ivoire. The current study will contribute to suggesting an appropriate response to onchocerciasis control in order to eliminate the disease in this area.

Study area

Kafolo is a village located in the north-east of Côte d’Ivoire in the department of Kong, in the Tchologo region (Figure 1). It used to be part of the Ferkessédougou Medical District, but since the end of 2019, Kafolo has been part of the Kong Medical District. It is located at about 700 km from Abidjan and more than 300 km from Bouaké. Its geographic coordinates are 09° 35’ 01’’.9 N and 04° 18’ 59’’.3 W.

Its climate is Sudanese with two (02) seasons: a dry season that goes from November to April and a rainy season that runs from May to October. The annual rainfall varies from 1,000 mm to 1,400 mm with an average annual rainfall of 1,203 mm (Ferkessédougou). Daily and annual thermal amplitudes are relatively significant. Humidity rate is lower than that of the southern part of the country. This area is characterized by an intermittent presence of cool and dry harmattan wind from December to February. The vegetation is made up of a mosaic of shrub or wooded savannah, and sparse woodlands. The main activity of the region is agriculture, followed by fishing and eco-tourism.

As a matter of fact, Kafolo hosts several sites of ecological and touristic interests. Some illustrative examples are (i)) the Comoé National Park is a protected zone that covers an area of 1,149,150 hectares provided with plentiful flora and fauna; which offers genuine touristic opportunities. Classified as a world heritage by UNESCO, it represents one of the biggest parks in West Africa. (ii) The second Safari touristic site is the Kafolo Lodge hotel resort. This Lodge is provided with all the amenities expected to be found in a modern residential resort. (iii) The Training Centre of the Ivorian Office of Parks and Reserves (OIPR) for poaching control is another attraction.

Kafolo is bordered by the Comoé River that springs up in Burkina Faso (Figure 2). The Comoé River meanders into Côte d’Ivoire at the level of Kafolo and drains the National Park of the Comoé southward in the country to plunge into the Atlantic Ocean. This river is a privileged place for black fly development, which is the aim of this study. Kafolo has been chosen also for this study because of its relatively high onchocerciasis prevalence (19.4 %) in 2010 and because of its location at the border with Burkina Faso, where the existence of onchocerciasis active transmission were brought to light in some of the villages, located on the other side of the border.

Regarding onchocerciasis control, Kafolo was part of the initial OCP area where vector control took place from 1975 to 1993 to eradicate Simulium damnosum S.S. / Simulium sirbanum vector pair, identified in this hotbed from which the blinding strain of O.volvulus was transmitted.

Selection of capture points and blackfly sampling method

Two (02) catching points were identified in the village according to some selection criteria previously [11-13]. The first point located at the level of a breeding site is called “Kafolo-Aval” (Kafolo-Downstream); its geographic coordinates are 09° 35’ 18’’N and 04° 18’ 25’’.9 W. The second catching point is «Kafolo-Amont» (Kafolo-Upstream). It is located upstream the breeding site between 09° 35’ 27’’.9 N and 04° 18’ 36’’.7 W). The catching time period ran from 7: am to 18:00 p.m., namely 11 hours catching time in a day.

Fly catchers, previously trained in this technique, were treated with ivermectin and vaccinated against yellow fever according to the recommendations of an Ethical Committee; moreover, some of the catchers also got anti-malarial treatment. Fly catches were done by human landing according to the catching technique previously described [14,15]. In brief, two catchers caught the flies, based on an hourly rotation system and packed them in humid cotton wool to maintain optimal temperature for their survival. Every two (02) hours, these flies were sent to a field laboratory for identification and dissection. The fly collection lasted two (02) consecutive days each month and for a full year, namely from January to December.

Method of identification and dissection of collected blackfly females

Caught flies kept in cool cotton wool were anesthetized with choloroform then identified with a binocular magnifier. The identification was done according to micromorphological criteria based on the coloration of the fly antennae, procoxa, wing tuft silk and the 9^th abdominal tergite silk. The criteria helped identify female flies of savannah and forest.

The dissection of previously choloroform-anesthetized flies was done with a binocular magnifier according to the method previously described [13,16]. The examination of the ovaries, the Malpighian tubules and the fatty substance provided some indication on the fly parturity. The dissection was continued when the female fly was pares.

Method of identification of Onchocerca infective larvae by molecular biology tools

The fly head was separated from the thorax and placed in a physiological water drop kept on a microscope slide. The head was then shredded in a physiological drop of water to look for onchocerca infective larvae (L3, head). The final step consisted in shredding the thorax and the abdomen to look for intermediate stages L1, L2 and L3, thorax. The number, the development stages and the location of these larvae were previously described [17]. The flies were declared infected when they hosted changing larvae (L1, L2 or L3), regardless of their location. Flies that host infective larvae (L3) in the head were declared infectious flies.

Infective larvae in the fly heads were collected and kept with some parts of their Malpighian tubules between a blade and a slide according to the technique described by Toé et al. (1997). These slides were sent to the biology molecular laboratory for the identification of species level. These miceoscope slides were transferred to the WHO/ESPEN laboratory in Ouagadougou to identify the onchocerca species, by specifying, animal origin (O. ochengi) or a human origin (O.volvulus) of the infective larvae by using the O-150 PCR technique [18-20] . In brief, DNA was extracted from individual infective larvae by adsorption to a silica slurry. The resulting DNA was then used as a template in a PCR targeting a tandemly repeated DNA sequence specific to parasites of the genus Onchocerca [20]. The sequence of the primers used in the PCR were GATTYTTCCGRCGAAXARCGC and CXRTRTAAATXTGXAAATTC, where R=A or G, Y=C or T and X=A, G, C or T. Amplifications were carried out in a volume of 50 ml in a solution containing 60 mM Tris–HCl (pH 9.0), 15 mM (NH4)2SO4, 2 mM MgCl2, 0.2 mM each dATP, dCTP, dGTP and dTTP, 0.5 mM of each primer, 2.5 units of Taq polymerase (Roche Diagnostics) and 2.5 ml of template DNA. Cycling conditions consisted of 30 repeats of 94°C for 30 sec, 37°C for 1 min, and 72°C for 30 sec. Amplification products were separated by electrophoresis in a 2% agarose gel, and the gel was used to prepare a Southern blot, following standard procedures. The amplification products were then characterized on the basis of hybridization to species-specific oligonucleotide probes [21] namly OVS2 (5’- AATCTCAAAAAACGGGTACATAC-3’) specific to Onchocerca volvulus, and OCH ( 5’– AAAAAACATTCAAAAAACGGGAAAATAC-3’) specific to Onchocerca ochengi, the bovine Onchocerca parasite, morphologically indistinguishable to O. volvulus and sharing with it, the same vector. The PCR products hybridizing to the O. volvulus-specific oligonucleotide probe OVS-2 were classified as O. volvulus, while those that did not hybridize to OVS-2 and hybridize to OCH probe were classified as O. ochengi.

At the end of this study, the entomological indices of onchocerciasis transmission (ATP, TP, Nb IF/ 1,000 pares, Nb IL /1,000 pares and ATP) were calculated.

Analysis of entomological data collected in Kafolo during vector control conducted by the OCP from 1975 to 1993

To better understand the entomological data of the present study, we analyzed raw OCP data relating to vector control operations carried out in Kafolo from 1975 to 1993. These data were extracted from the OCP database called VCU data manager which had been given to all OCP participating countries at the close of OCP and which is being installed on ESPEN Portal. We analysed time series over 19 years of vector control of the Annual Biting Rate (ABR), Annual Transmission Potential (ATP) and Annual Infective Biting Rate (AIBR) of blackfly populations collected at Kafolo. This analysis was made with regard to the successive phases of deployment and cessation of OCP vector control operations on both sides of Côte d’Ivoire and Burkina Faso border, figure 3.

In 2019 at Kafolo, 3,038 female flies were collected through standardized catches. The morphological identification of all the flies collected at Kafolo shows that they essentially belong (over 99%) to the flies of the savannah group. The relative proportions of savannah and forest blackflies have not varied over time, figure 4. All the collected flies were also dissected: 18 female flies were infected by a total of 25 onchocerca larvae among which 11 infectious female flies infected by 16 onchocerca infective larvae. All 16 infectious flies belonged to savannah group. Molecular analyses through the PCR technique O-150 of the 16 infective larvae help specify the fact that 6 of them were Onchocerca ochengi while the other 10 were Onchocerca volvulus. These data helped determine the entomological indicators of onchocerciasis transmission for each of the two (02) sites (Tables 1 and 2). Molecular analyses of five (05) L3 Thorax helped specify that four (04) among them were O. ochiengi and one (01) was O. volvulus, namely the one collected at Kafolo-Aval in September 2019. The results of analysis of inter annual variations in Annual Biting Rates (ABR), Annual Transmission Potential (ATP), and Annual Infective Bite Rates (AIBR) during vector control in Kafolo from 1975 to 1993 are shown in Figure 5. The fluctuations of these entomological indices are influenced by the phases of OCP vector control operations, figure 3.

The aim of this study consists of reporting the current entomological parameters relating to the quantification of human-vector contact and to the parasite transmission at Kafolo, after observation of the disease prevalence increase in both sides of the Ivorian-Burkinabe border. A retrospective analysis of onchocerciasis transmission in Kafolo during and after vector control is also done. This longitudinal entomological study will help take up this challenge by determining several entomological indicators that are discussed.

Monthly Bitting Rates (MBR) and Annual Bitting Rates (ABR)

It is observed that the bite rates are now very high compared to those found during vector control, 25,335 bites/person/year and 20,235 bites/person/year respectively at Kafolo Aval and Kafolo Amont in 2019, compared to fluctuations ranging between 9 bites/man in 1983 and 12,220 bites/man/in 1992, thus showing that the black fly populations returned to their natural density in the absence of larviciding pressure. The maximum rates of monthly bites in 2019 were recorded during the rainy season, from July to October with a peak of 3,795 bites/person/month in October at Kafolo–Aval and 4,065 bites/person/month at Kafolo-Amont in September. As a matter of fact, during the rainy seasons, the watercourse level increases. This results in an increase of immersed vegetation with abundant organic nutritive material. This establishes the most conducive conditions to the development of the fly preimaginal stages. That is the justification of the population growth and the increase of bite rates.

These observations corroborate the conclusions of a previous study carried out on the upper course of the Comoé River with density peaks in rainy seasons (April to October) at Bolibana, Badara Karaboro, and Badara Dogossè [10].

At Kafolo-Amont, the MBR (February and April put aside) exceeds the threshold of 1,000/bites/person/year. The same is valid for Kafolo–Aval where the MBR exceeds 1,000 (March, April, May, and November, excepted).

The tolerable threshold for annual bites rate (ABR) is 1,000 bites/person/year, as pre-defined by WHO [22,23]. ABRs at Kafolo–Amont and Kafolo–Aval were 25,335 and 20,235 bites/person/year respectively; that exceeds 20 to 25 times the threshold established by WHO [22,23]. This shows how serious the nuisance and the disease transmission risk are for the population of Kafolo.

It is known that in West Africa, there are large movements of blackflies which take place under the prevailing winds: a movement from the South-West to the North-East under the influence of the monsoon winds and a reverse movement from North-East to South-West thanks to harmattan winds [24-27]. The analysis of the OCP data seems to indicate that Kafolo and probably all this northern part of the Ivorian territory is under the influence of these movements of blackflies. Indeed, the south-western extension of vector control operations in Côte d'Ivoire has significantly impacted the densities and transmission rates in Kafolo. The beginning of the southwestern extension coincides with the fall of the ABR, ATP and AIBR at Kafolo, indicating that a large proportion of Kafolo blackflies come from breeding sites located in the southwestern extension area, figures 3 and 5, thus demonstrating the appropriate and rational decision of this extension which had precisely been put in place to protect the initial area from blackfly invasions [28]. Similarly, the cessation of vector control in 1989 on the rivers of Burkina Faso located north and north-east of Kafolo (Leraba, lower course of the Mouhoun and its tributaries) figure 3, coincided with increase in blackfly densities in Kafolo, also suggesting that a proportion of blackflies present in Kafolo would come from breeding sites located on these waterways figure 5. The scientific approach to highlight these movements of blackflies would be to characterize them by genetic markers that can discriminate local populations from migrant populations. Failing this, the opportunity to deploy then stop larviciding on a large scale makes it possible to put highlight these movements.

Analyzes of past OCP data also show that vector control operations continued in Kafolo for four years after the ATP was zero. In addition, the entomological studies of post controls carried out in the initial area of OCP in Côte d'Ivoire had confirmed the good results obtained when vector control ceased [29].

This study shows that the onchocerciasis area in Kafolo is not isolated but rather under the permanent influence of blackfly movements over relatively long distances. The delineation of transmission zones in West Africa would therefore be complicated by this phenomenon of black fly movements.

Black fly Infectivity rates

Raw infectivity rates, calculated based on these data, amounted to 4.91 IF/1,000 pares and 5.89 IL/1,000 pares for Kafolo–Amont; and to 8.05 IF/ 1,000 pares and 13.42 IL/1,000 pares for Kafolo-Aval.

The number of adjusted IF/1,000 pares at Kafolo-Amont is 3.93 IF/ 1,000 pares; that means about 4 times the tolerable threshold set up by WHO [22,23]. It is the same situation at Kafolo–Aval where an adjusted infectivity rate of 4.03 IF/1,000 pares dissected was reported. The adjusted number of infective larvae (IL)/1,000 pares follows the same trend of increase with 4.91 and 6.71 IL/1,000 pares at Kafolo–Amont and Kafolo–Aval respectively.

The adjusted infectivity rates of 3.93 IF/1,000 pares found at Kafolo-Amont and 4.03 IF/1,000 pares at Kafolo–Aval in 2019 certainly exceed those found at Bolibana (2IF/1,000 pares), but are quite below the rates at Bodadiougou (7 IF/1,000 pares) and at Badara Karaboro (11 IF/1,000 pares) [10].

Thanks to the vector control carried out by OCP, the vector infectivity rates in the entire initial area, including Kafolo, were below 1 infectious female fly/1,000 pares [6]. Additionally, post-control studies helped confirm this trend [29]. Updated transmission rates therefore indicate the resurgence of onchocerciasis transmission at Kafolo. It is therefore necessary to redo the study in the initial OCP area in Côte d’Ivoire to have a better understanding of the challenge.

Monthly and Annual Transmission Potential (MTP and ATP)

At Kafolo–Amont, the raw MTPs were 15, 30, 30 and 15 for May, June, July and October respectively. MTPs of the other months were null. After identification of collected infective larvae, the conclusion was that O. volvulus infective larvae were found only in June, July and October with adjusted MTPs of 30, 30 and 15 IL/person/year for these months respectively Hence, the raw ATP of this site dropped from 90 IL/person/year to an adjusted ATP of 75 IL/person/year. The ATP found at Kafolo-Amont is below the tolerated threshold of 100 IL/person/year, however this threshold significantly exceeds the rate of 20 IL/person/year set as the threshold for the interruption of the parasite transmission [30].

At Kafolo–Aval, raw MTPs were 60, 15, 15, and 60 for January, June, July, and October respectively. After adjustment, the MTPs were 15 and 60 IL/person//month for July and October respectively. Hence, the ATP at Kafolo–Aval dropped from 150 IL/person/year to 75 IL/person/year after adjustment by the means of molecular identification of collected IL. As a consequence, the adjusted ATP at Kafolo-Aval remains quite below the threshold of 100 IL/person/year but it significantly exceeds the threshold of 20 IL/person/year.

The ATP of 75 IL/person/year on both sites of Kafolo in 2019 is below that of Bolibana (30 IL/person/year), but it remains quite below those of Badara Karaboro (255 IL/person/year) and Bodadiougou (771 IL/person/year), evidenced in the Comoé focus in Burkina Faso [10].

Observations made for one-year help conclude that at Kafolo, onchocerciasis transmission generally takes place from May to October in the rainy season. As a matter of fact, it is in this time period that adjusted infection rates and MTPs were most significant, with a peak of 35.40 IL/1,000/pares and 60 Onchocerca volvulus IL /person noted in October at Kafolo–Aval, and 19.42 IL/1,000 pares and 30 Onchocerca volvulus IL /person observed in June and July at Kafolo –Amont.

As a reminder, at Kafolo as well as in the entire initial area, ivermectin had never been distributed during the OCP time period. CDTi really started in this area, following the increase of onchocerciasis prevalence in 2010. The first ivermectin distribution happened in 2012, with one administration per year. According to data provided by the National Programme for Eye Health and Onchocerciasis control (NPEHOC), the average therapeutic coverage at Kafolo for the first five-year-treatment was 69%; which is quite below the recommended 80% threshold for onchocerciasis elimination [30].

The results of this entomological study help conclude that active transmission of onchocerciasis is still at work at Kafolo, as testified by the infectivity rates. However, the scope of this active transmission is not well known. Under these circumstances, onchocerciasis elimination, as an aim, is still out of reach in this area, taking into account the levels of current entomological indicators that are 3 times and even 4 times above the threshold recommended for onchocerciasis elimination, coupled with inadequate CDTi therapeutic coverage in this border area. This study has been a great opportunity to gather entomological data that will be used as the basis for setting up entomological surveillance in the area within the framework of the verification of onchocerciasis elimination in coming years.

Following this study, we recommend that entomological studies be extended in the initial area in Côte d’Ivoire and that the disease prevalence levels in human populations be evaluated. Further recommendations, as of now, could be to intensify ivermectin treatment synchronically in both sides of the border in the regions of Tchologo (Côte d’Ivoire) and Cascades (Burkina Faso), to set up entomological surveillance in this area, and to conduct research studies at the national level in order to update onchocerciasis entomological and epidemiological data in Côte d’Ivoire.

Acknowledgements

Alassane Ouattara University (UAO) and the Medical and Veterinary Entomological Centre (CEMV) for hosting the project and providing transportation.
Ministry of Health and the NTD-CI control Programme for facilitating field deployment.
WHO Representation in Côte d’Ivoire for contributing to the transfer of research samples to Ouagadougou, at the time when country borders were closed due COVID in 2021.
Dr Koala Lassané from Burkina Faso for his precious contribution to this research study.
Health administrative authorities of Korhogo and Ferkessédougou for their warm welcome and willingness to help.
The population of Kafolo and the village chief, Mr. Bamba, for his warm welcome and reachability; and finally, the valued and valiant blackfly catchers for their strong sense of duty.
The state nurse (IDE) of the Kafolo health centre and his family, as well as the community health workers (ASC) of the centre for their warm welcome and readiness.

Funding

The Strategic Support Program for Scientific Research in Côte d'Ivoire (in French, le Programme d'Appui Stratégique à la Recherche Scientifique en Côte d'Ivoire “PASRES”) and the Joint International Laboratory on Vector-borne Diseases in Côte d’Ivoire (in French, Laboratoire Mixte International sur les maladies à Vecteurs en Côte d’Ivoire “LAMIVECT”) for contributing to this research study.

Availability of data and materials

The datasets used and/or analyzed during the current study are available from the first and corresponding authors on a reasonable request.

Author’s contributions

FC, GYY, KAK, JMCD, and AAGA designed the study. FC and DC performed and supervised field activities. KS, MSS, and AAGA performed the laboratory analysis of the samples. FC, DB and AAGA performed the data analysis and wrote the manuscript. RNN coordinated the transfer of samples to the ESPEN laboratory in Ouagadougou and contributed to the final draft of the manuscript. All authors read and approved the final manuscript.

Ethics approval and consent to participate

This project has received the approval of the National Ethics Committee for Life Sciences and Health (‎CNESVS‎), under its decision No. 118-18/MSHP/CNESVS-Km of September 4, 2018.

Consent for publication

Not applicable.

Competing interests

The authors declare that they have no competing interests.

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Table 1: Entomological indicators of onchocerciasis transmission in 2019 at Kafolo-Amont, Côte d’Ivoire.

	Jan	Feb	Marc	Apr	May	June	July	August	Sept	October	Nov	Dec	Total
Nb days	2	2	2	2	2	2	2	2	2	2	2	2	24.00
Nb caught fem	146	133	4	3	89	151	164	228	271	208	130	162	1,689.00
Nb bites/h/day	73	66.5	2	1.5	44.5	75.5	82	114	135.5	104	65	81	70.38
MBR/ABR	2,190	1,995	60	45	1,335	2,265	2,460	3,420	4,065	3,120	1,950	2,430	25,335
Nb fem identified and dissected	146	133	4	3	89	151	164	228	271	208	130	162	1,689.00
Nb of savanna flies	146	133	4	3	89	151	162	227	271	208	128	162	1,684
Nb of forest flies	0	0	0	0	0	0	2	1	0	0	2	0	5
Nb parous flies	56	105	4	2	53	103	126	147	70	94	124	135	1,019.00
% parous flies	38.36	78.95	100.0	66.67	59.55	68.21	76.83	64.47	25.83	45.19	95.38	83.33	60.33
Nb infected flies	2	0	0	0	1	2	1	0	0	1	1	0	8.00
Nb infected flies /1,000 parous	35.71	0	0	0	18.87	19.42	7.94	0	0	10.64	8.06	0	7.85
Nb fem with L1	0	0	0	0	0	0	0	0	0	0	0	0	0
Nb fem with L2	0	0	0	0	0	0	0	0	0	0	1	0	1
Nb fem with L3 thorax (FL3TH)	2	0	0	0	0	0	0	0	0	0	0	0	2
Nb fem with L3 Head (raw FL3H)	0	0	0	0	1	2	1	0	0	1	0	0	5.00
Nb FL3H /1,000 parous flies	0.00	0.00	0,00	0.00	18.87	19.42	7.94	0.00	0.00	10.64	0.00	0.00	4.91
Nb adjusted FL3H	0.00	0.00	0.00	0.00	0.00	2.00	1.00	0.00	0.00	1.00	0.00	0.00	4.00
Nb adjusted FL3H /1,000 parous flies	0.00	0.00	0.00	0.00	0.00	19.42	7.94	0.00	0.00	10.64	0.00	0.00	3.93
Nb Onchocerca larvae	3	0	0	0	1	2	2	0	0	1	1	0	10
Nb L1	0	0	0	0	0	0	0	0	0	0	0	0	0
Nb L2	0	0	0	0	0	0	0	0	0	0	1	0	1
Nb L3 thorax	3	0	0	0	0	0	0	0	0	0	0	0	3
Nb raw Infective Larvae (IL)	0.00	0.00	0.00	0.00	1.00	2.00	2.00	0.00	0.00	1.00	0.00	0.00	6.00
Nb adjusted IL	0.00	0.00	0.00	0.00	0.00	2.00	2.00	0.00	0.00	1.00	0.00	0.00	5.00
Nb raw IL/1000 parous flies	0.00	0.00	0.00	0.00	18.87	19.42	15.87	0.00	0.00	10.64	0.00	0.00	5.89
Nb adjusted IL /1000 parous flies	0.00	0.00	0.00	0.00	0.00	19.42	15.87	0.00	0.00	10.64	0.00	0.00	4.91
Raw MTP/ATP	0	0	0	0	15	30	30	0	0	15	0	0	90.00
Adjusted MTP/ATP	0	0	0	0	0	30	30	0	0	15	0	0	75.00

Table 2: Entomological indicators of onchocerciasis transmission in 2019 at Kafolo-Aval, Côte d’Ivoire

	Jan	Feb	Mar	April	May	June	Jul	Aug	Sept	Oct	Nov	Dec	Total
Nb days	2	2	2	2	2	2	2	2	2	2	2	2	24.00
Nb fem caught	135	106	14	3	42	68	154	167	233	253	66	108	1,349.00
Nb bites/h/day	67.5	53	7	1.5	21	34	77	83.5	116.5	126.5	33	54	56.21
MBR/ABR	2,025	1,590	210	45	630	1,020	2,310	2,505	3,495	3,795	990	1,620	20,235
Nb fem identified and dissected	135	106	14	3	42	68	154	167	233	253	66	108	1,349.00
Nb savannah flies	135	106	14	3	42	68	153	164	231	252	66	108	1,342
Nb forest flies	0	0	0	0	0	0	1	3	2	1	0	0	7
Nb parous flies	72	81	14	3	15	45	102	69	67	113	65	99	745.00
% parous flies	53.33	76.42	100.0	100.00	35.71	66.18	66.23	41.32	28.76	44.66	98.48	91.67	55.23
Nb infected flies	4	0	0	0	1	1	1	0	1	2	0	0	10.00
Nb infected flies /1,000 parous	55.56	0	0	0	66.67	22.22	9.80	0	14.92	17.70	0	0	13.42
Nb fem with L1	1	0	0	0	0	0	0	0	0	0	0	0	1
Nb fem with L2	0	0	0	0	0	0	0	0	0	0	0	0	0
Nb fem with L3 thorax (FL3TH)	1	0	0	0	1	0	0	0	1	0	0	0	3
Nb fem with L3 Head (raw FL3H)	2	0	0	0	0	1	1	0	0	2	0	0	6.00
Nb FL3H/1,000 1,000 parous flies	27.78	0.00	0.00	0.00	0.00	22.22	9.80	0.00	0.00	17.70	0.00	0.00	8.05
Nb adjusted FL3H	0.00	0.00	0.00	0.00	0.00	0.00	1.00	0.00	0.00	2.00	0.00	0.00	3.00
Nb adjusted FL3H /1,000 parous flies	0.00	0.00	0.00	0.00	0.00	0.00	9.80	0.00	0.00	17.70	0.00	0.00	4.03
*Nb Onchocerca* larvae**	6	0	0	0	2	1	1	0	1	4	0	0	15
Nb L1	1	0	0	0	0	0	0	0	0	0	0	0	1
Nb L2	0	0	0	0	0	0	0	0	0	0	0	0	0
Nb L3 thorax	1	0	0	0	2	0	0	0	1	0	0	0	4
Nb raw infective Larvae (IL)	4.00	0.00	0.00	0.00	0.00	1.00	1.00	0.00	0.00	4.00	0.00	0.00	10.00
Nb adjusted IL	0.00	0.00	0.00	0.00	0.00	0.00	1.00	0.00	0.00	4.00	0.00	0.00	5.00
NB raw IL/1,000 parous flies	55.56	0.00	0.00	0.00	0.00	22.22	9.80	0.00	0.00	35.40	0.00	0.00	13.42
Nb adjusted IL/1,000 parous flies	0.00	0.00	0.00	0.00	0.00	0.00	9.80	0.00	0.00	35.40	0.00	0.00	6.71
Raw MTP/ATP	60	0	0	0	0	15	15	0	0	60	0	0	150.00
Adjusted MTP/ATP	0	0	0	0	0	0	15	0	0	60	0	0	75.00

Nb days: Number of blackfly collection days. Nb caught fem: Number of caught females. NbFL1: Number of flies with L1 NbL1: Number of L1. NbFL2: Number of flies with L2

NbL2: Number of L2. NbFL3 Thorax : Number of flies with L3 Thorax. NbL3 Thorax: Number of L3 Thorax

NbFL3 Head: Number of flies with L3 Head. NbL3 Head: Number of L3 Head.

No competing interests reported.

Onchocerciasis transmission in Kafolo, located in the Côte d’Ivoire – Burkina Faso cross-border region of Tchologo

Status:

Version 1

Abstract

Figures

1. Background

2. Methods

3. Results

4. Discussion

Conclusion

Declarations

References

Tables

Additional Declarations

Status:

Version 1