Jaime Luna

Salvatore Metanmo

Farid Boumediene

Pascal Mbelesso

Emilie Auditeau

Daniel Ajzenberg

Professor Pierre-Marie Preux
email: pierre-marie.preux@unilim.fr

ONCHOCERCIASIS IN TROPICAL NEUROLOGY: A SCOPING REVIEW Authors

Keywords: Epidemiology, association, onchocerciasis, epilepsy, nodding syndrome, Nakalanga syndrome, encephalitis, scoping review

Background: Onchocerciasis is a serious problem in tropical areas. The role of the parasite as a factor associated with neurological diseases needs to be addressed because it might involve a reduction of the risk via elimination strategies. We performed a systematic scoping review to identify available studies on this association and put into perspective the different methodological approaches for interpreting the evidence.

Methodology: A literature search was conducted using MEDLINE (Pubmed) through October 1, 2020. We included all the studies evaluating the association between onchocerciasis and four neurological diseases (epilepsy, nodding syndrome, Nakalanga syndrome, and encephalitis) in tropical countries. A descriptive and critical summary of the results was conducted to provide an overview of the findings.

Results: Overall, 161 articles were identified in the literature search. After full-length examination, we included twelve articles for epilepsy and three for nodding syndrome. Two meta-analyses of case-control studies found a modest strength of the association between O. volvulus and epilepsy. Recent meta-analyses and original studies support a significant association. Epidemiological studies suggest an association between onchocerciasis and nodding syndrome, however, the level of evidence from case-control studies was relatively low. No measure of association was reported for Nakalanga syndrome. There was no specific study on the association between O. volvulus and encephalitis.

Conclusion:

The association between onchocerciasis and epilepsy seems increasingly likely.

However, there are still many unanswered questions about the different clinical presentations of this epilepsy. Strong international collaboration is essential to improve our understanding of risk factors and physiopathological mechanisms of these intriguing conditions.

Figures and tables

Introduction

Onchocerciasis is a neglected tropical disease caused by Onchocerca volvulus affecting millions of people in tropical areas. O. volvulus is a filarial nematode transmitted through the bites of infective blackflies of the genus Simulium [START_REF] Boussinesq | Human onchocerciasis in Africa][END_REF]. It is endemic in countries of West, East, and Central Africa, the Arabian Peninsula, and parts of South and Central America [START_REF]Onchocerciasis (river blindness)[END_REF].

The Global Burden of Disease Study reported around 20.9 million prevalent cases worldwide in 2017 [START_REF] James | Global, regional, and national incidence, prevalence, and years lived with disability for 354 diseases and injuries for 195 countries and territories, 1990-2017: a systematic analysis for the Global Burden of Disease Study 2017[END_REF]. The highest proportion of cases occurred in sub-Saharan Africa [START_REF] Amazigo | Disease and Mortality in Sub-Saharan Africa[END_REF].

O. volvulus infection leads to several clinical manifestations. The most frequent are skin lesions (e.g. pruritic dermatitis and nodules) and visual impairment (river blindness). In tropical neurology, onchocerciasis has been implicated as a potential risk factor in several disorders like epilepsy, nodding syndrome (NS), Nakalanga syndrome, and encephalitis [START_REF] Tekle-Haimanot | Chapter 10 -Epidemiology of Neurological Disorders in Sub-Saharan Africa[END_REF]. It has been particularly difficult to evaluate the evidence surrounding the association between O. volvulus and neurological tropical disorders due to different methods to assess onchocerciasis and define certain disorders (e.g. nodding syndrome, Nakalanga) along with heterogeneity in terms of study designs and the lack of understanding of related pathophysiological mechanisms. It is important to clearly establish this relation, as it could be a potentially modifiable component of risk.

In this context, we performed a scoping review of the existing literature regarding the association of O. volvulus and neurological disorders in tropical areas. The review aimed to identify the types of available epidemiological evidence, to examine how the research was conducted, and to analyze methodological advancement.

Methodology

The study was designed and conducted following the guidelines for scoping reviews [START_REF] Peters | Guidance for conducting systematic scoping reviews[END_REF][START_REF] Munn | Systematic review or scoping review? Guidance for authors when choosing between a systematic or scoping review approach[END_REF] and the Preferred Reporting Items for Systematic reviews and Meta-Analysis extension for Scoping Reviews [START_REF] Tricco | PRISMA Extension for Scoping Reviews (PRISMA-ScR): Checklist and Explanation[END_REF].

Search strategy and study selection

The search was conducted in MEDLINE (Pubmed). The following search strings and Boolean operators were entered to identify relevant studies: Onchocerciasis AND Epilepsy, Onchocerciasis AND Nodding syndrome, Onchocerciasis AND Nakalanga syndrome, Onchocerciasis AND Encephalitis. Systematic reviews and meta-analyses were also searched.

Rayyan QCRI (Qatar Computing Research Institute, Data Analytics Medical) [START_REF] Ouzzani | Rayyan-a web and mobile app for systematic reviews[END_REF] was used for managing and screening articles. Four authors (JL, SM, FB, DA) have independently performed an initial screening of title and abstract to select relevant studies. Full-length articles were examined to assess eligibility. We included publications evaluating the association between onchocerciasis and the previously mentioned neurological pathologies (epilepsy, nodding syndrome (NS), Nakalanga syndrome, and encephalitis) in tropical countries. Only studies that considered confirmed cases were included in the review. Case reports, editorials, and commentaries were excluded from the study selection. We also excluded opinion papers, reviews, and perspectives publications. There was no language restriction or restrictions on the publication date. The search was conducted through October 1, 2020.

Data extraction and synthesis of results

Data were extracted from included studies in an ad hoc document. The following standard information was collected: author, year of publication, country, study objective, study design, study population, numbers of cases, definition and confirmation methods (onchocerciasis and neurological disorders), main results, and the measure of association together with the corresponding 95% confidence interval (CI). The data were summarized in tabular form. We presented a descriptive and critical summary of the results to provide an overview of the epidemiological evidence.

Results

Onchocerciasis and epilepsy

Epilepsy is a chronic neurological disorder characterized by an enduring tendency to have recurrent seizures. Epilepsy constitutes a major public health problem especially in tropical areas where etiologies such as parasitological infections are frequent [START_REF] Newton | Chapter 11 -Epilepsy[END_REF]. The association between onchocerciasis and epilepsy has been suggested in several studies [START_REF] Preux | Onchocercose et épilepsie[END_REF][START_REF] Colebunders | From river blindness to river epilepsy: Implications for onchocerciasis elimination programmes[END_REF].

Literature search

The search strategy yielded a total of 130 articles (more than half of these articles have been published in the last years, 2018 -2020). After the initial screening of the title and abstract, 32

full-text articles were assessed for eligibility (28 original articles and 4 meta-analyses).

Twenty studies were excluded for several reasons (Figure 1). Ten studies were previously reviewed in the meta-analyses [START_REF] Kaiser | Association between onchocerciasis and epilepsy in the Itwara hyperendemic focus, West Uganda: controlling for time and intensity of exposure[END_REF][START_REF] König | The role of Onchocerca volvulus in the development of epilepsy in a rural area of Tanzania[END_REF][START_REF] Prischich | High prevalence of epilepsy in a village in the Littoral Province of Cameroon[END_REF][START_REF] Dozie | Onchocerciasis and epilepsy in parts of the Imo river basin, Nigeria: a preliminary report[END_REF][START_REF] Boussinesq | Relationship between onchocerciasis and epilepsy: a matched case-control study in the Mbam Valley, Republic of Cameroon[END_REF][START_REF] Farnarier | Onchocerciasis and epilepsy. Epidemiological survey in Mali][END_REF][START_REF] Druet-Cabanac | Onchocerciasis and epilepsy: a matched case-control study in the Central African Republic[END_REF][START_REF] Kaiser | High incidence of epilepsy related to onchocerciasis in West Uganda[END_REF][START_REF] Newell | Epilepsy, retarded growth and onchocerciasis, in two areas of different endemicity of onchocerciasis in Burundi[END_REF][START_REF] Kaiser | The prevalence of epilepsy follows the distribution of onchocerciasis in a west Ugandan focus[END_REF]. Four studies did not report a measure of association between onchocerciasis and epilepsy (relative risk, rate ratio, or odds ratio) [START_REF] Mukendi | High prevalence of epilepsy in an onchocerciasis endemic health zone in the Democratic Republic of the Congo, despite 14 years of community-directed treatment with ivermectin: A mixed-method assessment[END_REF][START_REF] Colebunders | Risk factors for epilepsy in Bas-Uélé Province, Democratic Republic of the Congo: a casecontrol study[END_REF][START_REF] Abd-Elfarag | Persons with onchocerciasis-associated epilepsy and nodding seizures have a more severe form of epilepsy with more cognitive impairment and higher levels of Onchocerca volvulus infection[END_REF][START_REF] Lakwo | The Role of the Maridi Dam in Causing an Onchocerciasis-Associated Epilepsy Epidemic in Maridi, South Sudan: An Epidemiological, Sociological, and Entomological Study[END_REF]. Two studies were randomized clinical trials [START_REF] Mandro | Single versus Multiple Dose Ivermectin Regimen in Onchocerciasis-Infected Persons with Epilepsy Treated with Phenobarbital: A Randomized Clinical Trial in the Democratic Republic of Congo[END_REF][START_REF] Mandro | Ivermectin as an adjuvant to anti-epileptic treatment in persons with onchocerciasisassociated epilepsy: A randomized proof-of-concept clinical trial[END_REF]. We excluded another study because the comparison of prevalence was conducted between neighbor villages at two time-periods [START_REF] Siewe | Low prevalence of epilepsy and onchocerciasis after more than 20 years of ivermectin treatment in the Imo River Basin in Nigeria[END_REF].

Three other studies were excluded because they included suspected cases of epilepsy rather than cases with a confirmed diagnosis of epilepsy: i) A study assessing the impact of repeated community-directed treatment with ivermectin (CDTI) campaigns on epilepsy prevalence after a 25-year interval (1991-1992 and 2017) [START_REF] Boullé | Impact of 19 years of mass drug administration with ivermectin on epilepsy burden in a hyperendemic onchocerciasis area in Cameroon[END_REF], ii) Two cohort studies evaluating the possibility of a temporal relationship between O. volvulus skin microfilarial density assessed during childhood and epilepsy incidence [START_REF] Chesnais | The temporal relationship between onchocerciasis and epilepsy: a populationbased cohort study[END_REF][START_REF] Chesnais | A Second Population-Based Cohort Study in Cameroon Confirms the Temporal Relationship Between Onchocerciasis and Epilepsy[END_REF]. Overall, twelve articles (4 meta-analyses and 8 new original studies) were included in the review [START_REF] Druet-Cabanac | Review of epidemiological studies searching for a relationship between onchocerciasis and epilepsy[END_REF][START_REF] Kaiser | Case-control studies on the relationship between onchocerciasis and epilepsy: systematic review and meta-analysis[END_REF][START_REF] Pion | Epilepsy in onchocerciasis endemic areas: systematic review and meta-analysis of population-based surveys[END_REF][START_REF] Fodjo | Meta-analysis of epilepsy prevalence in West Africa and its relationship with onchocerciasis endemicity and control[END_REF][START_REF] Burfeind | Baseline characterization of epilepsy in an onchocerciasis endemic area of the Democratic Republic of Congo[END_REF][START_REF] Mandro | Onchocerca volvulus as a risk factor for developing epilepsy in onchocerciasis endemic regions in the Democratic Republic of Congo: a case control study[END_REF][START_REF] Ngugi | Prevalence of active convulsive epilepsy in sub-Saharan Africa and associated risk factors: cross-sectional and case-control studies[END_REF][START_REF] Lenaerts | High prevalence of epilepsy in onchocerciasis endemic health areas in Democratic Republic of the Congo[END_REF][START_REF] Fodjo | Epidemiology of onchocerciasis-associated epilepsy in the Mbam and Sanaga river valleys of Cameroon: impact of more than 13 years of ivermectin[END_REF][START_REF] Mmbando | High prevalence of epilepsy in two rural onchocerciasis endemic villages in the Mahenge area, Tanzania, after 20 years of community directed treatment with ivermectin[END_REF][START_REF] Gumisiriza | Changes in epilepsy burden after onchocerciasis elimination in a hyperendemic focus of western Uganda: a comparison of two population-based, cross-sectional studies[END_REF][START_REF] Gumisiriza | Prevalence and incidence of nodding syndrome and other forms of epilepsy in onchocerciasis-endemic areas in northern Uganda after the implementation of onchocerciasis control measures[END_REF]. The PRISMA flowchart of the literature research and selection process is shown in Figure 1. The localization of included studies is displayed in the map presented in Figure 2.

Evidence from meta-analyses

The first meta-analysis was published in 2004 [START_REF] Druet-Cabanac | Review of epidemiological studies searching for a relationship between onchocerciasis and epilepsy[END_REF]. It included nine primary case-control studies using a cross-sectional approach. O. volvulus infection or exposure was determined using scarification, skin biopsies, or serological examination. A neurologist or a physician confirmed epilepsy diagnosis. An increased risk was reported with a common relative risk (RR) in the limit of significance. When the odds ratio (OR) was calculated using a randomeffect analysis based on the method described by DerSimonian and Laird [START_REF] Dersimonian | Meta-analysis in clinical trials[END_REF], the pooled OR was 1.70 (95% CI: 0.97 -2.98, p = 0.066). The test of heterogeneity was significant (p < 0.001) and the I 2 was 76.7% (unpublished data). The data was then re-analyzed after excluding two publications: a study in a hypo-endemic area possibly resulting in a low prevalence of onchocerciasis-associated epilepsy [START_REF] Kabore | Epilepsy in a focus of onchocerciasis in Burkina Faso[END_REF] and another study where participants with growth retardation (Nakalanga like symptoms) were considered as non-epileptic controls [START_REF] Ovuga | Epilepsy and retarded growth in a hyperendemic focus of onchocerciasis in rural western Uganda[END_REF]. The meta-analysis found a relative risk of 1.30 (95% confidence interval: 1.04 -1.62; p = 0.02). Significant heterogeneity was found (Cochran's Q test p < 0.001, I 2 = 92%) [START_REF] Fodjo | Epidemiological evidence concerning the association between onchocerciasis and epilepsy[END_REF].

In 2013, an updated meta-analysis was conducted [START_REF] Kaiser | Case-control studies on the relationship between onchocerciasis and epilepsy: systematic review and meta-analysis[END_REF]. Primary case-control studies were included using qualitative skin biopsies to determine onchocerciasis. Epilepsy diagnosis was established with several methods like non-standardized or standardized interviews and neurologist or physician examination. The pooled OR was 2.49 (95% CI: 1.61-3.86, p = 0.001). Significant heterogeneity was found (Cochrane's Q test: p = 0.001; I 2 = 65.7%). The analyses were restricted to studies achieving control for age, residence, and sex. The pooled OR was 1.29 (95% CI: 0.93 -1.79; p = 0.139) considering four studies using skin biopsies as an assessment method. No statistically significant heterogeneity between the studies was detected (p = 0.59; I 2 = 0%). The pooled OR was 2.80 (95% CI: 1.57 -5.00; p = 0.001) when two studies were considered using nodule palpation as a qualitative method of assessment.

In 2009, an interesting meta-analysis evaluated the relationship between onchocerciasis endemicity and epilepsy prevalence [START_REF] Pion | Epilepsy in onchocerciasis endemic areas: systematic review and meta-analysis of population-based surveys[END_REF]. Eight studies were included covering seven countries in Western Africa (Benin and Nigeria), Central Africa (Cameroon and Central African Republic), and Eastern Africa (Uganda, Tanzania, and Burundi). The prevalence of epilepsy was variable among those countries (ranging from zero to 8.7%). The logistic model found that the epilepsy prevalence was associated with the prevalence of onchocerciasis showing an odds-ratio of 1.044 (95% CI: 1.036 -1.052) [START_REF] Pion | Epilepsy in onchocerciasis endemic areas: systematic review and meta-analysis of population-based surveys[END_REF].

A recent meta-analysis estimated epilepsy prevalence and its relationship with onchocerciasis in West Africa [START_REF] Fodjo | Meta-analysis of epilepsy prevalence in West Africa and its relationship with onchocerciasis endemicity and control[END_REF]. Epilepsy prevalence was obtained from population-based surveys and the pre-control onchocerciasis endemicity was estimated from historical data. However, onchocerciasis prevalence at the time of the epilepsy survey was available for few endemic study sites. The overall pooled epilepsy prevalence was 13.14 per 1000 (95% CI 11.28 -15.00). A subgroup analysis was conducted based on endemicity and duration of onchocerciasis control. Epilepsy prevalence was 12.20 (95% CI 8. 53 -15.86) Multiple linear regression showed a significant relationship between the prevalence of epilepsy and the prevalence of microfilariae and the duration of control. The prevalence of epilepsy increases with the prevalence of microfilariae (p<0.001) but decreases with the duration of control (p<0.001).

Updated evidence

There has been a large increase in the number of research publications on this subject mainly in the last years. The eight new studies included in this scoping review were classified as follows: i) Case-control studies, ii) Spatial analysis, iii) Prevalence of epilepsy after onchocerciasis control campaigns.

Three studies were carried out with a case-control design [START_REF] Burfeind | Baseline characterization of epilepsy in an onchocerciasis endemic area of the Democratic Republic of Congo[END_REF][START_REF] Mandro | Onchocerca volvulus as a risk factor for developing epilepsy in onchocerciasis endemic regions in the Democratic Republic of Congo: a case control study[END_REF][START_REF] Ngugi | Prevalence of active convulsive epilepsy in sub-Saharan Africa and associated risk factors: cross-sectional and case-control studies[END_REF]. In 2014, a case-control study was performed in three villages (endemic areas): Kifuma, Kalama, and Lalu Sefu of the District of Kasangulu, Bas-Congo Province, Democratic Republic of Congo [START_REF] Burfeind | Baseline characterization of epilepsy in an onchocerciasis endemic area of the Democratic Republic of Congo[END_REF]. O. volvulus infection was determined via snip skin biopsies. The logistic regression showed a statistically significant association between epilepsy and onchocerciasis (OR = 4.5, 95% CI: 1.75 -11.54, p = 0.002), with no change after adjusting for age and gender. Another study was performed in three villages (endemic areas) of the Democratic Republic of Congo: village of Salambongo (Tshopo Province), village of Rassia, and village of Draju (Ituri Province) [START_REF] Mandro | Onchocerca volvulus as a risk factor for developing epilepsy in onchocerciasis endemic regions in the Democratic Republic of Congo: a case control study[END_REF]. CDTI campaigns were carried out for several years in the two first villages. At the time of the study, the epilepsy prevalence ranged from 2.6% to 6.2%. Microfilariae were identified through skin snip biopsies to establish onchocerciasis, while exposure to O. volvulus was determined using the serological test. A significant association was observed between epilepsy and onchocerciasis (OR = 2.44, 95% CI: 1. [START_REF] Winkler | MRI findings in people with epilepsy and nodding syndrome in an area endemic for onchocerciasis: an observational study[END_REF] and Ghana (2010-2011) [START_REF] Ngugi | Prevalence of active convulsive epilepsy in sub-Saharan Africa and associated risk factors: cross-sectional and case-control studies[END_REF]. Exposure to O. volvulus was established through serology.

There was a statistically significant association for active convulsive epilepsy in children (aOR = 1.67, 95% CI: 1.09 -2.57, p = 0.019) and active convulsive epilepsy in adults (aOR = 2.23, 95% CI: 1.56 -3.19, p < 0.0001). To quantify the contribution of risk to disease burden, the population attributable fraction was estimated at 0.06 (95% CI: 0.01 -0.10) for children and 0.14 (95% CI: 0.08 -0.20) for adults. Table 1 showed the description of updated casecontrol studies assessing the association between onchocerciasis and epilepsy.

A spatial analysis was performed in Ituri province (an onchocerciasis rural endemic area), Democratic Republic of Congo [START_REF] Lenaerts | High prevalence of epilepsy in onchocerciasis endemic health areas in Democratic Republic of the Congo[END_REF]. The households were randomly sampled and located via Geographic Information Systems. A three-stage approach -screening survey, clinical examination by a general practitioner, and consultation with a neurologist -identified people with epilepsy. Epilepsy diagnosis was based on the 2014 International League Against Epilepsy (ILAE) definition [START_REF] Fisher | ILAE official report: a practical clinical definition of epilepsy[END_REF]. Exposure to O. volvulus was evaluated in participants older than 3 years old (OV16 test). A significant association between epilepsy and exposure to O. volvulus was observed in the spatial analysis (aOR = 3.19, 95% CI: 1.63 -5.64, p = 0.001).

In recent years, it has been estimated the impact of CDTI in epilepsy prevalence. In Cameroon, epidemiological studies were performed to determine epilepsy prevalence in two villages (Bilomo, 1998 and Kelleng, 2004) followed by new surveys taken several years apart (Bilomo, 2017 and Kelleng, 2018) [START_REF] Fodjo | Epidemiology of onchocerciasis-associated epilepsy in the Mbam and Sanaga river valleys of Cameroon: impact of more than 13 years of ivermectin[END_REF]. These endemic areas have benefited from CDTI for more than a decade. The surveys were cross-sectional using a door-to-door approach.

However, the methodology to identify people with epilepsy was heterogeneous. The crude prevalence of epilepsy remained stable in Bilomo (1998 (4.9%) and 2017 (4.6%), p = 0.695), and there was no statistically significant decrease in Kelleng (2004 (10.5%) and 2018 (7.8%), p = 0.357). The age-and sex-standardized epilepsy prevalence was only reported for Kelleng with a statistically significant decrease (2004 (13.5%) and 2018 (9.3%), p < 0.001). The median age of people with epilepsy shifted in Bilomo from 17 years (IQR: 12-22) to 24 years (IQR: 20-30) between the two survey periods. The highest epilepsy prevalence was found in the 10-19 years age group in 1998 (12.9%), while the 20-29 years age group reported the highest epilepsy prevalence in 2017 (17.1%). The age shift suggests a reduction of epilepsy onset among young children after controlling onchocerciasis transmission using ivermectin. In Tanzania, another study compared epilepsy prevalence between two time periods (1989 [START_REF] Rwiza | Prevalence and incidence of epilepsy in Ulanga, a rural Tanzanian district: a community-based study[END_REF] and 2017 [START_REF] Mmbando | High prevalence of epilepsy in two rural onchocerciasis endemic villages in the Mahenge area, Tanzania, after 20 years of community directed treatment with ivermectin[END_REF]) in several villages in the Mahenge area (endemic area). The area was part of the annual CDTI since 1997. The authors reported no change in the epilepsy prevalence over time (Mdindo village 3.7% (1989) and 3.5% (2017); Vigoi and Matumbala 1.3% (1989) and 1.5% (2017)). It is important to note that the highest prevalence of epilepsy was observed in rural villages with the highest prevalence of OV16 prevalence among children, which suggests ongoing O. volvulus transmission. In Uganda, two publications compared the prevalence and incidence of epilepsy between two time periods in the same geographic endemic areas before and after the implementation of onchocerciasis control measures [START_REF] Gumisiriza | Changes in epilepsy burden after onchocerciasis elimination in a hyperendemic focus of western Uganda: a comparison of two population-based, cross-sectional studies[END_REF][START_REF] Gumisiriza | Prevalence and incidence of nodding syndrome and other forms of epilepsy in onchocerciasis-endemic areas in northern Uganda after the implementation of onchocerciasis control measures[END_REF]. The first publication concerns two cross-sectional studies performed in three villages of western Uganda (Kabende Centre, Masongora South, and Rwesenene) using a populationbased approach in 1994 and later, in 2018. According to the publication, onchocerciasis transmission was interrupted in 2001, and the elimination of onchocerciasis was declared in 2016. Epilepsy crude prevalence had significantly decreased from 3.0% in 1994 to 1.2% in 2018 (p = 0.0002). An age-specific epilepsy prevalence decrease was observed in the 10 to 19 year age group between 1994 and 2018 (p < 0.0001). The overall incidence also decreased between the two periods, 418 per 100 000 person-years (95% CI: 265 -626) in 1994 to 73 per 100 000 person-years (95% CI: 32 -114; p < 0.0001). Interestingly, no new cases of nodding syndrome or Nakalanga had occurred since the interruption of onchocerciasis transmission [START_REF] Gumisiriza | Changes in epilepsy burden after onchocerciasis elimination in a hyperendemic focus of western Uganda: a comparison of two population-based, cross-sectional studies[END_REF]. A similar study design was applied in selected villages of Kitgum and Pader districts (Northern Uganda). No significant decrease was observed in the epilepsy prevalence between 2012 and 2017. However, the cumulative incidence of epilepsy decreased from 675 to 87 per 100 000 persons per year (p = 0.024) and the number of new NS cases per year dropped from four in 2012 to zero in 2017 [START_REF] Gumisiriza | Prevalence and incidence of nodding syndrome and other forms of epilepsy in onchocerciasis-endemic areas in northern Uganda after the implementation of onchocerciasis control measures[END_REF].

Onchocerciasis and nodding syndrome

NS is a term that first appeared in 2003 to describe an atypical form of epilepsy in African children with head nodding as a signature sign at an early stage of the disease [START_REF] Lacey | Nodding disease: mystery of southern Sudan[END_REF]. The disease can progress to other complications such as stunting, wasting, delayed sexual development, psychiatric illness, and eventually death [START_REF] Sejvar | Clinical, neurological, and electrophysiological features of nodding syndrome in Kitgum, Uganda: an observational case series[END_REF][START_REF] Dowell | Nodding Syndrome[END_REF][START_REF] Idro | The natural history of nodding syndrome[END_REF].

The etiology of NS remains unknown and unclear. Because the disease occurs in settings with a high prevalence of epilepsy that are also endemic for onchocerciasis, NS could be part of a spectrum of onchocerciasis-associated epileptic (OAE) disorders that have in common to be triggered by a common factor, most likely O. volvulus (55).

Literature search

Our search strategy (Nodding syndrome AND Onchocerciasis) identified 61 articles. After reading titles and abstracts, eight articles were selected. After full reading, four of these articles did not study the association between NS and onchocerciasis and were therefore removed [START_REF] Gumisiriza | Prevalence and incidence of nodding syndrome and other forms of epilepsy in onchocerciasis-endemic areas in northern Uganda after the implementation of onchocerciasis control measures[END_REF][START_REF] Winkler | MRI findings in people with epilepsy and nodding syndrome in an area endemic for onchocerciasis: an observational study[END_REF][START_REF] Winkler | The head nodding syndrome--clinical classification and possible causes[END_REF][START_REF] Johnson | Nodding syndrome may be an autoimmune reaction to the parasitic worm Onchocerca volvulus[END_REF]. One paper (59) used data already published in another article [START_REF] Tumwine | Clinical and epidemiologic characteristics of nodding syndrome in Mundri County, southern Sudan[END_REF] and was therefore excluded. In total, three original articles were selected for our analysis [START_REF] Tumwine | Clinical and epidemiologic characteristics of nodding syndrome in Mundri County, southern Sudan[END_REF][START_REF]Nodding syndrome -South Sudan, 2011[END_REF][START_REF] Foltz | An Epidemiologic Investigation of Potential Risk Factors for Nodding Syndrome in Kitgum District, Uganda[END_REF].

The PRISMA flowchart is shown in Figure 1. The localization of included studies is presented in Figure 2.

Evidence

Eight case-control studies were reported in these three articles: five in different locations of Southern Sudan between 2001 and 2011 and three in one place of Northern Uganda in 2009 (Table 2). The eight studies all took place before 2012, i.e. before the first international conference on NS organized by WHO [START_REF] Idro | Proposed guidelines for the management of nodding syndrome[END_REF], which explains why the definition of NS cases may vary from one study to another. However, in all eight studies, the case definition included the occurrence of involuntary episodes of head nodding in a previously neurologically healthy resident of the study area who was less than 21 years of age at baseline.

Only two studies did not find a positive association between NS and onchocerciasis [START_REF]Nodding syndrome -South Sudan, 2011[END_REF][START_REF] Foltz | An Epidemiologic Investigation of Potential Risk Factors for Nodding Syndrome in Kitgum District, Uganda[END_REF].

When the US Centers for Disease Control and Prevention (CDC) investigated NS in Northern

Uganda in 2009 [START_REF] Foltz | An Epidemiologic Investigation of Potential Risk Factors for Nodding Syndrome in Kitgum District, Uganda[END_REF], they selected only one location but used three different tests for assessing O. volvulus infection: the skin snip and two serologic assays based on different antigens and techniques (Table 2). A positive association between NS and onchocerciasis was found only when the serological tests were used. Because the positivity of a skin snip depends on the viability of the microfilariae in the sample, serological tests are more sensitive for detecting O. volvulus past infections, and this lack of sensitivity of the skin snip may explain the absence of association. The CDC investigated NS with the same methodology in two different locations of Southern Soudan in 2011: Maridi town and Witto village [START_REF]Nodding syndrome -South Sudan, 2011[END_REF]. A positive association between NS and onchocerciasis was found in Maridi but not in Witto.

Since the number of case-control pairs was low in Witto (Table 2), one can reasonably expect that this lack of association in Witto be primarily due to a lack of statistical power rather than a true difference between settings.

Onchocerciasis and other neurological disorders: Nakalanga syndrome and encephalitis

Roscoe reported the first case of Nakalanga syndrome in Uganda at the beginning of the 20th Century [START_REF] Roscoe | The clans and their totems[END_REF]. Recently, it has been defined as a combination of stunted growth without a clear cause, delayed or absence of signs of sexual development, intellectual disability, epilepsy, and facial dysmorphia [START_REF] Föger | Nakalanga Syndrome: Clinical Characteristics, Potential Causes, and Its Relationship with Recently Described Nodding Syndrome[END_REF]. The search strategy identified thirteen studies (Figure 1). We did not identify any study reporting measures of association (relative risk or odds ratio) between onchocerciasis and Nakalanga syndrome. The difficulty lies in the definition of the pathology, as a specific syndrome or a particular clinical expression of another disease. Reports of Nakalanga cases mainly concern with their proportion among people living with epilepsy: 8.2% in Burundi ( 21), 0.6% in Cameroon (66), and 10.9% in Tanzania [START_REF] Bhwana | Clinical presentation of epilepsy in six villages in an onchocerciasis endemic area in Mahenge, Tanzania[END_REF]. The majority of Nakalanga syndrome cases were located in hyper-endemic areas of onchocerciasis, notably where NS has been described in recent years. A specific study was conducted to identify Nakalanga syndrome in Kabarole district, Western Uganda [START_REF] Kipp | The Nakalanga syndrome in Kabarole District, Western Uganda[END_REF]. Thirty-one cases of Nakalanga syndrome were identified. Controls were matched to cases in terms of age and sex.

Onchocerciasis was detected in all 31 cases of Nakalanga syndrome (100%) and 22 over 28 controls (79%), p <0.001.

Parasites can also cause encephalitis. We identified thirteen articles in our search strategy (Figure 1). There was no specific study on the association between onchocerciasis and encephalitis.

Discussion

This scoping review describes the association between O. volvulus and neurological disorders in tropical areas. Studies of particular interest were analyzed to put into perspective the methodological approaches for interpreting evidence.

Most of the literature available in the tropical area focuses on epilepsy, with a sharp increase in the number of publications in recent years. The latter has resulted in a proposal to define onchocerciasis-associated epilepsy (OAE) [START_REF] Colebunders | Onchocerciasis-associated epilepsy: From recent epidemiological and clinical findings to policy implications[END_REF] and the encouragement to carry out the First International Workshop on OAE [START_REF] Colebunders | Report of the first international workshop on onchocerciasis-associated epilepsy[END_REF]. According to Colebunders et al., the specific characteristics distinguishing OAE from other forms of epilepsy are time at onset (between the ages of 3 and 18 years), geographical clustering (onchocerciasis-endemic region), and certain kind of family clustering (siblings often affected by epilepsy), along with a normal neurological development before the onset of epilepsy [START_REF] Colebunders | Onchocerciasis-associated epilepsy: From recent epidemiological and clinical findings to policy implications[END_REF][START_REF] Colebunders | Report of the first international workshop on onchocerciasis-associated epilepsy[END_REF]. Clinically, the most common presentation is generalized tonic-clonic seizures showing no difference from seizures of another etiology. NS and the Nakalanga syndrome are also included as clinical presentations of onchocerciasis-associated epilepsy. In the Democratic Republic of Congo, a cross-sectional study including 420 people with epilepsy reported 32 (7.6%) cases of Nodding syndrome ( 14(43.7%) of them had a positive skin snip test) and 12 (3.1%) cases of Nakalanga syndrome (5 (41.7%) of them had a positive skin snip test) [START_REF] Fodjo | Onchocerciasis-associated epilepsy in the Democratic Republic of Congo: Clinical description and relationship with microfilarial density[END_REF].

The exact pathophysiological processes of OAE remain unknown. Several mechanisms have been suggested [START_REF] Marin | Onchocerciasis-related epilepsy? Prospects at a time of uncertainty[END_REF] including the presence of the parasite in the central nervous system (however, microfilaria have been not detected in the CSF ( 73)), the pathogenicity of O. volvulus strains [START_REF] Zimmerman | Onchocerca volvulus DNA probe classification correlates with epidemiologic patterns of blindness[END_REF][START_REF] Vuong | Forest and savanna onchocerciasis: comparative morphometric histopathology of skin lesions[END_REF], immunological mechanisms involving cross-reactive immunization or cytokine production [START_REF] Johnson | Nodding syndrome may be an autoimmune reaction to the parasitic worm Onchocerca volvulus[END_REF][START_REF] Brattig | Pathogenesis and host responses in human onchocerciasis: impact of Onchocerca filariae and Wolbachia endobacteria[END_REF][START_REF] Vezzani | Infections, inflammation and epilepsy[END_REF], the role of excretory-secretory products (secreted by O. volvulus) that may cross the blood-brain barrier and trigger CNS disease leading to OAE [START_REF] Hotterbeekx | From nodding syndrome to onchocerciasis-associated epilepsy[END_REF] and the triggering role of insomnia due to itching [START_REF] Méndez | Interactions between sleep and epilepsy[END_REF][START_REF] Lanigar | Sleep and Epilepsy: A Complex Interplay[END_REF].

It is important to note that meta-analyses of case-control studies found a modest strength of the association between O. volvulus and epilepsy. Several methodological limitations were observed in the included studies. Additionally, the restricted analysis showed different results.

Several considerations need to be taken into account for case-control studies. Epidemiological studies used a variety of methods for participant recruitment (population-based/health records), assessment of exposure (Skin snip/OV16), and matching (no individual matching/age/sex/area). This heterogeneity complicates the interpretation of the new evidence. We have to keep in mind the difficulty to recruit a sample of controls that provides a representation of the subjects without the disease. Another crucial point is an adequate assessment of the exposure (O. volvulus) and outcome (epilepsy) using standard and welldetermined criteria. The gold standard test for the diagnosis of onchocerciasis remains the skin snip biopsy (81), however, it depends on the viability of the microfilaria in the sample.

Serological tests measure exposure to O. volvulus and therefore indirectly detect the transmission, but do not discriminate between past or active infections [START_REF]Guidelines for stopping mass drug administration and verifying elimination of human onchocerciasis[END_REF]. To improve our understanding of the relationship between exposure and outcome, the temporal relation is mandatory, although not easy to achieve, notably due to the cross-sectional approach. In this scoping review, we excluded two cohort studies in Cameroon assessing the incidence of epilepsy relative to O. volvulus skin microfilarial density measured during childhood because suspected cases of epilepsy were considered for the analysis instead of confirmed cases of epilepsy [START_REF] Chesnais | The temporal relationship between onchocerciasis and epilepsy: a populationbased cohort study[END_REF][START_REF] Chesnais | A Second Population-Based Cohort Study in Cameroon Confirms the Temporal Relationship Between Onchocerciasis and Epilepsy[END_REF]. However, these two cohorts suggest a temporal relationship between onchocerciasis and epilepsy. The first cohort found that the risk of an individual being a suspected case of epilepsy increased rapidly with the increase in baseline microfilarial density [START_REF] Chesnais | The temporal relationship between onchocerciasis and epilepsy: a populationbased cohort study[END_REF]. Similar results were found in the second cohort reporting a gradual increase of suspected cases incidence rate based on the initial microfilarial density with values ranging from 117.8 per 100 000 person-years for individuals without skin microfilarial to 952.8 per 100 000 persons-years for those with a microfilarial density >200 microfilarial/snip (p < 0.0001) [START_REF] Chesnais | A Second Population-Based Cohort Study in Cameroon Confirms the Temporal Relationship Between Onchocerciasis and Epilepsy[END_REF].

At this point, it is difficult to draw firm conclusions. It is equally important to say that the most reliable epidemiological evidence of association will not necessarily imply causation.

Another interesting issue is the relationship between seizure frequency and microfilarial density. A cross-sectional study was conducted to assess this relationship in two health zones (Logo and Aketi) in the Democratic Republic of Congo [START_REF] Fodjo | Onchocerciasis-associated epilepsy in the Democratic Republic of Congo: Clinical description and relationship with microfilarial density[END_REF]. No statistically significant association was found between seizure frequency and microfilarial density in the Logo health zone (p = 0.148). However, the association was statistically significant in the Aketi health zone (p = 0.025).

Although there are innovative ways of approaching this topic, the potential limitations of these methods need to be accounted for. Many efforts have focused on directed treatment with ivermectin in onchocerciasis endemic countries [START_REF] Colebunders | Onchocerciasis-Associated Epilepsy, an Additional Reason for Strengthening Onchocerciasis Elimination Programs[END_REF][START_REF] Hopkins | Neglected tropical diseases in Africa: a new paradigm[END_REF]. Ivermectin has been proposed as an adjuvant to anti-epileptic treatment, however, a randomized clinical trial carried out in the Democratic Republic of Congo found that ivermectin did not have an added beneficial effect over anti-epileptic treatment (phenobarbital) [START_REF] Mandro | Ivermectin as an adjuvant to anti-epileptic treatment in persons with onchocerciasisassociated epilepsy: A randomized proof-of-concept clinical trial[END_REF]. Another clinical trial conducted by the same research team in the Democratic Republic of Congo showed that seizure reduction was more likely among those treated with multiple doses of ivermectin [START_REF] Mandro | Single versus Multiple Dose Ivermectin Regimen in Onchocerciasis-Infected Persons with Epilepsy Treated with Phenobarbital: A Randomized Clinical Trial in the Democratic Republic of Congo[END_REF]. Regarding the community-led treatment with ivermectin, it is difficult to estimate the impact of the program on epilepsy prevalence over time. The methodology used was heterogeneous across surveys to identify people with epilepsy and age-and sex-standardized epilepsy prevalence was not systematically reported for all the areas. Furthermore, it is extremely challenging to assess the impact of risk factors (family history of seizures, previous febrile seizures, perinatal trauma, head injury, and CNS infections [START_REF] Ba-Diop | Epidemiology, causes, and treatment of epilepsy in sub-Saharan Africa[END_REF]) and their relation with epidemiological indicators of epilepsy over time.

Endemicity is a key concept in onchocerciasis epidemiology [START_REF] Zouré | The geographic distribution of onchocerciasis in the 20 participating countries of the African Programme for Onchocerciasis Control: (2) pre-control endemicity levels and estimated number infected[END_REF]. The severity of the disease and the public health importance in a given area are directly related to the local level of endemicity [START_REF] Murdoch | Onchocerciasis: the clinical and epidemiological burden of skin disease in Africa[END_REF][START_REF] Remme | Ocular onchocerciasis and intensity of infection in the community. I. West African savanna[END_REF]. This is also a key issue in tropical neurology. Publications support a high prevalence of epilepsy in onchocerciasis endemic areas (villages close to rivers or dams considered blackfly breeding sites). For instance, studies from Maridi County in South Sudan showed a significantly higher prevalence of epilepsy in villages <2 km from the Maridi River compared to those more distant from the river (p < 0.001) [START_REF] Colebunders | High prevalence of onchocerciasis-associated epilepsy in villages in Maridi County, Republic of South Sudan: A community-based survey[END_REF]. A prevalence of 11.9% of epilepsy was observed in a village close to the Maridi dam, which is a hotspot for O. volvulus transmission [START_REF] Lakwo | The Role of the Maridi Dam in Causing an Onchocerciasis-Associated Epilepsy Epidemic in Maridi, South Sudan: An Epidemiological, Sociological, and Entomological Study[END_REF]. The authors also reported a high percentage of people with epilepsy who met the diagnostic criteria for OAE in the area. Cysticercosis was considered unlikely given the absence of pigs in the Maridi area [START_REF] Colebunders | Clinical characteristics of onchocerciasis-associated epilepsy in villages in Maridi County, Republic of South Sudan[END_REF]. Another study showed a high prevalence of epilepsy in areas with very high ongoing O. volvulus transmission in the Democratic Republic of the Congo (villages of Wela (6.8%) and Makoko (8.4%) and the town of Aketi (5.7%)) [START_REF] Mukendi | High prevalence of epilepsy in an onchocerciasis endemic health zone in the Democratic Republic of the Congo, despite 14 years of community-directed treatment with ivermectin: A mixed-method assessment[END_REF]. The relationship of neurological diseases with endemicity level (hypo, meso-and hyperendemic) according to geographic areas is intriguing.

To improve our understanding of the association between onchocerciasis and epilepsy, it could be interesting to analyze the relationship according to seizure types. There is a lack of information on the type of seizure in these studies. However, we recognize the challenging context to perform epidemiological research in tropical areas for a variety of reasons (methodological, logistic, political, economic, ethical, and often the low perceived value of such works (91)).

Regarding NS, it seems quite clear that there is an association with onchocerciasis. However, association does not imply causation and it is still unknown whether NS is directly or indirectly caused by O. volvulus or not. The level of evidence from case-control studies is relatively low because it is often impossible to prove that exposure to a factor was anterior to the occurrence of the disease. Thus, even if studies reported an association between O. volvulus and NS, it is impossible to know whether O. volvulus infection preceded or followed NS onset [START_REF] Tumwine | Clinical and epidemiologic characteristics of nodding syndrome in Mundri County, southern Sudan[END_REF][START_REF]Nodding syndrome -South Sudan, 2011[END_REF][START_REF] Foltz | An Epidemiologic Investigation of Potential Risk Factors for Nodding Syndrome in Kitgum District, Uganda[END_REF]. Another limitation of case-control studies is the adjustment variables used in the different models, especially for the variable age since the disease develops in children aged 3 to 18 years with previously normal development. In the Ugandan study, the authors stated that the results were unchanged when three models were used, including an ageadjusted one and two models adjusted for exposure factors [START_REF] Foltz | An Epidemiologic Investigation of Potential Risk Factors for Nodding Syndrome in Kitgum District, Uganda[END_REF]. However, in the Sudanese included within a scoping review is generally not performed (unless there is a specific requirement due to the nature of the scoping review aim) [START_REF] Peters | Guidance for conducting systematic scoping reviews[END_REF][START_REF] Munn | Systematic review or scoping review? Guidance for authors when choosing between a systematic or scoping review approach[END_REF]. Further reviews need to perform a careful assessment of biases using standard tools (e.g. the Newcastle-Ottawa Scale).

The Bradford Hill criteria need to be considered to evaluate the likelihood of a causal association.

Conclusion

Given that the incidence and mortality of epilepsy are highest in tropical countries, it is of fundamental importance to understand specific risk factors in these areas as a crucial milestone to reduce disease burden. The association between onchocerciasis and epilepsy is more and more likely based on recent epidemiological studies. However, there are still many unanswered questions about the different clinical presentations of this epilepsy.

Multidisciplinary research teams are now taking a step forward to further understand these conditions in tropical neurology.

Tables and Figures

Table 1. Description of updated case-control studies assessing the association between onchocerciasis and epilepsy Table 2. Description of case-control studies assessing the association between onchocerciasis and Nodding syndrome

of tables: 2 Number of figures: 2

 22 Number of references: 93Supplemental DataPreferred Reporting Items for Systematic reviews and Meta-Analyses extension for Scoping Reviews (PRISMA-ScR) Checklist

  -3.84, p = 0.0001), and epilepsy and exposure to O. volvulus (OR = 1.98, 95% CI: 1.25 -3.13, p = 0.0037). A large study was conducted to evaluate risk factors of active convulsive epilepsy in five African countries: Kenya (2007-2008), South Africa (2008-2009), Uganda (2009-2009), Tanzania (2009-2009)

Figure 1 .Figure 2 .

 12 Figure 1. Flow diagram for the scoping review process

  in non-endemic areas, 24.45 (95% CI 17.76 -31.14) in endemic areas with no onchocerciasis control, 13.99 (95% CI 8.45 -19.53) in endemic areas with 1 to 9 years of onchocerciasis control and 9.44 (95% CI 4.83 -14.05) in endemic areas with more than 10 years of onchocerciasis control.

Table 1 . Description of updated case-control studies assessing the association between onchocerciasis and epilepsy

 1 Previous Meta-analysis ; NAR: No Association Reported ; RCT: Randomized Controlled Trial ; SCE: Suspected Cases of Epilepsy ; PCI: Prevalence Comparison Issues; DAP: Data Already Published

	Excluded (n=98)	Excluded (n=53)	Excluded (n=10)	Excluded (n=7)
	R: 19 CR/CD: 5	AT: 50 P/E: 24	R: 13 CR/CD: 4 AT: 21	P/E: 14 D: 1	R: 7 AT: 3	R: 3 AT: 2 P/E: 2
	RCT: 2 NAR: 4 PM: 10 Excluded (n=20) PCI: 1 SCE: 3	DAP: 1 NAR: 4 Excluded (n=5)	Excluded (n=3) NAR: 3	Excluded (n=6) NAR: 6
	4 meta-analyses
	8 new original articles

R: Review ; CR/CD: Case Report/Clinical Description ; AT: Another Topic ; P/E: Protocol/Editorial ; D: duplicate PM:

Funding source

No funding source was involved in the preparation of this paper or the decision to submit it for publication.

studies, cases and controls were matched by age and area but the authors did not specify either the model used or not the adjustment variables [START_REF] Tumwine | Clinical and epidemiologic characteristics of nodding syndrome in Mundri County, southern Sudan[END_REF][START_REF]Nodding syndrome -South Sudan, 2011[END_REF].

NS is still a challenging condition to understand. The pathogenesis is still unclear. One hypothesis suggests that NS is an autoimmune disorder triggered by cross-reacting antibodies between neuronal proteins and O. volvulus antigens [START_REF] Johnson | Nodding syndrome may be an autoimmune reaction to the parasitic worm Onchocerca volvulus[END_REF][START_REF] Colebunders | Nodding syndrome, other forms of epilepsy, and the Nakalanga syndrome most likely directly or indirectly caused by Onchocerca volvulus[END_REF]. Postmortem examinations showed pathological changes in the brain characterized by neuronal loss especially in the cerebellum at the top of the folia, with associated hyperplasia of Bergmann glia and foci of gliosis [START_REF] Hotterbeekx | Neuroinflammation and Not Tauopathy Is a Predominant Pathological Signature of Nodding Syndrome[END_REF].

The specific geographic distribution of NS is another intriguing aspect. Onchocerciasis is endemic in many areas of Africa and Latin America for centuries but NS has the pattern of an epidemic strictly limited in children and a few areas of Africa.

The Nakalanga syndrome is another clinical condition observed in onchocerciasis-endemic regions of Africa and is considered a form of onchocerciasis-associated epilepsy by several authors [START_REF] Colebunders | Onchocerciasis-associated epilepsy: From recent epidemiological and clinical findings to policy implications[END_REF]. The causes and physiopathological mechanisms of the disease are not yet known. There is still a knowledge gap in understanding this syndrome.

The major strength of this work relies on the critical review of key studies focusing on methodological aspects to establish a link with the findings for their interpretation. The major limitation of this study is the use of only one major database (Medline/Pubmed), which could lead to a lack of inclusion of certain studies, mainly in the grey literature. We only included studies providing evidence from confirmed cases and we have placed particular emphasis on the measure of association to quantify the relationship. However, relevant data from studies that did not meet the inclusion criteria were also described and discussed. Another limitation is the lack of formal assessment of the risk of bias. Scoping reviews aim to provide an overview of the evidence. Due to this, an assessment of the risk of bias of the evidence

Disclosures of conflicts of interests

The authors declare no financial or other conflicts of interest.