Overcoming challenges for designing and implementing the One Health approach: A systematic review of the literature

Collaborative approaches in health, such as One Health (OH), are promising; nevertheless, several authors point at persistent challenges for designing and implementing OH initiatives. Among other challenges, OH practitioners struggle in their efforts to collaborate across disciplines and domains. This paper aims to provide insights into the existing challenges for designing and implementing OH initiatives, their causes and solutions, and points out strategic solutions with the potential to solve practical challenges. A systematic literature search was performed for emerging challenges and proposed solutions in the process of conducting OH initiatives. Next, a thematic and a causal analysis were performed to unravel challenges and their causes. Finally, solutions were discriminated on whether they were only recommended, or implemented as a proof-of-principle. The 56 included papers describe 21 challenges endured by OH initiatives that relate to different themes (policy and funding; education and training; surveillance; multi-actor, multi-domain, and multi-level collaborations; and evidence). These challenges occur in three different phases: the acquisition of sufficient conditions to start an initiative, its execution, and its monitoring and evaluation. The findings indicate that individual challenges share overlapping causes and crosscutting causal relations. Accordingly, solutions for the successful performance of OH initiatives should be implemented to tackle simultaneously different types of challenges occurring in different phases. Still, promoting collaboration between the wide diversity of stakeholders, as a fundamental aspect in the OH approach, is still by far the most challenging factor in performing OH initiatives. Causes for that are the difficulties in promoting meaningful and equal participation from diverse actors. Solutions proposed for this challenge focused on guiding stakeholders to think and collaborate beyond their professional and cultural silos to generate knowledge co-creation and innovative methodologies and frameworks. Finally, the biggest knowledge gap identified, in terms of proposed solutions, was for monitoring and evaluating OH initiatives. This highlights the need for future research on evaluation methods and tools specific for the OH approach, to provide credible evidence on its added value. When considering challenges endured by former OH initiatives and the proposed solutions for these challenges, practitioners should be able to plan and structure such initiatives in a more successful way, through the strategic pre-consideration of solutions or simply by avoiding known barriers.

designing and implementing OH initiatives, many of which are related to the necessity to include a wide range of relevant professionals, since each contribute valuable skills and perspectives that will ultimately advance the cause [7]. Despite the consensual OH definition that emerged in recent years, the level of collaboration required in the OH approach is not yet agreed upon. Most of the OH initiatives are based on interdisciplinary (ID) collaborations, implying the integration of different disciplines and cooperation between diverse experts, creating possibilities for knowledge co-creation. More recent initiatives, however, advocate for taking a whole society approach, which implies a transdisciplinary (TD) level of collaboration, with the inclusion of stakeholders beyond the academic domain [8,9]. Some authors advocate that taking a TD approach can better address complex global health challenges, due to the consideration of local contexts and the inclusion of community stakeholders, which contribute to the adoption and sustainability of OH initiatives [8,9].
The inclusion of diverse stakeholders under the OH approach, however, can lead to conflicts due to their manifold interests and priorities. Examples of collaborative conflicts are power imbalances, conflicts of interest and coordination gaps that can occur at an ID level but especially at a cross-sectoral, participatory TD level [10]. Besides these collaboration barriers, the different interpretations of the OH scope translate into diversity in the implementation of OH initiatives, generating challenges for the development of standards and guidelines for OH practitioners. This empirical gap and the manifold challenges endured by OH initiatives have been identified in the literature [2,4,11], highlighting the need for guidance on how to address collaboration and implementation challenges for the OH approach.
Reviewing and understanding such challenges is therefore essential to address the complexity in performing OH initiatives. Zoonotic and emerging infectious diseases require fast actions that are only possible through the design of OH initiatives in line with the acknowledgement of potential challenges and possible solutions [12]. Through a systematic literature review, this paper identifies challenges for designing and implementing OH initiatives, their causes and proposed solutions. The objective is to support the development and performance of the OH approach by providing insights on existing challenges, besides identifying proposed and implemented solutions that have the potential to solve multiple challenges.

Materials and methods
A systematic review of the literature was carried out to identify potential challenges and possible solutions for designing and implementing OH initiatives.

Search strategy
In order to provide an overview of challenges to designing and implementing OH initiatives and enable further discussion on how those initiatives attempt to address such challenges, we sampled the population of published peer-reviewed papers that mentioned OH initiatives and the challenges endured by them. For the systematic search, the online databases of PubMed and Web of Science were used, due to their broad scope of publication and multi-disciplinary contents. The key terms applied on the search were "one health" and "challenges". The final searching syntax was as the following: (challenges OR difficulties OR barriers OR problems) AND ("one health" OR "eco health" OR "one medicine"). The final search was performed on August 31, 2017.

Eligibility criteria and selection of studies
After the systematic search, different inclusion and exclusion criteria were used to select the papers for this review. Papers were included when they considered OH initiatives according to the definition and scope applied by the OH initiative movement [1]. This scope consists of two aspects: 1) aiming to enhance collaboration and communication in all aspects of health and 2) recognizing the link between animal, human and/or environmental domains. Papers were excluded when they described OH as an outcome (e.g. did not discuss challenges endured by OH initiatives, but the challenges that lead to their creation), or when they did not present any challenges. The search was not restricted by year of publication, study design or any further factors, but language (English). Fig. 1 depicts the selection process for the inclusion and exclusion of papers.
After removing duplicates that were identified by both databases, the papers were screened in two phases: first the title and abstract, and second the full text of the papers were screened. Papers were excluded if they did not meet the inclusion criteria by not referring to OH initiatives, or by employing a OH definition that was not in accordance with the definition and scope previously described (e.g. discussed challenges from a single disciplinary perspective not engaging in crossdisciplinary collaborations). Additionally, papers that did not discuss empirical challenges endured by OH initiatives or discussed challenges that lead to their creation (OH as an outcome) were excluded. After conducting the systematic search, the authors identified extra articles of relevance to the topic of this review; those were added manually to the dataset. A total of 56 papers were included in this review, which were diverse in type of study, geographic location, approach, and field of publication (see Table A.1 in the Appendix). The included articles were compiled for analysis with the aid of the referencing software Endnote.

Data extraction and analysis
After the selection of studies, papers were screened systematically to identify challenges and proposed solutions within the breadth of the literature. Secondly, a thematic analysis was performed to classify the identified challenges and solutions following both chronological phases of realization and emerging themes. Solutions were classified on their extent of implementation indicating whether they were only recommended or also implemented in specific cases. Thirdly, a causal analysis was conducted, through the construction of a causal tree, to unravel arguments for defining causal relations between challenges [13][14][15]. Finally, a semi-quantitative analysis was performed to measure the frequency in which challenges and solutions were mentioned by the included papers. The authors highlight here that although the search strategy was focused on identifying challenges in the performance of One Health initiatives, most of the included papers either proposed or discussed solutions to address such challenges, which are later elaborated in this review by discriminating practical examples of implementation.

Defining OH challenges
From the analysis of the 56 included papers, 21 single challenges for designing and implementing OH initiatives were identified. Based on the thematic analysis, the challenges were classified in different themes that emerged during the data analysis. Firstly, challenges were grouped in three distinct chronological process phases: (a) conditions for starting, (b) execution, and (c) monitoring and evaluation. Although chronological process phases were used to group challenges, a pertinent point is that in practice such phases are not necessarily consecutive and unrelated, but rather interactive and iterative [16]. In other words, phases can and usually do occur in parallel mode, overlapping from time to time. Secondly, challenges were classified in the emerging themes of policy and funding, education and training, surveillance, multi-actor (ID) collaborations, multi-domain (TD) collaborations, multi-level collaborations (across different institutional levels), and evidence.

Conditions for starting
Challenges under this group relate to difficulties for OH practitioners to acquire and establish the necessary conditions for starting their initiatives. Conditions for starting and executing projects have to be created at different levels: at an external and institutional context level (systemic); and at an internal local context within projects [16]. Still, the acquisition of conditions has to be performed not only in the beginning of a project but throughout the whole process (including execution and monitoring and evaluation). In relation to the thematic classification, challenges for acquiring conditions for starting OH initiatives comprise the themes of insufficient policy prioritization and funding, and the lack of OH educational and training programs, as described in Table 1.
Policy support, access to funds, and trained professionals, able to understand and implement the OH approach, are basic conditions to facilitate the start and smooth execution of OH initiatives. Therefore, addressing condition challenges is considered an important first step towards the successful design and implementation of OH initiatives [2]. Some authors also elaborate on the interdependency of these themes [26,29,[44][45][46]53]. While on the one hand, without policy support and funding, educational and training programs cannot be enhanced; on the other hand, educated and trained personnel are necessary for improving and increasing OH advocacy and message development for policy-makers and the public, and therefore enhancing policy support and funding.

Execution
For those initiatives that are able to meet the necessary conditions for starting, many practitioners also encounter challenges on the next step of their execution (see Table 2). Next to the difficulties in performing OH surveillance, execution challenges relate to problems in collaboration between multiple actors, in multiple domains and at multiple levels. Most of these challenges are perceived at the local (project) level, while performing the experimental initiatives. Nevertheless, the roots of such challenges usually come from structures institutionalized at a systemic level.
The challenges affecting the execution of OH initiatives are highly influenced by problems in acquiring conditions, which already started in the initial phase and persist until the monitoring and evaluation. This is especially the case for the difficulties in performing OH surveillance, which are mainly caused by the poor availability of resources and personnel [2,9,19,30,31,36]. The challenges in stakeholder collaboration relate to the fact that a multidisciplinary team of scientists, working together but within their own silo, is not enough for the knowledge co-creation proposed in OH innovations. Nevertheless, such superficial collaborations are a long-term heritage of fragmented systems and practices that influence the flexibility of actors and their organizations to collaborate and integrate diverse ideas, methods and actions [2,22,48,52].

Monitoring & evaluation
To facilitate the adoption, upscaling and institutionalization of OH initiatives, clear evidence of the added value of performing such initiatives needs to be provided. However, a range of challenges relate specifically to the difficulties in performing monitoring and evaluation of OH initiatives (see Table 3).
Poor monitoring and evaluation of OH initiatives hamper performance assessment of prior (implemented) initiatives and therefore the gathering of evidence on their effectivity and efficiency. Challenges in monitoring and evaluating OH initiatives were mentioned as major, and it was argued that they hinder more widespread political interest and support for the OH approach [24,52].

Causal relation of challenges
After the identification of individual challenges, a causal analysis was performed to unravel the causal relations between the different challenges, through the construction of a causal tree. For an overview of the complete causal tree of challenges, refer to Fig. A.1 in the Appendix. The tree follows a line of argumentation in which causal factors for challenges are hierarchically clustered. As a result causes that are (perceived as) more tangible are at the top and causes that are more fundamental and embedded in the system are at the bottom [66]. In Fig. 2, a simplified version of the causal tree is presented depicting the interconnection between the main (groups of) challenges and positioning them according to the previously described causal line of argumentation. The root-causes for all types of challenges are placed at two different types of fragmentations. The institutional-academic fragmentation relates to differences in educational and professional pathways, and organizational structure, resulting in diverse methodological preferences, practices, background assumptions and normative orientations of actors [67][68][69]. The geographic-cultural fragmentation is embedded in historical, geographic and social factors, resulting in conflicting actions, values and even disparities in knowledge and capacity.
Added to the qualitative analysis, a semi-quantitative assessment of the frequency in which the different challenges were mentioned by the included papers was performed. Most of the included papers mentioned challenges located at the phase of execution, followed by challenges in the conditions for starting and finally challenges for the monitoring and evaluation phases. Regarding the thematic classification, most papers mentioned challenges related to multi-actor collaborations (n = 36); followed by policy and funding (n = 31), and multi-domain collaborations (n = 27), respectively. Equally mentioned were the themes of surveillance, and education and training (n = 22). The least mentioned themes were evidence (n = 19) and multi-level collaborations (n = 13).

Defining solutions for OH challenges
Next, the 56 included papers were analysed for solutions addressing the challenges in designing and implementing OH initiatives. Following the same thematic analysis used to classify challenges, solutions were organized according to phases and themes. In addition, solutions were discriminated on their status of implementation by indicating whether they were only recommended by the authors or also implemented in practice. The implemented solutions serve as examples of strategies that attempted to solve challenges for performing OH initiatives in practice. Notably, the papers not always described solutions for the exact same challenges mentioned in their content. Some articles proposed solutions for only few of the mentioned challenges, while others presented solutions beyond mentioned challenges. For this reason, solutions are not linked to individual challenges but rather broadly related to the thematic groups.

Solutions for improving conditions for starting
The solutions proposed for boosting the start of OH initiatives aim at acquiring and establishing the necessary conditions such as improving political support and access to funds, and enhancing the educational and training opportunities for OH practitioners (see Table 4).
The solutions proposed to improve policy support and funding for OH initiatives elaborate especially on enhancing OH advocacy through better message development and collaborations with policy-makers. The solutions proposed to improve OH education and training emphasize the need of implementing ID and TD training programs and educational departments, as well as taking a more holistic approach to educational programs with the inclusion of different disciplines and courses on interpersonal skills. For only two solutions no implemented examples were mentioned: 1) strategies to increase policy-makers awareness about the struggles to implement OH initiatives (1b); and 2) the need to restructure how researchers are evaluated and rewarded within academic institutions (2 m). Although the first solution seems easy to implement, meaningful results would only be generated if there is a change in behaviour, in which policy makers increase their support for OH. The second solution is clearly a challenging suggestion, since that would imply changing the way the academic and scientific community is structured for many years, which can face a lot of resistance from the stakeholders involved.

Solutions for improving execution
The solutions proposed for the execution challenges focus on the following themes: improving OH (integrated) surveillance; and enhancing multi-actor, multi-domain, and multi-level collaborations (see Table 5).
The solutions proposed for improving OH surveillance focus mainly on enhancing the integration and sharing of surveillance data, added to capacity building of infrastructure and human resources. In this group, only for the suggestion of improving laboratory and operational capacities for OH surveillance (3j) no implemented example was identified; indicating that capacity building in OH surveillance, although important, has been extremely challenging. Improvements to the number and quality of OH facilitators, who can promote better collaboration, Table 1 Challenges for acquiring conditions for starting OH initiatives, their causes and definitions. Policy prioritization and funding, and educational and training programs are essential conditions for initiating OH initiatives. The reference numbers follow the notation presented in the Reference list.

Solutions for improving monitoring & evaluation
The solutions proposed for improving OH monitoring and evaluation aim at generating reliable evidence on the benefits of performing OH initiatives (see Table 6).
The solutions proposed for improving OH monitoring and evaluation elaborate on different gaps on the existing evidence on OH outcomes: the absence of studies and guidelines for OH monitoring and evaluation, and the need of developing holistic OH metrics and indicators that account for both quantitative and qualitative aspects. Within this group, only one solution was not accompanied by an example of implementation: the establishment of a network of experts to develop evaluation protocols for OH (7b). Although a proof-of-principle for this solution was not provided, it might be an interesting approach to try as an attempt to improve OH monitoring and evaluation. • Political fragmentation [45] • Hard to incorporate input from multiple actors in research design and analysis [20,47,52,58] • Hard to build trust between stakeholders [33] • Hard to find consensus due to multiple agendas [33] • The engagement of multiple actors can be time consuming [4,10,33,44] • Hard to engage the private sector [39,61] • Competition between stakeholders [57] • Different backgrounds/power/languages/knowledge among relevant stakeholders [4,17,22,33,48,57] • It is hard to integrate diverse perspectives and at the same time respect differences [53,65] Difficulties to include context-specific factors in OH initiatives [17,27,33,39,41,44,58,59] • Hard to promote community engagement [17,41,44] • Hard to find tailored solutions and promote changes [27,33,39,58,59] • Hard to consider contextual factors in all its complexity in OH projects [33] Multi-level collaborations [2,17,21,22,25,37,45,46,48,52,58,64,65] Causes and defining arguments Institutional and academic fragmentation [17,21,22,25,37,46,57,58] • The integration of high-level health management strategies generate extra costs [21] • Bureaucracy and administrative hurdles as complex web of mandates and jurisdictions make integration difficult [60] • Different organizational structures as administrative locations, availability of personnel and resources [17,25,37,46,57] • Lack of a coordinating body able to promote collaboration and integration of structures and strategies [22] Geographic and cultural fragmentation [2,22,48,52] • Territorial and nationalistic behaviour [2,22,48,52] • Global differences especially in cultural practices and disparities in terms of capacity [2,22,48,52] Table 3 Challenges for monitoring and evaluating OH initiatives, their cases and definitions. In order to prove the benefits of performing OH initiatives, OH monitoring and evaluation needs to be improved, by tackling challenges in performing evaluation studies and developing specific OH indicators and metrics. The reference numbers follow the notation presented in the Reference list.

Solutions at a multi-level perspective
The solutions were also plotted in the framework of the causal tree of challenges (see Fig. A.2 in the Appendix) in order to visualize how they relate to the causal line of argumentation. In fact, the causal tree (displayed in Figs. 2 and A.1) is essential to understand why and how interventions and solutions may or may not work. Since more tangible challenges are at the top of the tree, solutions that focus on these rows tend to be superficial, targeting symptoms rather than the real causes. On the other hand, solutions focused on the bottom rows of the tree tend to be hard to implement, since they tackle fundamental problems embedded in institutions and systems. It follows that most of the solutions for which no examples of successful implementation (i.e. recommended solutions) were provided, were located either at the very beginning (1b and 7b) or at the very end (2 m, 3j, 5 h and 6 g) of the tree. Tackling causes in the middle of the causal tree tends to be easier and therefore more implemented examples were provided. In addition, adopting solutions that address a cause for multiple symptomatic challenges is also beneficial, since it can solve several challenges at once, having the potential to generate faster results [66]. Furthermore, even if the papers mentioned solutions that were successfully implemented, those usually occurred on a small-scale and in specific contexts. Moreover, although a solution worked in a specific context, it did not solve all the challenges that the specific OH initiative was facing; most papers still described enduring challenges even after the solutions were implemented. These enduring challenges were usually related to a different theme, and sometimes even phase, from the one that the implemented solution addressed. This confirms the interdependency of challenges, which indicates that in order to successfully implement OH initiatives, a range of solutions need to be considered to tackle simultaneously the different themes pertaining to the different process phases.
The frequency in which the included papers mentioned specific solutions was also assessed. The majority of papers propose solutions at the phase of execution, followed by the conditions for starting and finally the monitoring and evaluation phase. In relation to the thematic classification of challenges, most papers mentioned solutions related to the theme of multi-actor collaborations (n = 39), followed policy and funding (n = 29). Closely mentioned were the themes of multi-domain collaborations (n = 28) and multi-level collaborations (n = 26). The next most mentioned themes were education and training (n = 25) followed by surveillance (n = 22). Solutions for the theme of evidence were least mentioned (n = 17).

Discussion
In this paper, we showed that a variety of challenges endured by OH initiatives affect their performance in different process phases and across different themes. The causal analysis revealed that challenges are interconnected through overlapping causes, crosscutting causal relations and even direct links, emphasizing the need for integrative approaches as the OH. The striking majority of mentioned challenges related to problems in the collaboration between the different stakeholders. The root causes for all types of challenges are at the Simplified causal tree. This simplified version of the causal tree depicts groups of challenges showing that although challenges were organized in different process phases and themes, they are interconnected through overlapping causes, crosscutting causal relations and even direct links. The colour code scheme represents the frequency in which the included papers mentioned the challenges, with darker colours representing highly mentioned and lighter colours representing less frequently mentioned.

Table 4
Solutions proposed for improving conditions for starting OH initiatives. In order to start and further execute OH initiatives, policy support and access to funds need to be improved, as well as the amount and quality of OH educational and training programs. The reference numbers follow the notation presented in the Reference list.

One Health 7 (2019) 100085
institutional and systemic level, and they relate to two different fragmentations: either the institutional-academic (e.g. professional and organizational silos), or the geographic-cultural (e.g. diverse preferences, values and capacity). Still, many solutions are proposed for tackling the challenges in performing OH initiatives and, for most of these solutions, practical examples of implementation were described. However, even for the challenges of which examples of implemented solutions were described, the initiatives still faced persistent challenges to be addressed in the different phases and themes. The biggest knowledge gap, in terms of proposed solutions for overcoming the challenges endured by OH initiatives, was notably for performing monitoring and evaluation.
A key finding from the causal analysis is that challenges and their causes are interrelated across phases and themes. While the lack of conditions (policy and funding, and education and training) causes difficulties in the execution of OH initiatives, a poor execution is a cause for the difficulties in producing OH evidence; as monitoring and evaluation is better performed when planned already during the design and execution phases. Furthermore, the lack of evidence on the benefits of performing OH initiatives is a cause for the lack of conditions, since to guarantee policy support and funding, evidence about the benefits of performing the OH approach needs to be provided.
In addition, the process phases for performing OH initiatives, hereby delineated, do not happen in a demarcated and unrelated fashion, but rather in an interactive and iterative way. Creating the necessary conditions, for instance, is important not only for boosting the start of initiatives but should be a constant effort in place throughout the whole process and at different levels. For instance, relevant competences by practitioners, and methodological principles at project level are essential conditions for starting the project; still, conditions in the institutional context (organizational flexibility) and correlations with the wider societal context (community engagement and consideration of local context) are essential for generating intended changes. Therefore, efforts have to be made simultaneously to create conditions at project level, and to achieve embedding and support at institutional level [16]. Although this may be true, this paper showed that projects are still initiated and executed even when conditions are not completely present [16]. In fact, even without conducive institutional and cultural conditions at the system level, experiments and initiatives happen. However, under this lack of proper conditions, practitioners struggle with enduring barriers for the implementation and scaling-up of such projects, as demonstrated in this paper for OH initiatives.
This unclear delineation of challenges across categories also applies to their classification in themes. Albeit the thematic classification of challenges has been broadly used by different fields of integrative approaches [68,[70][71][72][73][74], it does not reflect how challenges work in practice. This finding also affects the strategic implementation of solutions.
Although many examples of implemented solutions were identified, they did not address all problems faced by the described OH initiatives, most authors reflected in their papers on enduring problems on the overall performance of the OH approach. To improve the performance of OH initiatives, a range of solutions should be implemented to tackle simultaneously challenges at the different phases and from different themes. The surveillance of zoonotic diseases, for instance, could benefit significantly from the OH approach, through the integration of data and analysis from human, veterinary and environmental domains. Nevertheless, difficulties in collaboration between these actors lead to inefficiencies in data sharing, integration and collective analysis [30,40,54,55]. Therefore, solutions should focus at the same time on improving the standardization and integration of surveillance data through the enhanced collaboration of relevant stakeholders across domains. Other authors similarly claimed that neglecting the interconnection between different thematic aspects (such as epistemic, regulatory, and practical aspects) can generate inefficiencies in the execution of projects [69].
Not surprisingly, among the manifold challenges endured by OH initiatives that persist in different phases and relate to different themes, this paper shows that challenges for promoting collaboration between the wide diversity of stakeholders, as a fundamental aspect in the OH approach, are by far the most mentioned in the literature. Collaboration challenges are also recognized in collaborative approaches from other ID and TD fields as transition management [70][71][72], and resilience thinking [75][76][77]. The emergence of the OH approach challenged the pre-conception that global health problems could be solved through insights from experts working exclusively within their own discipline, proposing a multidisciplinary engagement of stakeholders [69]. However, as showed in the collaboration challenges, the difficulties in promoting real and equal participation from diverse actors persists. The intended innovation proposed by the OH approach, necessary to solve global health challenges, will only occur when stakeholders manage to overcome their professional and cultural silos to work together and therefore generate the co-creation of new knowledge, methodologies and practices [17,25,47,61]. Multi-level collaborations are also problematic, since they request the engagement of policy-makers and Table 6 Solutions proposed for improving the monitoring and evaluation of OH initiatives. In order to improve OH monitoring and evaluation, more studies need to be performed, guidelines developed, and specific OH metrics and indicators proposed. The reference numbers follow the notation presented in the Reference list.

Improving monitoring & evaluation
Status of implementation 7. Evidence [5,17,23,24,29,30,32,38,40,42,44,45,53,[61][62][63][64] 7.a Develop a standardized framework for systematic evaluation and reporting of OH outcomes [24,30,38] Implemented [24] 7.b Establish a network, as a community of experts, to develop science-based evaluation protocols for OH [42] Recommended 7.c Perform cost-effectiveness analysis and develop a OH business case [5,32,38,40,42,45] Implemented [5,32,38,42,45] 7.d Consider evaluation before program implementation (in the design phase) [23,38] Implemented [23,38] 7.e Include measures relevant to each sector in the monitoring and evaluation [38] Implemented [38] 7.f Develop standardized quantitative indicators for OH evaluation [24,42] Implemented [24] 7.g Use examples from other disciplines for improving monitoring and evaluation (e.g. epidemiology, environmental impact, socio-economics) [29,38,53,61] • Use the outcome mapping technique [30] Implemented [29,38,53,61] 7.h Understand health as a "quantitative and qualitative interaction and outcome process in social-ecological systems" [17,29,38,42,44,53,[61][62][63] Implemented [29,38,42,44,53,61,63] 7.i Test and monitor the development and use of OH metrics and indicators [29,45] Implemented [29] C. dos S. Ribeiro, et al. One Health 7 (2019) 100085 international organizations, who tend to interpret the scope of OH within the context of their mandate and activities. Their reluctance to broaden their vision and scope affects the wider acceptance and implementation of collaborative approaches as OH. In this context, some authors argue that this conservative attitude at a preeminent level influences the performance of mainstream OH initiatives, because practitioners tend to reproduce the narrowed pursue of specific goals with the prioritization of self-interests, leading to reductionism and fragmentation [22]. Notably, the root causes not only for collaboration, but all types of challenges are embedded in two different types of institutional and systemic fragmentations. Firstly, institutional-academic fragmentation is reflected on disciplinary and organizational silos, with each institution having specific working practices and methods. Secondly, the geographic-cultural fragmentation is reflected in diverse social and cultural preferences, values and even disparities in knowledge and capacity; which is particularly, but not exclusively, evident in institutions from developed nations adjacent to developing countries. Building upon similar root causes found in the fields of OH data sharing [78] and rabies innovation [14], this study elaborates on fundamental challenges that generate persistent and complex problems for OH initiatives.
Recognizing that the search strategy for including papers in this review focused on identifying challenges for designing and implementing OH initiatives, this review does not provide an exhaustive overview of successfully implemented OH initiatives. Still, the striking majority of the included papers either proposed or discussed solutions to address challenges, and even described examples implemented in practice. Interestingly, most of the solutions for which no example of implementation was found were located either at the beginning or the end of the causal tree. This reflects the logic of the fundamentality of the different challenges and their causes, solutions at the middle level have a focus on local and immediate problems, being easier to implement with the potential for generating faster results. Still, several solutions were implemented at the bottom level. Solutions proposed for the bottom (systemic) level aim at the root causes of the OH challenges, having the potential to generate long-term and sustainable results. Nevertheless, individual OH practitioners can do little in influencing the adoption and implementation of solutions at this institutional and systemic level. Hence, the organization of OH practitioners (e.g. in projects or networks) and engagement of policy-makers, international organizations, and community members are essential preconditions for supporting solutions that propose deeper institutional and cultural changes.
Finally, a mismatch was noticed in terms of most mentioned challenges and proposed solutions. This is especially the case for the challenges under the themes of multi-level collaborations, and evidence. Few papers mentioned specific challenges for multi-level collaborations, however, several papers proposed solutions to address these challenges. As aforementioned, a possible reason for that is the preference for solutions that can cause deeper and sustained changes (at a systemic level), even if they are hard to implement. The opposite happened for the challenges in monitoring and evaluating OH initiatives, where the biggest gap in the literature in terms of proposed solutions was identified. The adoption of the OH approach is unlikely to increase unless a clear sign of its benefits is provided. Therefore, solving these challenges represents the "unfinished agenda" of OH, in the sense that future studies need to be performed aiming to develop a framework for better monitoring and evaluating OH initiatives.

Conclusion
This paper indicates that many challenges exist and persist for designing and implementing OH initiatives. Although many solutions have been proposed, they were mostly implemented in a small scale and within a specific context. The success of the OH approach does not rely exclusively upon efforts within local initiatives, but also upon changes in cultural, social and institutional practices, at an institutionalized and systemic level [79][80][81]. In addition, in order to generate a paradigm shift for solving global health problems, a merely multidisciplinary team of experts is not sufficient. Stakeholders should work at an ID and TD level through the integration of academic and 'real world' expertise for knowledge co-creation to address OH challenges in an innovative way. Based on the acknowledgement of possible challenges endured by the OH approach and proposed solutions for these challenges, OH practitioners are able to plan and structure the designing and implementation of OH initiatives in a more successful way, by avoiding barriers and/or through the strategic pre-consideration of solutions. Nevertheless, a knowledge gap was identified for solutions proposed to solve challenges in monitoring and evaluating OH initiatives. Future research should focus on this theme in order to provide clear evidence on benefits of using the OH approach.

Declaration of interest
The authors declare no conflict of interest.

Funding source
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Authors' contribution
CdSR performed the data collection and wrote the first draft of the manuscript. LHMvdB and BJR critically revised and edited the manuscript. All authors were involved in conceptualizing and designing the study; analysis and interpretation of data; and final approval of the manuscript.  This paper illustrates how knowledge generated from participatory research does not necessarily imply solving a public health problem. This study aimed to contribute to the understanding of the benefits and barriers of following the basic principles of the Ecohealth approach.

Ecohealth in the TD sense, beyond academic actors Peruvian Amazon
Heavy metal exposure [12] Atlas This article discusses about the future of OH. There is a need to more clearly define its boundaries and demonstrate its benefits. Interestingly, the greatest acceptance of OH is seen in the developing world where it is having significant impacts on control of infectious diseases.
OH in the ID sense, with the integration of veterinary, medical and environmental sciences Non-geographically located The OH approach in itself [36] Asokan & Asokan The major obstacles to control zoonotic diseases include insensitive systems and unreliable data. This article argues that intelligent handling big data can accomplish the overall goals of OH.
OH in the ID sense, with the integration of veterinary, medical and environmental sciences Non-geographically located

Zoonosis [63] Barrett & Bouley
The authors assessed the current integration of environmental issues in OH publications and leadership, to understand its maturation; they gauge environmental representation in OH; and suggest ways to enhance integration of environmental drivers, data, and disciplines into the future development of OH.
OH in the ID sense and advocate for the adoption of the Ecohealth approach in the TD sense Non-geographically located The OH approach in itself The aim of the study is to offer a vision for improving tertiary education to prepare environmental and health professionals to address a changing world.
OH in the ID sense, with the integration of veterinary, medical and environmental sciences  In this paper it is presented an evaluation of an integrated health messaging intervention, for five bacterial and dog-associated zoonotic diseases. The aim is to encourage sequential adaptation of images, key messages, and delivery strategies using autoevaluation and end-user feedback. The authors describe the challenges and opportunities of this approach. They conclude by discussing the merits of incorporating the validated education approach into the school curriculum in order to influence longterm behaviour change.
OH in the TD sense, beyond academic actors Sidi Kacem Province, northwest Morocco Neglected tropical diseases [39] Durski, Jancloes, Chowdhary & Bertherat The authors present the Global Leptospirosis Environmental Action Network (GLEAN) developed to improve global and local strategies of how to predict, prevent, detect, and intervene in leptospirosis outbreaks in order to prevent and control leptospirosis in high-risk populations.
One Medicine in the multidisciplinary sense with the involvement of actors from veterinary and medical sciences   This paper is a conference abstract about Caribbean regional challenges related to community-based approaches for zoonotic disease control and prevention, and mitigation of problems at the interface of wildlife, domestic animals, and humans. Participants suggested a framework for practicing OH in the Caribbean that emphasized capacity building and sustainability.
OH in the ID sense with the inclusion of veterinary, medical and environmental sciences Caribbean countries

Zoonosis [62] Wolf
This essay locates the one health discussion on disease ecologies in a more than human world within recent developments in cultural and medical anthropology that focus on the entanglements between health and a multitude of animals, plants or microbes, as they are characteristic of a globalized modernity. The paper aims to examine the social dimensions of human-animal-disease-interactions, claiming that disease is a biocultural phenomenon and that social factors generally play a crucial role in the emergence, spread and management of (infectious) disease.
OH in the TD sense, with the involvement of actors beyond the academic domain Non-geographically located OH in itself [65] Yates-Doerr This article, through its descriptions of the contingencies scientists face when producing edibility, suggests that "the global," at least when it comes to food security, cannot be a singular thing and thus cannot be addressed by a universal approach or commonly shared solution.
One World One Health in the ID sense, with the involvement of human, animal and environmental sciences This paper firstly recalls briefly the history of integrative thinking on human and animal health, secondly it reviews "one medicine" and "ecosystem approaches to health" in the conceptual landscape of comparable and neighbouring approaches, and thirdly it explores avenues of systemic approaches to the health of animal and humans and their potential to address the challenges ahead. Causal tree of challenges. The causal relations between challenges for performing OH initiatives are depicted through the representation of the logical structure of causal arguments [15]. Challenges are broadly classified in process phases (conditions for starting, execution and monitoring and evaluation), and grouped in themes (policy and funding, education and training, surveillance, multi-domain collaborations, multi-actor collaboration, multi-level collaborations, and evidence).