Preventing zoonotic spillover from a One Health perspective

Zoonotic spillover, defined as the transmission of a pathogen from an animal to a human, depends on the alignment of ecological, epidemiological and behavioural factors.8 Zoonotic pathogens must meet a series of conditions in order to induce spillover infections in humans, including appropriate density and distribution of reservoir hosts, pathogen prevalence, infection intensity and human exposure.8 Across this transmission pathway, a number of drivers of zoonotic spillover have been identified, including changes in wildlife and livestock populations;9 deforestation, urbanisation and other forms of land use change;10 and a variety of human practices including hunting, farming, animal husbandry, keeping of exotic pets and trade.6 7 11 12 These large-scale changes have on multiple occasions given rise to spillover events, sometimes involving pathogens with epidemic or pandemic potential.

A One Health perspective, which recognises the health of humans, animals and ecosystems as being closely linked and interdependent,13 can be useful in conceptualising a range of potential determinants of spillover events. From this perspective, interventions could include surveillance of pools of viruses in wildlife and management of wildlife populations;14 enhanced food safety measures in both the wildlife and livestock value chain, prefarm and postfarm gate;12 15–17 replacement of traditional ‘wet’ markets with supermarkets;18 controls on wildlife hunting, trade and consumption;11 19 20 and phasing out of unsustainable agriculture practices.6 21

While some evaluative evidence exists around the effectiveness of interventions,22–25 they have often been implemented as short-term to medium-term programmes or academic investigations.6 In some cases, zoonoses have re-emerged after successful programmes have ended.25 As a result, experts have argued for the incorporation of successful interventions into policy frameworks, providing interventions with the sustainability required for long-term disease control.6

Governance, systems and the role of multisectoral actors

Public policy is ‘a set of interrelated decisions taken by a political actor or group of actors concerning the selection of goals and the means of achieving them’.26 Public policy decisions are ultimately in the hands of government and supranational governing bodies, and have greater longevity compared with many programmes, which are often implemented for a fixed term. Non-government actors, including vested interest stakeholders, can also play a powerful role in shaping government decisions.27 28

Although the longevity and scope of government actions may make policy an effective vehicle for prevention of emergent diseases, implementing policy is a complex process involving numerous stakeholders with competing views and interests.29 The responsibility for addressing zoonotic disease frequently spans multiple sectors of governance due to its relevance for both animals and humans. Where relevant policies are designed and implemented in isolation, opportunities for synergy may be missed and efforts may even be counter-productive.

Successful policy measures require both a sound evidence base, and also governance structures that enable action to be taken. Given the range of possible risk factors that might contribute to emerging zoonoses, and the possible impacts of policies to prevent zoonotic spillover, a One Health response has been advocated, requiring coordination between institutions and government departments involved in human and animal health, trade, agriculture and the environment.30 At the international level, the WHO, the Food and Agriculture Organisation and the World Organisation for Animal Health have endorsed a One Health policy framework to respond to zoonotic infectious diseases, emphasising collaboration between agencies.31 Within countries, national and local governments have also emphasised the need for multisectoral efforts, although many report that further integration is still required.32

Furthermore, given the complex social–ecological systems within which policies to prevent zoonotic spillover are implemented, the risk of unintended consequences is high. For example, region-specific closures of live animal markets have been shown to spread pathogens further afield as vendors seek new venues to sell their animals.33 Meanwhile, attempts to manage populations of wild animals may alter pathogen dynamics, unintentionally increasing the risk of spillover into livestock or people.34

Given these particular characteristics of policy development and implementation, they may be usefully considered as a particular case of intervention, and the evidence around them assessed accordingly. Different types of interventions might be more or less feasibly implemented by governments (or their partners), and their impacts might be different given potentially more complex implementation contexts, longer timespans and broader geographic ranges. Evaluations of these policies should also include consideration and monitoring of potential unintended consequences. In order to facilitate this, multisectoral involvement in both policy development and evaluation may be required.

Aims and scope

Approaches to managing epidemic and pandemic infectious pathogens once they have entered human populations have been systematically catalogued in the medical literature.35–41 These measures include hand washing, face masks, school closures and contact tracing and case isolation. Further upstream, systematic reviews of interventions targeting the spillover pathway have predominantly focused on programmes rather than policies, and have been restricted by various characteristics such as geographic region24 or pathogen type,25 or focused on programmes with an explicit endorsement of a One Health approach.23 In consequence, a comprehensive understanding of how policies to prevent zoonotic spillover have been evaluated, and what evidence there is of their effectiveness, is lacking. To address these research gaps, our objectives are to do the following:

1. Identify evaluations of policies that target the determinants of zoonotic spillover included in the spillover pathway8 (ie, human and animal health and interactions).

2. Identify insights around policy success and failure, and unintended consequences of policy implementation.

3. Describe approaches to evaluation and key barriers and facilitators to evaluating policies to reduce the risk of zoonotic spillover.

Our approach to identifying and analysing this literature will be informed by a One Health lens, acknowledging the interconnectedness of human, animal and environmental health.

Methods and analysis

We will conduct a systematic scoping review of evaluations of policies aimed at preventing zoonotic spillover events. The scoping review will be conducted in line with guidelines published by Arksey and O’Malley and refined by Levac and colleagues,42–44 which emphasise an iterative approach suited to an exploratory research question.

Stage 1: identifying the research question

The aim of this review is to use a One Health lens to identify and describe the range of policies that have been evaluated, the approaches to evaluation and the evaluative evidence. Informed by this aim, our research questions are as follows:

1. What policies aimed at preventing zoonotic spillover have been evaluated?

   1. What are the types of policies?

   2. Which policy actors (single department, multisectoral, whole of government) are engaged?

2. What are the reasons for policy success and failure, and the unintended consequences of implementing these policies?

3. How has evaluation of these policies been approached in the literature?

   1. What are the methods or study designs used?

   2. What are the outcomes?

   3. What are the barriers and facilitators to evaluation?

Stage 2: identifying relevant studies

We searched four electronic databases (Medline, Scopus, Web of Science and Global Health) in May 2021. The search strategy is organised by the main concepts in our research question: the spillover pathway; public policy; prevention and zoonotic pathogens. The search strategy was developed iteratively, informed by existing systematic reviews focused on related concepts24 45–49 and known indicator papers meeting inclusion criteria. We also searched the websites of 18 organisations involved in the prevention of zoonotic spillover to identify relevant grey literature. See online supplemental file 1 for details of search strategy and websites searched.

Stage 3: study selection

Records identified through the searches will be collated and double screened using the online platform Covidence.50 Studies will be included where they meet all of the following criteria:

1. Primary empirical study from any country or region with English-language abstracts.

2. Report empirical findings from an evaluation of any sort.

3. Focus on a policy implemented by government that targets a determinant of zoonotic spillover located on the spillover pathway (see figure 1).

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Figure 1

Spillover pathway adapted from Plowright et al.8 22

Titles and abstracts will initially be screened, followed by full-text screening. Title and abstract screening of an initial set of 100 papers will be undertaken by two independent researchers. Results will be compared in order to ensure consistency in decisions around study eligibility, and discrepancies resolved through discussion of the inclusion criteria. This process will be repeated until an acceptable level of agreement (>90%) is reached. The remaining papers will then be screened by one of the two reviewers. Full-text screening will be undertaken by two independent researchers and discrepancies will be resolved by discussing reasons for inclusion or exclusions among the screeners. Studies with full-texts in languages other than English will be eligible for inclusion if they include an English-language abstract. Full-text studies published in French, Spanish or Chinese will be single-screened by a member of the research team fluent in that language. Studies published in other languages will be translated as necessary.

In line with published guidelines, the approach to study selection may be refined iteratively when reviewing articles for inclusion.42–44 Reporting on the search and screening process will follow the guidelines provided in the Preferred Reporting Items for Systematic Reviews and Meta-Analyses Extension for Scoping Reviews.51

Stage 4: charting the data

Data charting will be conducted using a data charting form designed to identify the information required to answer the research question and subresearch questions (see online supplemental file 2). Data charting focused on characteristics of the study, the policy and the evaluation. For each policy, this included identifying which determinant of zoonotic spillover situated along the spillover pathway was being targeted. For the purpose of this study, we used a model of the spillover pathway adapted from Plowright et al.’s work,8 22 in which we differentiated between wildlife and domesticated animals (figure 1). This differentiation is important in the policy context, as the wildlife-domesticated animal interface is an important site for intervention, as well as the human–animal interface.

As recommended, the data charting form will be piloted with 10 records to ensure that it is consistent with the research question, and the data charting form will be revised iteratively in order to ensure the purpose of the research is being met.42–44

Stage 5: collating, summarising and reporting the results

We will undertake quality assessment of the included studies using the Quality Assessment Tool for Quantitative Studies developed by the Effective Public Health Practice Project,52 which has previously been used to assess the quality of natural experiments including public policy evaluations.53

We will analyse the extracted data, presenting a numerical summary of the included studies in table form, allowing us to describe the range of policy interventions that have been evaluated, approaches to evaluation and evidence of effectiveness. We will also conduct a thematic analysis of the contents of the included articles in order to identify, if possible, barriers and facilitators to implementing and evaluating these policies, as well as insights into why policies succeeded or failed in achieving their aims.

Patient and public involvement

This scoping review is being undertaken as part of a larger project involving policy actors at national and international levels as research team members, knowledge users and participants. Insights from the project have informed protocol development and stakeholders are able to provide input and perspectives on the results of the review. Project-level dissemination events involving policy stakeholders are also planned, where findings from the proposed review will be shared.