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Keywords:

  • Systematic review;
  • meta-analysis;
  • narrative review

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Differences Between Systematic Reviews and Traditional Narrative Reviews
  5. Systematic Reviews in Human Health Care
  6. Systematic Reviews in Animal Agriculture and Veterinary Medicine
  7. Conclusion
  8. Acknowledgements
  9. References

This article is the first in a series of six articles related to systematic reviews in animal agriculture and veterinary medicine. In this article, we overview the methodology of systematic reviews and provide a discussion of their use. Systematic reviews differ qualitatively from traditional reviews by explicitly defining a specific review question, employing methods to reduce bias in the selection and inclusion of studies that address the review question (including a systematic and specified search strategy, and selection of studies based on explicit eligibility criteria), an assessment of the risk of bias for included studies and objectively summarizing the results qualitatively or quantitatively (i.e. via meta-analysis). Systematic reviews have been widely used to address human healthcare questions and are increasingly being used in veterinary medicine. Systematic reviews can provide veterinarians and other decision-makers with a scientifically defensible summary of the current state of knowledge on a topic without the need for the end-user to read the vast amount of primary research related to that topic.

Impacts
  • Systematic reviews are a structured method to synthesize existing knowledge on a topic.
  • Systematic reviews are widely used in a variety of fields, including human healthcare.
  • With some modifications to the methods, systematic reviews are a valuable tool in animal agriculture and veterinary medicine.

Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Differences Between Systematic Reviews and Traditional Narrative Reviews
  5. Systematic Reviews in Human Health Care
  6. Systematic Reviews in Animal Agriculture and Veterinary Medicine
  7. Conclusion
  8. Acknowledgements
  9. References

The use of reviews in decision-making

In human health care, and increasingly in animal agriculture and veterinary medicine, the evidence-based medicine movement encourages an evidence-based approach to decision-making, whereby the clinician precisely identifies a clinical problem, searches for and critically appraises all relevant information and decides whether or how to use this information. A major challenge to this approach is how to manage the thousands of research results that become available every year. One way to address this challenge is to use reviews of scientific evidence that have been performed by others. The clear advantage for decision-makers is that it is faster and more time efficient than identifying, obtaining and reading all of the relevant primary (i.e. original) research on a topic themselves. Some of the disadvantages to using reviews are that they are sometimes not easy to adapt to individual cases, they become outdated as new research is published, the quality of the review itself needs to be addressed, and one can often find conflicting results between reviews (Barnes and Bero, 1998; Hoving et al., 2001). Evaluations of traditional narrative reviews in the medical literature (Mulrow, 1987; McAlister et al., 1999) and on-farm food safety literature (Sargeant et al., 2006b) illustrate that these reviews do not report the use of scientific methods to identify, assess and synthesize the literature available on the review topic, potentially resulting in invalid conclusions.

Overview of systematic reviews

Well-executed systematic reviews provide a rigorous and replicable method of identifying, evaluating and summarizing evidence (Cook et al., 1997; Sargeant et al., 2006a). Integral to current definitions of systematic review is the emphasis on employing systematic methods to reduce bias in the identification and evaluation of studies to be included in the review, and a critical appraisal of the risk of bias in the studies included in the review. Therefore, systematic reviews can provide veterinarians and other decision-makers with a scientifically defensible summary of the current state of knowledge on a topic, without the need to read the vast amount of primary research.

Systematic reviews address a targeted question or hypothesis using a structured series of steps with multiple reviewers used at several steps to reduce the potential for bias (Fig. 1) (Sargeant et al., 2006a; European Food Safety Authority (EFSA), 2010). A comprehensive and explicit search strategy is used to identify primary studies (i.e. original research) for inclusion in the review, minimizing the potential for selection bias. Selection of studies for inclusion in a systematic review should be based on specific criteria that are applied to abstracts from all citations identified by the search. Data on study characteristics and results are extracted from the primary studies, and risk of bias is assessed for each study. This requires good reporting of study features and results within the primary studies: guidelines are available for human clinical trials (Moher et al., 2010; Schulz et al., 2010) and for trials in large animals and food safety (O'Connor et al., 2010; Sargeant et al., 2010). When a sufficient number of studies with similar outcomes are available, a formal statistical combination of the data from multiple studies (meta-analysis) can be performed. Meta-analysis is an effective way of increasing power and detecting intervention effects where the results in individual studies are unclear or conflicting. This may be particularly valuable in animal trials, which may be small and under-powered. When meta-analysis is not possible, qualitative summarization of the available data may still provide valuable input to answer the review question or to identify knowledge gaps.

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Figure 1. Structured steps used to conduct systematic reviews of the literature (EFSA 2010).

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Uses of systematic reviews

Systematic reviews have been used increasingly to inform practice and policy in areas ranging from medicine, to education, to public safety. Systematic reviews have been used in health care to address a range of health-related issues such as disease prevalence / incidence, aetiology and risk factors, diagnostic test accuracy and evaluation of preventive or therapeutic interventions (EFSA, 2010). The focus of this article, and other articles in this series, will be on systematic reviews as they pertain to the evaluation of interventions. In this context, ‘intervention’ refers to a treatment used to prevent, reduce or treat an adverse health outcome or event in animal populations and encompasses strategies such as antimicrobials, biologics and dietary or management manipulation.

The range of intervention questions that have been answered by systematic reviews includes the following: Does Penicillin reduce the risk of streptococcal throat infections and attacks of rheumatic fever in people who have already had a bout of rheumatic fever? (Manyemba and Mayosi, 2003); Do school-based drug education programs reduce drug use? (McBride, 2003); Do sobriety checkpoints by law enforcement officers reduce the number of alcohol related traffic accidents? (Elder et al., 2003). Similarly, systematic reviews in veterinary medicine have been used to answer a broad range of questions such as What are the best interventions to treat canine atopic dermatitis? (Olivry and Mueller, 2003); What is the efficacy of vaccines for the prevention of pinkeye in cattle? (Burns and O'Connor, 2008); What is the evidence for the effectiveness of acupuncture in veterinary medicine? (Habacher et al., 2006); and What is the evidence for efficacy of selected pre-harvest interventions to reduce faecal shedding of Escherichia coli O157 in ruminants? (Sargeant et al., 2007; Snedeker et al., 2011).

Differences Between Systematic Reviews and Traditional Narrative Reviews

  1. Top of page
  2. Summary
  3. Introduction
  4. Differences Between Systematic Reviews and Traditional Narrative Reviews
  5. Systematic Reviews in Human Health Care
  6. Systematic Reviews in Animal Agriculture and Veterinary Medicine
  7. Conclusion
  8. Acknowledgements
  9. References

There is a common but erroneous belief that systematic reviews are the same as traditional narrative reviews, only more comprehensive (Petticrew, 2001). Systematic reviews are not just big literature reviews, and their main objective is not simply to search more databases. Rather, systematic reviews are designed to answer a specific question, to reduce bias in the selection and inclusion of studies, to appraise the risk of bias in the included studies and to summarize them objectively. They also differ in the measures taken to reduce bias, for instance using several reviewers working independently to screen papers for inclusion, extract study outcomes and assess the risk of bias in the studies. Even small systematic reviews are likely to involve several reviewers screening thousands of abstracts. As a result, systematic reviews commonly require more time, staff and money than traditional reviews. Systematic reviews are not simply ‘bigger’, they are qualitatively different, as illustrated in Table 1.

Table 1. Good quality systematic reviews and traditional narrative reviews compared
 Good quality systematic reviewsTraditional narrative reviews
  1. Sources: adapted from Cook et al. (1997) and Petticrew (2001).

Review topic – what is the question?Clear question to be answered or hypothesis to be testedOften broad in scope addressing general topics such the epidemiology and or pathology of a disease
Finding data – sources and strategiesComprehensive search of electronic databases, hand searching of relevant journals, review of references lists and contact with researchers. Explicit search strategy provided including attempts to access unpublished dataNot usually specified, often no attempt to find all relevant literature. Therefore, potential for selection bias
Selection of studies to includeExplicit description of the types of studies to be included; other criteria specified to limit reviewer biasNot usually specified. Therefore, potential for selection bias
Assessing risk of bias in the included studiesFormal assessment of the risk of bias in the original research studies is examined in a systematic mannerOften do not evaluate risk of bias or differentiate between methodologically sound studies and those with design flaws or potential for biased results
Synthesizing study resultsHeterogeneity (difference in results) between studies is explicitly described; results are statistically pooled where data and methods are similarGenerally no formal statistical pooling of results; Often do not consider differences in study methods or risk of bias in the included studies
InferencesConclusions are based on totality of evidence with a consideration of the risk of biasConclusions not necessarily based on totality of evidence

Systematic Reviews in Human Health Care

  1. Top of page
  2. Summary
  3. Introduction
  4. Differences Between Systematic Reviews and Traditional Narrative Reviews
  5. Systematic Reviews in Human Health Care
  6. Systematic Reviews in Animal Agriculture and Veterinary Medicine
  7. Conclusion
  8. Acknowledgements
  9. References

Historical perspectives

Although there are a limited number of written examples, the critical consideration of all scientific research or data on a topic has been an important component of scientific enquiry since the Enlightenment (Hunt, 1997). In many historical examples, attempts were made to synthesize available data or place new data in the context of current knowledge. In a 1904 report published in the British Medical Journal, statistician Karl Pearson synthesized data from several studies on the efficacy of typhoid vaccination. His rationale for pooling data was that ‘Many of the groups… are far too small to allow for any definite opinion being formed at all, having regard to size of the probable error involved’ (Pearson, 1904). In the years leading up to and during the Second World War, noted statistician Frank Yates wrote about pooling results from groups of studies using agricultural and livestock examples (Chalmers et al., 2002).

Thus, attempts to search for and synthesize evidence to support clinical practice and policy decisions are not new. However, formal approaches and systematic methods for appraising and collating evidence have only been developed in recent decades. This has been in response to calls from the ‘evidence movement’ to organize knowledge into a useable and reliable format (Hansen and Rieper, 2009). Critical appraisal and synthesis of research findings in a systematic manner emerged in the 1970s under the term ‘meta-analysis’ (Chalmers et al., 2002). The phrase was coined by Glass and colleagues who conducted research syntheses to determine the benefits of psychotherapy (Smith and Glass, 1977) and the impact of class size on student achievement (Glass and Smith, 1979). The concept of ‘systematic review’ is newer still and is defined to emphasize the importance of systematic methods to reduce bias in the identification and selection of research results which are to be synthesized through ‘the application of strategies that limit bias in the assembly, critical appraisal and synthesis of all relevant studies on a specific topic’ (Last, 2001).

Although initial published syntheses were conducted in the areas of public policy and social interventions, a common early use of systematic research synthesis was in human therapeutics. One important leader in this movement was epidemiologist Archie Cochrane who wrote ‘it is surely a great criticism of our profession that we have not organized a critical summary, by specialty or subspecialty, adapted periodically, of all relevant randomized controlled trials’ (Cochrane, 1979). These influences helped to inform the evidence-based medicine movement of the late twentieth century. Evidence-based medicine has been defined by Sackett et al. (1996) as ‘the conscientious, explicit, judicious use of current best evidence in making decisions about the care of individual patients’. An important feature of this movement has been the emphasis on research synthesis, with healthcare decision-making based on systematically reviewed and critically appraised research evidence. Benefits of this approach include the ability to balance research evidence collected in a systematic and rigorous manner with the experiences and expertise of stakeholders (Sackett et al., 1996). This applied research appro-ach to practice and policy has contributed to the increasing popularity of structured abstracts and secondary journals summarizing studies of high relevance and methodological quality, the creation of the Cochrane Collaboration and its methodology for systematic reviews, and the publication of texts emphasizing evidence-based decision-making (Guyatt et al., 2004). The principles of evidence-based practice have become core concepts of undergraduate, postgraduate and continuing education in an increasing number of fields including veterinary medicine.

Methods for conducting systematic reviews have been designed for the use by those who want to make more informed decisions in clinical practice, healthcare research and public health policy. Examples of groups involved in the methodology and conduct of systematic reviews include the Cochrane Collaboration (www.cochrane.org), the Agency for Healthcare Research and Quality (www.ahrq.gov) and The Centre for Reviews and Dissemination (http://www.york.ac.uk/inst/crd/index.htm). Although there are some differences in the style and focus of these groups, the basic methodological components of their systematic reviews are the same. The websites for these organizations provide detailed guidance documents.

Key organizations for human healthcare systematic reviews

The Cochrane Collaboration opened its centre in Oxford in 1992 and is now an international network of researchers, academics, practitioners and users committed to the principles of managing healthcare knowledge in such a way that it is quality assured, accessible and cumulative. As of September 2012, the Cochrane Library includes the Cochrane Central Register of Controlled Trials, which now includes more than 680 000 trials, and the Cochrane Database of Systematic Reviews, which currently contains more than 7500 complete Cochrane systematic reviews (Cochrane Collaboration, 2013). The Cochrane Library (including protocols and full reviews) is freely and publically available in many countries. Structured abstracts for all Cochrane reviews are available free of charge to anyone with Internet access. The systematic review format developed and used by the Cochrane Collaboration is internationally recognized as the gold standard for systematic reviews of the effects of healthcare intervention studies. A handbook describing the process of preparing and maintaining Cochrane systematic reviews is available online (Higgins and Green, 2011).

The Agency for Healthcare Research and Quality (AHRQ) is the health services research arm of the United States Department of Health and Human Services, complementing the biomedical research mission of its sister agency, the National Institutes of Health. AHRQ is home to research centres that specialize in major areas of healthcare research such as quality improvement and patient safety, outcomes and effectiveness of care, clinical practice and technology assessment, and healthcare organization and delivery systems. This agency provides research support and policy guidance in health services research and systematic reviews.

The Centre for Reviews and Dissemination (CRD) is part of the United Kingdom's National Institute for Health Research (NIHR) and is a department of the University of York. Established in 1994, CRD is one of the largest groups in the world engaged exclusively in evidence synthesis in the health field. The centre comprises experienced health researchers, medical information specialists, health economists and a knowledge dissemination team. In addition to producing a large range of systematic reviews, CRD also produces internationally accepted guidelines for the conduct of systematic reviews.

Systematic Reviews in Animal Agriculture and Veterinary Medicine

  1. Top of page
  2. Summary
  3. Introduction
  4. Differences Between Systematic Reviews and Traditional Narrative Reviews
  5. Systematic Reviews in Human Health Care
  6. Systematic Reviews in Animal Agriculture and Veterinary Medicine
  7. Conclusion
  8. Acknowledgements
  9. References

Systematic reviews are beginning to be used in veterinary medicine and food safety for companion animals, food animals and wildlife, with several hundred systematic reviews on animal health topics published between 2000 and 2013 (a database that includes systematic reviews in veterinary medicine is available at: http://webapps.nottingham.ac.uk/refbase/).

The findings of systematic reviews can offer valuable information on whether interventions are efficacious, compare the efficacy between interventions or can provide data as input into risk assessment models. For instance, a systematic review of multiple pharmacological interventions for canine atopic dermatitis reported good evidence for recommending the use of oral glucocorticoids and cyclosporine, fair evidence for topical triamcinolone spray, topical tacrolimus lotion, oral pentoxifylline or oral misoprostol and insufficient evidence for or against several other interventions including type-1 histamine receptor antagonists, tricyclic antidepressants and cyproheptadine (Olivry and Mueller, 2003). This review also concluded that there was fair evidence against recommending the use of other interventions.

Systematic reviews can also highlight areas where there is insufficient evidence to assess the efficacy of interventions or where there are common methodological flaws in the available research, thereby providing direction and impetus for future basic and applied research in a specific area. For instance, a systematic review of surgical interventions for canine cranial cruciate ligament injury concluded that none of the surgical procedures evaluated in the review had sufficient data available in the literature to support them giving a consistent return of dogs to normal function (Aragon and Budsberg, 2005). In a systematic review of on-farm interventions to reduce faecal shedding of E. coli O157 in domestic ruminants, several of the interventions considered had only been evaluated using small scale field trials, in many cases using an artificial disease challenge (Sargeant et al., 2007). While the findings provide useful preliminary evidence that an intervention may work, they do not provide strong evidence that the intervention will work under real-life conditions. The same review noted that there were methodological flaws in several of the publications, including failure to report randomization to intervention groups in clinical trials. Thus, systematic reviews can identify knowledge gaps to target needed research and can also identify methodological issues that need to be considered when reading the existing literature and addressed when designing additional primary research studies.

However, although the methods for developing systematic reviews of interventions in human health care are well developed, these methods may not be directly applicable to evaluate issues in animal agriculture and veterinary medicine (Sargeant et al., 2006a). An obvious difference between human health care and animal sciences is that animal studies encompass multiple species, and extrapolating results of studies from one species to another may not be appropriate. There are also important differences between human and veterinary medicine in the study designs used to address clinical questions. Randomized controlled trials (RCTs) provide the highest level of evidence for evaluating efficacy of interventions under real-world conditions (Higgins and Green, 2011) and are therefore preferred for inclusion in systematic reviews in both human and veterinary medicine. In human medicine, when a sufficient number of RCTs exist on a topic, systematic reviews of interventions often are restricted to include only results from RCTs. However, there are comparatively few RCTs published in veterinary medicine compared with human medicine, and therefore, observational studies are commonly used, particularly for interventions related to management practices. Additionally, challenge studies, where the investigator controls both the allocation to intervention and the disease occurrence (e.g. by deliberately challenging all study subjects with an infectious disease agent of interest and then randomly allocating study subjects to intervention groups), are often conducted in veterinary research but are generally not used in human healthcare. Animal models of non-infectious diseases can be developed and used in the species of interest in veterinary medicine to evaluate potential interventions. In human medicine, studies using animal models can provide proof of concept for an intervention, but are not included in systematic reviews to evaluate real-world efficacy in humans.

In human health care, patients are recruited for a study based on specified eligibility criteria and, with the exception of studies involving children or individuals with cognitive disabilities, provide informed consent for their own participation. In veterinary medicine, animal owners'/care-givers' consent to participation on behalf of the animals that actually participate in the trial. This means that, in veterinary studies, there often are two ‘levels’ of selection and eligibility: that of the owner and that of the animal subjects. This may have implications for the comparability of trials evaluating the same intervention.

There are additional factors to consider when conducting systematic reviews related to livestock species. Agricultural and husbandry practices vary substantively between countries, and livestock generally passes through distinct production stages during their lives which may involve very different dietary, housing and other management practices. Livestock populations are grouped and housed differently from human populations, meaning that aspects of trial design and analyses may differ between trials in livestock species and human trials. Research studies in livestock, particularly clinical trials, may be performed with individual animal intervention allocation and housing (often using an artificial disease challenge), small group intervention allocation and housing, or under typical commercial (field) conditions. When animals are housed in groups, the outcome may be determined based on group level variables, measurements on all animals within the group or subsampling of individuals within the group. Each of these methodological approaches will be associated with different statistical analyses. Therefore, when conducting systematic reviews of livestock-related questions, differences in study designs between studies addressing the same basic question will need to be considered when summarizing and interpreting results from multiple studies.

Finally, human medical decisions are made based on the combination of scientific evidence about efficacy of the intervention, the balance of benefits and harms, and patient values and preferences. For livestock species, the balance of benefits and harms to the animal is important. However, there may be additional considerations, such as the cost-benefit associated with the use of an intervention, food safety or animal welfare considerations, the producer or owner's values and preferences, and for some issues the consumer's values and preferences. Therefore, it is necessary to modify the existing protocols used for systematic reviews in the human healthcare field for use in systematically evaluating animal research.

Despite these challenges, systematic reviews in animal agriculture and veterinary medicine will allow researchers to synthesize the current body of knowledge on targeted issues and lend increased credibility to findings in the field.

Conclusion

  1. Top of page
  2. Summary
  3. Introduction
  4. Differences Between Systematic Reviews and Traditional Narrative Reviews
  5. Systematic Reviews in Human Health Care
  6. Systematic Reviews in Animal Agriculture and Veterinary Medicine
  7. Conclusion
  8. Acknowledgements
  9. References

Systematic review is a structured method that has been widely used in human health care to summarize knowledge about the efficacy of interventions. The use of systematic reviews in animal agriculture and veterinary medicine is more recent, and there are difference between human and animal research that need to be considered. However, systematic reviews increasingly are being used in animal agriculture and veterinary medicine and offer an evidence-based method of summarizing primary research. Further articles in this series will overview study design issues when conducting systematic reviews, as well as providing details for each step of the systematic review process.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Differences Between Systematic Reviews and Traditional Narrative Reviews
  5. Systematic Reviews in Human Health Care
  6. Systematic Reviews in Animal Agriculture and Veterinary Medicine
  7. Conclusion
  8. Acknowledgements
  9. References

The authors thank Annette Wilkins for assistance with this manuscript. Funding and in-kind support was obtained from the Laboratory for Foodborne Zoonoses, Public Health Agency of Canada and the Canadian Institutes of Health Research (CIHR) Institute of Population and Public Health/Public Health Agency of Canada Applied Public Health Chair awarded to JM Sargeant.

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  2. Summary
  3. Introduction
  4. Differences Between Systematic Reviews and Traditional Narrative Reviews
  5. Systematic Reviews in Human Health Care
  6. Systematic Reviews in Animal Agriculture and Veterinary Medicine
  7. Conclusion
  8. Acknowledgements
  9. References
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