By Dr Gary Barker

Systematic review has emerged as a significant form of scientific publication, initially in medicine but increasingly in human and animal health (One Health). Systematic review tries to aggregate multiple scientific publications from independent studies with a single focus to deliver an unbiased overview and sometimes ‘meta-analysis’ in relation to well-defined research questions.

It was inevitable that, ultimately, there would be systematic reviews of systematic reviews, and this has proved to be the case. But is there a central limit theorem? Does repeated review home in on essential information or are the review steps like secret whispers that get distorted as they get further from their source?

Bovine value chains

One of the early examples of this review hierarchy concerns African One Health projects. ‘Where literature is scarce: observations and lessons learnt from four systematic reviews of zoonoses in African countries’ (Alonso et al. Animal Health Research Reviews 2016) – is particularly relevant for Livestock Livelihoods and Health because one of its component reviews addresses research questions about the bovine value chain in Tanzania. This second-level systematic review crystallises generic research questions that can be addressed by systematic review and which are relevant to a variety of specific (pathogen, host) zoonotic hazards in low-income countries. For example: What is the prevalence? What are the risk factors? What are the impacts? What control strategies are available?

However, the review of reviews also highlights the small quantity of literature that relates to some specific value chains and shows how this means that reviewers often embrace multiple hazards and hosts so that their focus can become compromised. Reviewing the reviews highlights important patterns for the sources that are suitable for inclusion in an aggregated view. In particular international databases, such as PubMed and Web of Science, provide strong coverage of research on value chains in low-income countries, and accessing additional specialist databases, such as African Journals Online, does not have a significant impact. Data from the second-level systematic review shows that, for the African value chains that are considered, both the accessibility and the quality of relevant science publications is improving rapidly.

Research questions

A network of keywords associated with research publications about zoonoses in Tanzania ©Gary Barker

A network of keywords associated with research publications about zoonoses in Tanzania  Gary Barker

The collected systematic reviews show that research relating to the prevalence and the risk factors is by far the most abundant but, in some respects, the bovine chain in Tanzania is an exception to this rule because several publications address control and impact. Hazards associated with the bovine chain in Tanzania, and particularly brucellosis and mycobacterium spp., contributed most publications to the systematic reviews that were considered by the second-stage review process.

The review of reviews shows that publications relating to diseases that have an international impact, such as brucellosis or trypanosomiasis, outweigh those corresponding to disease that might be more important from a public health perspective. For example, the recent report from the WHO foodborne disease burden epidemiology reference group indicates that the disease burden associated with non-typhoidal salmonellosis in East Africa is more than 500 times that associated with brucellosis but this is not reflected by the number of publications found by a systematic review process.

Currently, it is difficult to estimate the value of nested reviews in general. However, in relation to research on zoonoses in Africa, the recent second-level review prototyped by Alonso et al provides some valuable conclusions that resonate clearly within LLH, and in particular the meat risks project.

As the authors state: “Our study seems to suggest that most of the focus of research on the livestock value chains has been placed on production threats and diseases associated with historically important public health impacts. The increasing literature in all countries from the 1990s on other zoonotic pathogens (i.e. foodborne) for which livestock are a healthy reservoir suggests a shift in priorities, and represents a step forward towards the widely promoted One Health approach to research and policy.”

Dr Gary Barker is a research leader at the Institute of Food Research and a partner in the meat risks project of LLH.

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