Diarrhoeal disease trends in the GBD 2015 study: optimism tempered by scepticism
Authors: Christopher J Gill, Donald M Thea, Patricia Hibberd
Journal: The Lancet Infectious Diseases (2017)

A now-deceased iconoclastic colleague of ours long enjoyed provoking his public health students with the statement, “Public health is the art of deciding who lives and who dies, and with what degree of misery”. Cynical? Yes, but essentially correct: at its core, public health is all about the allocation of sparse resources, and nowhere is this more so than in global public health. To make progress, there must be an accurate and recurring record of the burden of disease so that resources are used cost-effectively and progress tracked systematically. In this regard, the Global Burden of Disease (GBD) project from the Institute of Health Metrics and Evaluation (IHME) in Seattle has emerged as the leading authority on global health outcomes.1GBD 2015 Mortality and Causes of Death CollaboratorsGlobal, regional, and national life expectancy, all-cause mortality, and cause-specific mortality for 249 causes of death, 1980-2015: a systematic analysis for the Global Burden of Disease Study 2015.Lancet. 2016; 388: 1459-1544Summary Full Text Full Text PDF PubMed Scopus (4142) Google Scholar The Article in the Lancet Infectious Diseases on the GBD analysis of the burden of diarrhoeal disease draws attention to this pervasive killer of young children, and asserts an impressive 10% decline in disability adjusted life years (DALYs) due to reductions in childhood undernutrition and a 20·8% reducition in overall mortality worldwide since 2005.2GBD Diarrhoeal Diseases CollaboratorsEstimates of global, regional, and national morbidity, mortality, and aetiologies of diarrhoeal diseases: a systematic analysis for the Global Burden of Disease Study 2015.Lancet Infect Dis. 2017; (published online June 1.)http://dx.doi.org/10.1016/S1473-3099(17)30276-1Google Scholar Overall, the analysis finds diarrhoeal disease to be the ninth leading cause of death globally, at about 1·3 million deaths per year, but the fourth leading cause in children under 5 years, with roughly 499 000 deaths, or 38% of all deaths due to diarrhoea. Integrating aetiological data from the Global Enteric Multicenter Study (GEMS), the key pathogens were, in declining order of incidence, rotavirus, Cryptosporidium spp, and Shigella spp.3Kotloff KL Nataro JP Blackwelder WC et al.Burden and aetiology of diarrhoeal disease in infants and young children in developing countries (the Global Enteric Multicenter Study, GEMS): a prospective, case-control study.Lancet. 2013; 382: 209-222Summary Full Text Full Text PDF PubMed Scopus (2291) Google Scholar Overall, these results accord well with other recent modelling estimates.4Walker CL Rudan I Liu L et al.Global burden of childhood pneumonia and diarrhoea.Lancet. 2013; 381: 1405-1416Summary Full Text Full Text PDF PubMed Scopus (1454) Google Scholar There are limitations to the GBD study, which have been voiced before;5Kelly SL Wilson DP GBD 2015 and HIV estimates from the Optima model.Lancet HIV. 2016; 3: e558Summary Full Text Full Text PDF PubMed Scopus (6) Google Scholar, 6Byass P Cause-specific mortality findings from the Global Burden of Disease project and the INDEPTH Network.Lancet Glob Health. 2016; 4: e785-e786Summary Full Text Full Text PDF PubMed Scopus (14) Google Scholar, 7Liu L Black RE Cousens S Mathers C Lawn JE Hogan DR Causes of child death: comparison of MCEE and GBD 2013 estimates.Lancet. 2015; 385: 2461-2462Summary Full Text Full Text PDF PubMed Scopus (20) Google Scholar in particular, that the data by aetiology for each country need to be interpreted with care. The central challenge to the GBD project is that the source data for their model-generated estimates are often sparse, out of date, or entirely absent in exactly the same places where diarrhoeal disease exacts its greatest toll. In short, the absence of robust country-level surveillance statistics might itself be a symptom of a frail or failing public health system in those countries. This limitation is openly acknowledged by the GBD team, but the situation remains that their modelled country-specific estimates rely heavily on extrapolating spatiotemporal trends from countries or regions with better data to those with weak or absent data. For example, the GBD study estimates that disability adjusted life years (DALYs) due to diarrhoeal disease have fallen by 35% in Liberia. Yet Liberia has experienced calamitous shocks in recent years that make such reported improvements hard to reconcile. Only three data sources for Liberia were listed:8IHMEGlobal health data exchange.http://ghdx.healthdata.org/Date: 2017Google Scholar a decades old verbal autopsy study involving 25 individuals,9Pacque-Margolis S Pacque M Dukuly Z Boateng J Taylor HR Application of the verbal autopsy during a clinical trial.Soc Sci Med. 1990; 31: 585-591Crossref PubMed Scopus (26) Google Scholar a survey encompassing two Liberian counties in 1984 (preceding two decades of civil war from 1989),10Becker SR Thornton JN Holder W Infant and child mortality estimates in two counties of Liberia: 1984.Int J Epidemiol. 1993; 22: S42-S49Crossref PubMed Scopus (8) Google Scholar and a WHO website summarising cholera statistics that itself provided estimated data for Liberia with scant detail about its sources.11WHOHealth statistics and information systems. Disease and injury country estimates.http://www.who.int/healthinfo/global_burden_disease/estimates_country/en/Date: 2017Google Scholar In other words, there were no recent, representative, and robust data about diarrhoeal deaths in Liberia. Similar shortcomings apply to estimates for other hard-stressed countries, such as Libya, Afghanistan, Syria, and Haiti, all of which were estimated to have reduced diarrhoea DALYs by 10–60%. The GBD team has been commendably transparent in making its data sources, models, and modelling assumptions freely available. But few of the consumers of these data have sufficient backgrounds in epidemiological modelling to adjudicate them, nor the time or patience to scrutinise the underlying data and modelling assumptions. Instead, most users will accept the GBD estimates at face value. The GBD model's complexity is unavoidable, and their models fit well against verifiable data sources,12Vos T Barber R Phillips DE Lopez AD Murray CJ Causes of child death: comparison of MCEE and GBD 2013 estimates—Authors' reply.Lancet. 2015; 385: 2462-2464Summary Full Text Full Text PDF PubMed Scopus (6) Google Scholar, 13Kassebaum NJ Wang H Lopez AD Murray CJ Lozano R Maternal mortality estimates—Authors' reply.Lancet. 2014; 384: 2211-2212Summary Full Text Full Text PDF PubMed Scopus (5) Google Scholar but this does not address the problem of dated, sparse, or poor data, or the implications of imputing at the whole-country level. In the GBD data tables, this uncertainty is captured statistically in the 95% uncertainty intervals for each country and for each outcome. But is this uncertainty because of imprecision around country-specific data? Or, is it because data were extrapolated from other countries—ie, is Liberia really Ghana in disguise? These do not seem to be equivalent cases. As a practical matter, better transparency could be achieved by stratifying GBD results by the quality of data sources. Alternatively, the GBD team could simply decide to provide no estimates at all when country-specific data are inadequate, acknowledging the cases where nobody really knows. These points of scepticism should not detract from the many things that are right and important about this analysis and about the GBD project as a whole. And it is also clear that the GBD investigators are aware of these challenges and continually seeking ways to improve their models and move, iteratively, towards a clearer view of the truth. Globally, progress is being made to reduce diarrhoeal disease. If the GBD data could be used to also identify areas where data are particularly thin or dated, and so reduce uncertainties in the case of individual countries, that also would be a huge advance. As child survival researchers, we are hugely encouraged by this report, even as it reminds us that there are roughly 1·3 million excellent reasons why our job remains incomplete. Estimates of global, regional, and national morbidity, mortality, and aetiologies of diarrhoeal diseases: a systematic analysis for the Global Burden of Disease Study 2015At the global level, deaths due to diarrhoeal diseases have decreased substantially in the past 25 years, although progress has been faster in some countries than others. Diarrhoea remains a largely preventable disease and cause of death, and continued efforts to improve access to safe water, sanitation, and childhood nutrition will be important in reducing the global burden of diarrhoea. Full-Text PDF Open Access