<< Back To Home

The benefits of mass deworming on health outcomes: new evidence synthesis, the debate persists

Tuesday, 20th of December 2016 Print


The Lancet Global Health, Volume 5, No. 1, e4–e5, January 2017


The benefits of mass deworming on health outcomes: new evidence synthesis, the debate persists

Jason R AndrewsPress enter key to Email the author

Isaac I Bogoch, 

Jürg Utzinger

Open Access

DOI: http://dx.doi.org/10.1016/S2214-109X(16)30333-3

Soil-transmitted helminthiasis and schistosomiasis affect more than 1 billion people, with the greatest burden in the poorest regions of the world. The global strategy for addressing these parasitic worm infections is mass deworming with anthelminthic drugs that are periodically administered to school-aged children and other high-risk groups. Traditionally, mass deworming is provided to all school-aged children, rather than a screen-and-treat approach. The goal is to control morbidity at the population level, which might reduce disease transmission. WHO has issued treatment guidelines based on infection prevalence,1 and several governments, donors, technical agencies, and pharmaceutical companies jointly signed The London Declaration2 committing to a roadmap for control and elimination of soil-transmitted helminthiasis, schistosomiasis, and other neglected tropical diseases.

Despite broad support for mass deworming—initially considered one of the best buys in public health—several concerns have been raised about this strategy. Mass deworming alone is unlikely to lead to helminthiasis elimination. Experts agree that complementary strategies, including improved access to clean water, sanitation, and hygiene and snail control for schistosomiasis, have played a crucial role in settings that have achieved interruption of disease transmission.3 More controversially, a debate is ongoing as to whether mass deworming confers population health benefits at all. In 2015, a landmark study4 investigating the long-term health and educational effects of school-based deworming came under scrutiny when data were re-analysed by a second group5, 6 who challenged conclusions about its benefits. Additionally, a Cochrane systematic review7 found little or no effects of mass deworming for soil-transmitted helminths on nutrition, haemoglobin, school attendance, and school performance, though the quality of evidence was mainly low or very low. However, helminthiasis experts raised concerns about the standard Cochrane methodology for the issue at hand and thus tempered the conclusions.8, 9

In this issue of The Lancet Global Health, Vivian A Welch and colleagues10 aimed to address some of these concerns with a new systematic review and meta-analysis of mass deworming for soil-transmitted helminthiasis and schistosomiasis, following a Campbell Collaboration approach. New features of this analysis were use of network meta-analytic methods, separate consideration of schistosomiasis, additional subgroup analyses, and the inclusion of some observational studies, though the latter received little attention. The primary findings were consistent with the Cochrane review: mass deworming for soil-transmitted helminths does not improve nutritional or cognitive outcomes, school attendance, or mortality, whereas screening and selective treatment might improve children´s weight. The authors found that mass deworming for schistosomiasis might slightly increase weight, but not height. The effect of treatment for schistosomiasis on haemoglobin was not presented, despite a significant effect in most trials including praziquantel and positive findings of earlier meta-analysis.11 Welch and colleagues´ analyses to evaluate effect modification added little apart from robustness checks and did not address the main concerns about the Cochrane review.

Two fundamental objections to the conclusion that mass deworming for helminths has no benefits and could therefore be abandoned have been repeatedly raised. First, the underlying studies might have been insufficient to detect benefits, because the follow-up for most studies was too short to detect helminthiasis-associated morbidity (eg, cancer, portal hypertension, and infertility) that takes a long time to accrue.8 Simultaneously, many short-term symptoms, including diarrhoea and abdominal pain, contribute to global disability burden estimates11 but are not measured. Second, the conclusion that mass deworming is not beneficial at the population level is logically inconsistent with findings of individual-level benefits for infected individuals. Effects, if present for infected individuals, are diluted when measured across a population that receives little (lightly infected individuals) or no (uninfected) direct benefits from deworming; however, a diluted effect is still a positive effect, and failure to detect this is a problem of measurement or statistical power.

The debate is not new, having repeated itself with little variation for almost two decades,12, 13 and further meta-analyses are unlikely to resolve these fundamental concerns about what is being measured. However, decisions must be made, often amid conflicting evidence or opinions. Low-income and middle-income countries, where helminthiases are rife, need to determine whether mass deworming should be prioritised over other interventions when allocating scarce health resources. Disease control experts will note that both meta-analyses found benefits of screening and targeted treatment for helminthiases, but mass deworming remains a more logistically feasible and inexpensive approach to reach those who would benefit. As Duflo and colleagues noted, “the only reasons to prefer a screening approach is if deworming drugs had negative effects on uninfected children (they do not), or if the costs of treating uninfected children in a mass campaign were greater than the costs of individually testing children to determine whether they required treatment (in fact it is much cheaper to mass treat than to diagnose and treat).”14

Parasitic worms cause human diseases. Mass deworming and complementary measures might treat infected individuals and reduce community-level burden, preventing new infections. If we accept these premises, this epistemological debate on the nature of evidence shifts to pragmatic challenges of how to optimise control of these neglected tropical diseases and where to prioritise specific control interventions among the many health needs of impoverished communities.

We declare no competing interests.


  1. WHO. Helminth control in school-age children: a guide for managers of control programmes. World Health Organization, Geneva; 2011
  1. The London Declaration on Neglected Tropical Diseases. London, UK.http://unitingtocombatntds.org/resource/london-declaration; 2012. ((accessed Nov 3, 2016).)
  1. Rollinson, D, Knopp, S, Levitz, S et al. Time to set the agenda for schistosomiasis elimination. Acta Trop. 2013; 128: 423–440



  1. Miguel, E and Kremer, M. Worms: identifying impacts on education and health in the presence of treatment externalities. Econometrica. 2004; 72: 159–217
  1. Aiken, AM, Davey, C, Hargreaves, JR, and Hayes, RJ. Re-analysis of health and educational impacts of a school-based deworming programme in western Kenya: a pure replication. Int J Epidemiol. 2015;44: 1572–1580


  1. Davey, C, Aiken, AM, Hayes, RJ, and Hargreaves, JR. Re-analysis of health and educational impacts of a school-based deworming programme in western Kenya: a statistical replication of a cluster quasi-randomized stepped-wedge trial. Int J Epidemiol. 2015; 44: 1581–1592


  1. Taylor-Robinson, DC, Maayan, N, Soares-Weiser, K, Donegan, S, and Garner, P. Deworming drugs for soil-transmitted intestinal worms in children: effects on nutritional indicators, haemoglobin, and school performance. Cochrane Database Syst Rev. 2015; 7 (CD000371)
  1. Montresor, A, Addiss, D, Albonico, M et al. Methodological bias can lead the Cochrane collaboration to irrelevance in public health decision-making. PLoS Negl Trop Dis. 2015; 9: e0004165


  1. Campbell, SJ, Nery, SV, Doi, SA et al. Complexities and perplexities: a critical appraisal of the evidence for soil-transmitted helminth infection-related morbidity. PLoS Negl Trop Dis. 2016; 10: e0004566
  1. Welch, VA, Ghongomu, E, Hossain, A et al. Mass deworming to improve developmental health and wellbeing of children in low-income and middle-income countries: a systematic review and network meta-analysis. Lancet Glob Health. 2016; 5: 40–50
  1. King, CH, Dickman, K, and Tisch, DJ. Reassessment of the cost of chronic helmintic infection: a meta-analysis of disability-related outcomes in endemic schistosomiasis. Lancet. 2005; 365: 1561–1569





  1. Cooper, E. Treatment for intestinal helminth infection. Message does not follow from systematic review´s findings. BMJ. 2000; 321: 1225–1226
  1. Bundy, D and Peto, R. Treatment for intestinal helminth infection. Studies of short term treatment cannot assess long term benefits of regular treatment. BMJ. 2000; 321: 1225
  1. Duflo, A, Fishbane, A, Glennerster, R, Kremer, M, Madon, T, and Miguel, E. Cochrane´s incomplete and misleading summary of the evidence on deworming.http://blogs.berkeley.edu/2012/07/20/cochranes-incomplete-and-misleading-summary-of-the-evidence-on-deworming/; July 20, 2012. ((accessed Nov 18, 2016).)