The evidence for mass school-based deworming

Parasitic worms are debilitating, widespread, and under-treated. School-based deworming is safe, cost-effective and scaleable. There is a robust evidence base for the work of the Deworm the World Initiative undertakes.

Deworming has important impacts on school participation, cognition and nutrition, and future earnings. Multiple rigorous studies have shown strong evidence of the effects of deworming, providing confidence in the benefits of treatment.  We summarize the evidence base for mass school-based deworming in this post.

School Participation

Parasitic worms limit educational outcomes for children. Not only are infected children less likely to be enrolled in school, but they are also less likely to attend school and more likely to perform lower on testing.

  • A long-term follow-up study linking aggregate infection data with individual socioeconomic data from the southern US in the 1910s found that a non-infected child was 20 percentage points more likely to be enrolled in school than an infected child, and was also 13 percentage points more likely to be literate.
  • Miguel and Kremer’s experimental evaluation in Western Kenya found that deworming treatment resulted in a 25% decrease in absenteeism at treatment schools.
  • In a long-term follow-up study in Kenya, evidence shows that among females, deworming increased the rate of passing the national primary school exit exam by 9.5 percentage points on a base of 41%.

Nutrition and Cognition

Children with parasitic worms suffer from nutritional impairment, impacting their growth and physical development. Deworming treatment leads to significant weight gains and allows more energy to be focused on growth and development.

  • A meta-analysis authored by Croke et al. (2016) finds a substantial and highly robust positive effect on child weight resulting from deworming. The effects are particularly large in areas with at least 20% prevalence; this is the same threshold at which the WHO currently recommends mass treatment.
  • A randomized controlled trial in Uganda finds that the provision of periodic anthelmintic treatment as a part of child health services resulted in an increase in weight gain of about 10% above expected weight gain when treatments were given twice a year, and an increase of 5% when the treatment was given annually.
  • Deworming has positive externalities even for children who are not directly treated. Owen Ozier’s 2016 study finds that younger siblings of children who were treated show cognitive gains comparable to between 0.5 and 0.8 years of schooling ten years later.

Future Earnings

Children who were dewormed have higher earnings in adulthood. Higher earnings contribute to improved economies and significant returns on investment for governments, especially considering the extremely low cost of treatment.

  • Hookworm infections could have explained as much as 22% of the income gap between the U.S. North and South in the early 1900s.
  • In Kenya, men who were treated as children worked 3.4 more hours per week, spent more time in entrepreneurial activities, and were more likely to work in higher-wage manufacturing jobs compared to their untreated peers. This long-term impact study in Kenya calculates a rate of return between 32-52% for governments who invest in deworming.

  • Is mass treatment justified? On cost-effectiveness grounds we believe that it clearly is, as the cost of screening is four to ten times that of the treatment itself. Everyone agrees that children infected with worms should be treated. Because the drugs are very effective in treating worms, and are safe even for the uninfected, the WHO recommends mass drug administration as the clinical standard of care in areas where more than 20% of children are infected.
  • Deworming through schools, and preschools in some areas, provides the greatest opportunity to reach a high proportion of the at-risk population while minimizing costs through the use of existing infrastructure.

Why Mass Treatment