Intestinal helminths—including hookworm, roundworm, whipworm, and schistosomiasis—infect more than one in four people worldwide. Researchers evaluated the long-run impacts of a mass school-based deworming program in Western Kenya. Approximately 10 years after treatment, researchers found that the program increased women’s educational attainment and men’s labor supply, with accompanying shifts in occupation choice.

Policy Issue 

Intestinal helminths—including hookworm, roundworm, whipworm, and schistosomiasis—infect more than one in four people worldwide and are particularly prevalent among school-aged children in less-developed countries. While light worm infections are often asymptomatic, more intense infections can lead to lethargy, anemia, and growth stunting, and may weaken the immune system. There is a growing body of evidence that suggests that school-based deworming can generate immediate improvements in child appetite, growth, and overall health, and subsequent improvements in school attendance. However, in order to understand what level of public investment is worthwhile to support such public health measures, it is necessary to assess the long-run impacts of such programs. 

Context of the Evaluation 

This study is a follow-up of the Primary School Deworming Program (PSDP) launched by International Child Support (ICS) in Busia district in 1998. Busia district is a poor and densely-settled farming region in western Kenya adjacent to Lake Victoria. Parasitological surveys indicated that in 1998 this area had a helminth infection rate of over 90 percent, and over one-third of the sample had moderate to heavy infections with at least one helminth. Helminth infections can be transmitted several ways: schistosomiasis is easily contracted from contaminated lake water, while soil-transmitted helminths are transmitted through contact with or ingestion of fecal matter. This can occur, for example, if children do not have access to a latrine and instead defecate in the fields near their home or school, where they also play. .

Details of the Intervention 

In January 1998, 75 schools were randomly divided into three groups of 25 schools each. Researchers first stratified the schools by their administrative zone and by their involvement with other NGO programs. They then listed the schools alphabetically and assigned every third school to a given project group. All groups were phased into treatment over three years. Schools with worm prevalence over 50 percent were mass treated with deworming drugs every six months. While all children in high-prevalence schools received deworming treatment by year three, children in treatment schools received two to three more years of deworming than the comparison group.

A 2004 evaluation by J-PAL affiliates Edward Miguel and Michael Kremer examined the 1-2 year impacts of PSDP and found that the deworming treatment led to large gains in school attendance and health outcomes. Children in treatment schools who did not receive deworming pills, as well as those living within three kilometers of treatment schools, also had lower worm infection rates and higher school participation.

In the current analysis, researchers examined health, education, and labor market outcomes a decade after treatment, at which point most subjects were 19 to 26 years old. The Kenyan Life Panel Survey (KLPS-2) was collected during 2007-2009, and tracked a representative sample of approximately 7,500 respondents who were enrolled in grades 2-7 in the PSDP schools at baseline in 1998. 84 percent of individuals in the original study sample who were still alive were successfully contacted for the follow-up survey.

Results and Policy Lessons 

One decade after receiving the deworming treatment, education and labor market outcomes improved. Evidence from this study suggests that health interventions among school-aged children, delivered too late in life to affect cognition or height, can have long-run impacts on labor outcomes by affecting the amount of time people spend in school or work.

Impacts on health: There were no long-term effects on physical growth or body mass index. However, there is some evidence of improved self-reported health and reduced miscarriage. Respondents in the treatment group were 4.0 percentage points more likely to report that their health was “very good” relative to the comparison group mean of 67.3 percent.

Impacts on education: A long-term follow-up found that deworming also improved school performance and future earnings. Deworming led to large academic gains for girls, increasing the rate at which girls passed the secondary school entrance exam by 9.6 percentage points over the comparison group mean of 41 percent.  This increase of roughly 25 percent halved the existing gender gap in exam performance. Men who were dewormed as children worked 3.5 more hours per week, spent more time in entrepreneurial activities, and were more likely to work in higher-wage manufacturing jobs than their untreated peers.

Impacts on labor supply and occupation: Deworming increased time men spent working by 17 percent, or 3.49 hours per week. Men and women spent 1.41 more hours (45 percent) in non-agricultural self-employment, and the deworming treatment led to a shift in occupation choice: respondents were three times more likely to work in manufacturing and less likely to work in casual labor.

Impact on living standards: Deworming led to fewer missed meals per week, as well as increased earnings. Among male and female wage earners, earnings increased by more than 20 percent. The estimated differences in earnings was larger than those of hours, suggesting that the deworming treatment led men to shift into jobs that required more work hours and that payed better.

Social returns: The social returns to mass school-based deworming in terms of measured earnings gains versus the costs of treatment are high, ranging from 32.2 percent to 51.6 percent depending on estimation approach. These estimates suggest that the program generated a financial rate of return higher than the cost of the program, and which likely exceeds conventional hurdles for public health investment. 


1. Kremer, Michael and Edward Miguel. 2004. “Worms: Identifying Impacts on Education and Health in the Presence of Treatment Externalities.” Econometrica 72(1): 159-217.