Economic Benefits of Intervention
The characteristics of helminth infections make a compelling case for public sector intervention if based only on the evidence related to the intervention's effect on health. From an economic perspective, the public sector has several reasons to become involved in improving health outcomes. First, other benefits may be gained, in addition to the benefit for the treated individual. Second, some forms of intervention are almost pure public goods; that is, no one can be excluded from using the goods or services the interventions deliver, and the private sector is thus unlikely to deliver them. Finally, preventive measures, such as information on the value of washing hands,may not be delivered through the private sector. The lack of knowledge about infections and subclinical symptoms may make individuals less likely to seek treatment. In analyzing the gains of interventions for worm control, one should account for the burden of helminth infections, which extends well beyond the health impacts and DALYs. The economic implications may be quite large.
The negative correlation between helminth infections and income level is clearly demonstrated both within and between countries (de Silva and others 2003). However, causality cannot be inferred from this established relationship; poverty promotes higher worm burdens, yet poor health induced by helminths can lead to lower income. There may also be opportunity costs to uninfected household members residing with infected persons. Few studies have been designed to evaluate, either directly or indirectly, the magnitude of the effect of deworming on economic productivity. The indirect evidence at the micro level suggests that helminth infection has a significant impact on adult productivity and, subsequently, on earnings. More direct evidence for children shows that helminth infection has long-term implications for educational attainment and economic status.
Studies are increasingly documenting a causal impact of adult health (broadly defined) on labor force participation, wages, and productivity in developing countries (Thomas and Strauss 1997). Moreover, helminth infection is known to affect some of the health conditions related to productivity—namely, iron deficiency anemia and wasting. Guyatt (2000) reviews numerous studies relating these conditions to physical fitness and productivity; Haas and Brownlie (2001) review studies on the effect of iron supplementation on work. The studies generally show productivity gains linked to better health along the various health dimensions studied. However, although some evidence points to the indirect impact of STHs on income earnings, these relationships have not been adequately studied, either directly or indirectly.
More compelling links between helminth infection and economic well-being may exist for children. The strong association between worm burden and poor health outcomes for children suggests that infections may affect school enrollment, attendance, grade repetition, and grade attainment. In turn, the potential impact on educational outcomes has implications for the assessment of the economic benefits of intervention. Numerous studies have demonstrated the benefits of schooling, showing that the return on education is quite high. Increased education is associated with, among other things, higher worker productivity and generally higher productivity in nonmarket production activities, including greater farmer efficiency and productivity (Psacharopoulos and Patrinos 2002).
Although observational studies show that lower levels of learning and schooling are linked to helminth infection (World Bank 2003), establishing a causal relationship requires adequately controlling for all unobserved or confounding factors. Miguel and Kremer (2003) note that several methodological issues hamper many existing randomized treatment-control evaluations. First, externalities associated with interventions can lead to underestimating impacts among the untreated population. Second, sample selection and attrition issues can affect the validity of findings, although the direction of this effect is ambiguous. Third, existing studies typically evaluate the impact of deworming on cognitive skills, likely the culmination of several years of health and education investments, as assessed by tests administered to treated and untreated children. Although studies find an effect on cognitive skills for those with the heaviest worm burden, they do not focus on other important education outcomes, which are likely to be more affected in the short run by health improvements, such as school enrollment and school attendance.
The study by Miguel and Kremer (2003) in Kenya attempts to address those shortcomings through improved study design and analytical methods. In addition to providing health gains, deworming reduced total primary school absenteeism by at least one-quarter in the first two years of the project. The gains were largest for the youngest children, who suffered from more intense worm infections. Externalities would cause a substantial underestimation of this effect. In terms of cost-effectiveness as an educational intervention, deworming proved to be far more effective at improving school attendance than other educational interventions implemented in a study in Kenya. Deworming offers a high rate of return, increasing the net present value of discounted wages by more than US$30 per treated child compared with per treatment costs of under US$1. For realistic estimates of returns to schooling, these results show in general that the net present discounted value of lifetime earnings is high compared with the costs of treatment even for small gains in school participation (figure 24.3).
[Figure
24.3]
Bleakley (2003) examined the effect of a hookworm control program undertaken about 1910 in the southern part of the United States. Hookworm infection was estimated to cause a 23 percent drop in the probability of school attendance, and children with greater exposure to the hookworm eradication campaign were more likely to be literate. Moreover, the long-term follow-up of affected cohorts showed that hookworm infection in childhood led to significantly lower wages in adulthood.
Helminth infections in preschool-age children can have consequences for subsequent schooling, such as delaying primary school enrollment and school attainment, thereby affecting future labor market outcomes. Bobonis, Miguel, and Sharma (2003) conducted a study of preschool-age children, using iron supplementation and deworming drugs administered to children two to six years of age. Preliminary results indicate that, in addition to the weight gain associated with treatment, average preschool participation rates increased sharply by 6.3 percentage points among assisted children older than two, reducing preschool absenteeism by roughly one-fifth.
Beyond the current impacts on schooling and implications for cognition, helminth infection in children can have long-term implications for economic outcomes in adulthood through its effect on physical growth. Height has been shown to affect wage-earning capacity as well as participation in the labor force for men and women (Thomas and Strauss 1997). This relationship may be strongest in settings where infection rates are highest—that is, low-income areas, where physical endurance yields high returns in the labor market.
