by Katherine Hoffmann


Advocates of public health interventions to reduce parasite prevalence often focus on how poverty increases susceptibility to parasitic disease, framing medical interventions as a means of providing a healthier and more equitable life for the extreme poor. Nonetheless, an equally interesting (albeit less straightforward) question of interest is whether the causal link may go in the other direction, with high worm burden acting to reinforce or worsen situations of poverty. This question is of particular relevance to those allocating resources to development aid programs, for it implies that reducing worm burden may not only improve population health, but also act as a long-run mechanism to increase economic growth and break the vicious cycle of poverty in the developing world.


In this paper, I discuss the effects of intestinal helminth infections – specifically roundworm, hookworm, and whipworm – on educational attainment, a key mechanism of “escape” from poverty. I hypothesize that parasite infections have a variety of direct and indirect effects on educational attainment, and I review the literature supporting the theoretical links between the two. I conclude with a discussion of why school deworming programs may be especially practical and cost-effective interventions for reducing disease burden and increasing educational outcomes in low-income communities.





The global prevalence of helminth infection is astoundingly high. Scholars estimate that over a quarter of the world’s population is infected with an intestinal worm of some sort, with roundworm, hookworm, and whipworm infecting 1.47 billion people, 1.05 billion people, and 1.30 billion people, respectively.[1] Furthermore, the World Bank estimates that 100 million people may experience stunting or wasting as a result of infection.[2]


Because of their high mobility and lower standards of hygiene, school-age children are particularly vulnerable to these parasites.[3] Overall, it is estimated that 400 million, 170 million, and 300 million children are infected with roundworm, hookworm, and whipworm.[4] Children may also be particularly susceptible to the adverse effects of helminth infections due to their incomplete physical development and their greater immunological vulnerability.[5]


While the high prevalence of parasitic worm infections presents a clear opportunity for action, there is also significant anecdotal evidence that deworming interventions work. Public health campaigns to reduce helminth infections in the US may be traced as far back as 1910, when the Rockefeller Foundation began the fight against hookworm – the so-called “germ of laziness” – in the American South.[6] This campaign was enthusiastically received by educators throughout the region; as one Virginian school observed: “‘children who were listless and dull are now active and alert; children who could not study a year ago are not only studying now, but are finding joy in learning...for the first time in their lives their cheeks show the glow of health.’”[7] From Louisiana, a grateful school board added:


‘As a result of your treatment...their lessons are not so hard for them: they pay better attention in class and they have more energy...In short, we have here in our school-rooms today about 120 bright, rosy-faced children, whereas had you not been sent here to treat them we would have had that many pale-faced, stupid children.’ [8]


Similar (albeit somewhat more imperialist) reports emerged from various other regions of the developing world at the time; for example, two scholars in Puerto Rico found that:


‘Over all the varied symptoms with which the unfortunate jibaro [peasant], infected by  uncinaria [hookworm], is plagued, hangs the pall of a drowsy intellect, of a mind that has received a stunning blow...There is a hypochondriacal, melancholy, hopeless expression, which in severe cases deepens to apparent dense stupidity, with indifference to surroundings and lack of all ambition.’[9]


Such observations made an intuitive connection between worm burden and intellectual performance, but even today this link is anything but well-established. While it seems that worms may impair cognition in some way, the mechanisms driving this relationship are still hotly debated.





Perhaps the most obvious means by which intestinal helminths may impair the development of their human hosts is through their impact on nutrition. Intestinal helminth infection has been associated with problems such as vitamin deficiencies, stunting, anemia, and protein-energy malnutrition, which in turn affect cognitive ability and intellectual development.[10] This relationship is particularly alarming because it is gradual and often relatively asymptomatic.[11]


Parasite infection may affect nutrition in several ways. On the one hand, some scholars argue that worms may compete directly with their hosts for access to nutrients; both whipworm[12] and roundworm[13] are believed to impact their hosts in this way. Nonetheless, Watkins and Pollitt argue that the magnitude of this effect is likely to be minimal; after all, the nutritional requirements of these intestinal worms is small when compared with that of their host organism.[14]


A more probable source of infection-induced malnutrition is the nutrient malabsorption associated with parasite presence in the body. For example, in both pigs and humans, Ascaris has been tied to temporarily induced lactose intolerance and Vitamin A,[15] nitrogen, and fat malabsorption.[16] Impaired nutrient uptake may result from direct damage to the intestine’s mucosal walls as a result of the worms’ presence, but it may also be a consequence of more nuanced changes such as chemical imbalances caused by the body’s reaction to the helminths.[17] Alternately, Watkins and Pollitt suggest that the worms’ release of protease inhibitors to defend against the body’s digestive process may impair the breakdown of other, nutritious substances as well.[18] Finally, worm infections may also cause diarrhea and speed “transit time” through the intestinal system, further reducing the body’s opportunity to capture and retain the nutrients in food.[19]


Worms may also contribute to malnutrition by creating anorexia. A decline in appetite and food consumption due to helminthic infection is widely recognized by the literature,[20] with a recent study of 459 children in Zanzibar reporting that even mothers noticed spontaneous increases in appetite after their children underwent a deworming regime.[21] Although the exact cause of such anorexia is not known, researchers believe that it may be a side effect of body’s immune response to the worm and the stress of combating infection. [22]


Helminths may also affect nutrition by inducing iron-deficiency anemia. This is most severe in heavy hookworm infections, as N. Americanus and A. Duodenale feed directly on the blood of their host. Although the impact of individual worms is limited (each consumes about .02-.07 ml and .14-.26 ml of blood daily, respectively), this may nonetheless add up in individuals with heavy infections, since they may carry hundreds of worms at a given time.[23] One scholar went so far as to predict that “the blood loss caused by hookworm was equivalent to the daily exsanguination of 1.5 million people,”[24] while a study in Zanzibar showed that a 15Ę triannual application of Mebendazole could avert 0.25 l of blood loss per child per year.[25] Although whipworm is milder in its effects, it may also induce anemia as a result of the bleeding caused by its damage to the small intestine.[26]


The connection between worm burden and malnutrition is further supported by studies indicating that deworming programs lead to sharp increases in growth; the presence of this result even in older children has lead some scholars to conclude that “it may be easier to reverse stunting in older children than was previously believed.”[27]  Other, less clearly causal studies (see Oberhelman 1998) also show a strong correlation between worm burden and malnourishment among school-age children.


Once the links between helminth infection and various forms of malnutrition are established, there are a number of pathways by which parasite burden may affect cognition. For example, poor performance on normal growth indicators appears to be correlated with lower school achievement and enrollment, worse results on some forms of testing, and a decreased ability to focus; on the other hand, iron deficiency may result in “mild growth retardation,” difficulty with abstract cognitive tasks, and “lower scores...on tests of mental and motor development...[as well as] increased fearfulness, inattentiveness, and decreased social responsiveness” among very young children.[28] Anemia has also been associated with reduced stamina for physical labor, a decline in the ability to learn new information, and “apathy, irritability, and fatigue.”[29]


These connections are supported by a number of deworming studies. For example, using 47 students from the Democratic Republic of the Congo Boivin and Giardani (1993) found that iron supplements acted as a complement to deworming medication, producing better effects on mental cognition when they were applied in conjunction than when they were individually administered.[30] He hypothesized that this result was due to the fact that iron supplements may “improve [students’] physical well-being to the point of enhancing attentional or arousal mechanisms influential in learning and cognitive performance,” with deworming medication only acting to extend these benefits by further reducing the tendency to anemia.[31]


Perhaps even more fascinating are a number of papers that take the study of intestinal helminth beyond the malnutrition-cognition link to focus on the connections between worm infections and memory formation. For example, Nokes et. al. (1992) find that interventions to reduce whipworm infection in 159 Jamaican schoolchildren led to better “auditory short-term memory” and “scanning and retrieval of long-term memory;” particularly fascinating was his discovery that a nine-week period was all that was necessary for dewormed students to “catch up” to their worm-free peers in test performance.[32] Nokes’ optimistic conclusion that “whipworm infection[‘s]...adverse effect on certain cognitive reversible by therapy” is particularly significant because it suggests that the effects of worms on intellectual performance may not be restricted to the mechanism of long-term malnutrition, since the physical and developmental effects of such malnutrition would theoretically be irreversible.[33]


Also worth noting are the studies of Ezeamama et. al. (2005) and Sakti et. al. (1999), which studied worm burden in the Philippines and Indonesia, respectively. Both authors found significant negative impacts of helminthic infection on memory and fluency, findings that are particularly meaningful because they included controls for socioeconomic status, hemoglobin levels, and proxies of nutrition (nutritional status and stunting, respectively). As Ezeamama observes, these studies suggest “that undernutrition is not the primary mediator of the observed relationships” between worm infection and intellectual performance, particularly because their findings were significant in aspects of intellect that went beyond mere cognition and reaction time.[34]





The studies cited above suggest that it may be necessary to look beyond the nutrition-cognition link and explore other elements of helminth infection that could impact intellectual development. For the purpose of this analysis, these will include the immediate physical side effects of parasitic disease, school absenteeism, and the increase in medical costs associated with parasite infection.


Watkins and Pollitt note that much as physical activity is “nutritionally mediated” as patients with heavy worm burden struggle to preserve energy and fight malnutrition, so too could “the poorly nourished mind similarly reducing mental effort in the form of arousal and sustained attention.”[35] While they find little evidence that this adaptation would provide benefits in the form of energy conservation, it is clear that the active course of ongoing parasitic disease could impose other, more direct limitations on an individual’s attention span.


On the one hand, the parasite infection process is frequently symptomatic. Conditions associated with intestinal helminth infection include intestinal obstruction, insomnia, vomiting, weakness, and stomach pains;[36] while the natural movement of worms and their attachment to the intestine may be generally uncomfortable for their hosts.[37] The migration of Ascaris larvae through the respiratory passageways can also lead to temporary asthma and other respiratory symptoms.[38] These side effects may all act to distract students in an academic setting and reduce their clarity of mind and intellectual productivity.


Parasitic infection may also make individuals extremely sick. In addition to the low-level costs of chronic infection, helminth infection may be punctuated by the need for more serious, urgent care; for example, the World Health Organization found that worm infection is common reason for seeking medical help in a variety of countries, with up to 4.9% of hospital admissions in some areas resulting from the complications of intestinal worm infections and as many as 3% of hospitalizations attributable to ascariasis alone.[39] Also worth considering is the fact that the immune response triggered by helminth infection may drain the body’s ability to fight other diseases, making affected individuals more prone to co-infection.[40]


The day-to-day costs of illness also provide a strong explanation for the third practical reality of helminth infection, or the observation that it acts as “a very real barrier to children’s progress in school” as quantified by “outcome measures such as absenteeism, under-enrollment, and attrition.”[41] Parasite-heavy students may be too weak to attend classes, or their families may be too indebted by medical bills and low worker productivity to pay for school enrollment fees. This effect may be conceptually distinct from previous findings about the impact of parasitism on cognition and learning; for example, Miguel and Kremer (2004) find that deworming programs improve school attendance by 25% without affecting test outcomes at all.[42] Nonetheless, these effects may also be related: Bleakley (2007) found that school attendance and enrollment grew significantly in the school-age populations that benefited most from the Rockefeller Foundation’s deworming programs, leading to a long-term increase in income as well as a rise in literacy rates.[43]






Thus far, I have examined the global burden of parasitic worms and enumerated two mechanisms by which helminth infection may impose a concrete economic cost on heavily infected victims. On the one hand, chronic infection may lead to indirect, long-term nutritional consequences for parasite hosts, such as anemia, stunting, malabsorption, and protein-energy malnutrition; these, in turn, impact cognitive ability and memory. On the other hand, acute parasitic infection may have real, immediate consequences for its victims, including the burdens of discomfort, distraction, and illness; the expense of seeking medical attention; and the toll that these costs may take on school enrollment and attendance rates.


Having made the case for why parasitic disease must be addressed if we wish to insure truly equal access to education for all students, the question remains: why are school deworming programs an optimal approach? 


School deworming programs have a number of advantages. On the one hand, they allow health policymakers to take advantage of existing infrastructure and institutions for the dispensation of medical treatment; students already plan to attend school on a somewhat regular basis, and teachers can easily distribute the medication to their students without receiving any medical training.[44]


On the other hand, school deworming programs have been shown to have strong positive externalities. Miguel and Kramer (2004) used a difference-in-difference model to prove that deworming programs in some schools reduced the burden of disease in neighboring, untreated schools; other evidence suggests that deworming children also has strong benefits for adult infection rates, since children are a significant source of transmission.[45]


The nature of the intestinal helminths and the medications available to treat them

also favor universal deworming programs. Infection is generally diffuse, so it is worth treating a wide sample of the population; furthermore, a drug like Albendazole is a cheap, safe intervention that is not particularly specific, and so can be used fairly effectively against all three of the main intestinal helminths (or any coinfection of them).[46] Finally, because these worms cannot replicate inside of their hosts, reducing transmission may be the best way to reduce prevalence, and mass interventions on an annual or biannual basis may in fact be a reasonable means of achieving this goal.[47]


In conclusion, I have argued that reductions in intestinal helminth infection may be a key tool of economic development. There is considerable evidence that reduced worm load may enhance cognitive development, memory retention, and intellectual capacity in school-age children; it also may confer long-term benefits in the form of improved population health due to reduced malnutrition, anemia, and stunting. On the other hand, the direct and immediate health benefits of deworming programs – in the form of decreased illness and discomfort, reduced hospitalizations and doctor’s visits, and fewer coinfections – may also improve school attendance, enrollment, and performance while increasing the quality of life of school-aged children and their communities. Deworming programs should be a natural accompaniment to broader efforts to improve the quality and accessibility of education, as such programs will naturally increase the returns to investment in the educational sector while enhancing the capacity of the poor to take full advantage of the resources available to them.



1. Watkins and Pollitt, 171.

2.World Development Report 1993, 79.

3. Montresor, 9.

4. Levinger, 15. Please note that this estimate is less current than the Watkins and Pollitt estimate, leading Levinger to underestimate the number infected.

5. Montresor, 9.

6. Watkins and Pollitt, 174-5.

7. Rockefeller Sanitation Committee (1915), cited in Bleakley, 79.

8. Rockefeller Sanitation Committee (1912), cited in Bleakley, 78.

9. Ashford and Igaravidez (1911), cited in Watkins and Pollitt, 175. More colorfully, the Rockefeller Foundation referred to Tamil workers’ “disinclination to use latrines” as the “‘White Man’s burden,’” “‘a direct menace to the community,’” and “‘an indirect menace to the world.’”

[1]0. Report of a WHO Expert Committee (1987), 14.

11. Del Rosso and Marek, 5.

12. Levinger, 17.

13. World Development Report (1993), 79.

[1]4. Watkins and Pollitt, 174.

[1]5. Humans only.

[1]6. Report of a WHO Expert Committee (1987), 15.

[1]7. Crompton, S24.

[1]8. Watkins and Pollitt, 174. Levinger mentions this briefly in the case of whipworm (17).

19. Report of a WHO Expert Committee (1987), 15; Levinger, 17.

20. World Development Report (1993), 79; Del Rosso and Marek, 5.

21. Stolzfus, 348.

22. Watkins and Pollitt, 185. Specifically, some of the cytokines released in the immune response have been tied to anorexic reactions in animals.

23. Report of a WHO Expert Committee (1987), 18.

24. Watkins and Pollitt, 174.

25. “School-based Deworming Interventions: An Overview,” 1.

26. Watkins and Pollitt, 174; Report of a WHO Expert Committee (1987), 19.

27. World Development Report (1993), 79.

28. Levinger, 9, 13.

29. Report of a WHO Expert Committee (1987), 29. Watkins and Pollitt, 186.

30. Boivin, 261.

31. Boivin, 261.

32. Nokes, 77.

33. Nokes, 77.

34. Sakti, 322.

35. Watkins and Pollitt, 185.

36. John and Petri, 242, 254, 264; Watkins and Pollitt, 185.

37. Watkins and Pollitt, 185, 174.

38. John and Petri, 242.

39. Report of a WHO Expert Committee (1987), 31.

40. Watkins and Pollitt, 174.

41. Levinger, 15.

42. Miguel and Kremer, 163.

43. Bleakley, 73, 75-6.

44. Watkins and Pollitt, 173.

45. Del Rosso and Marek, 19.

46. World Development Report (1993), 74.

47. Del Rosso and Marek, 20.




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