ReviewParasites and poverty: The case of schistosomiasis
Introduction
The problem of helminthic parasitism has not gone away. Fortunately, the issue is gaining renewed prominence in the discussions public health policy, a phenomenon that is based on several factors: first, there is increasing appreciation of the health and social burden of long-term chronic infections (Guerrant et al., 2002, Reidpath et al., 2003, King et al., 2005); second, newer, more sensitive diagnostics indicate that concurrent polyparasitism is much more common than we previously thought (Kasehagen et al., 2006, Raso et al., 2006b); and third, new, inexpensive approaches are making parasite treatment and transmission control increasingly more accessible (Molyneux et al., 2005, Fenwick, 2006, Ottesen, 2006).
Until recently, the conventional wisdom on helminth infections was that ‘light worm burdens remain asymptomatic’—implying (erroneously) that they do not provoke disease nor do they specifically require medical care (Warren, 1982, Gryseels, 1989). However, recent studies on the immunopathology of parasite infection and its chronic disease formation (Wamachi et al., 2004, Coutinho et al., 2005, Leenstra et al., 2006, Coutinho et al., 2007), indicate that it is the presence, as well as the intensity of infection, that drives morbidity due to chronic parasites such as Schistosoma spp. (King et al., 2006). Under-recognized, ‘subtle’ morbidities such as caloric malnutrition, growth stunting, anaemia, and poor school performance are all significant correlates of both helminthic and protozoan parasitic infections (Guyatt, 2000, Coutinho et al., 2005, Ezeamama et al., 2005, King et al., 2005, Fernando et al., 2006, Leenstra et al., 2006).
Internationally, concern is developing that combined health effects of multiple concurrent parasite infections are the source, as well as the effect, of poverty (Fig. 1) (Sachs, 2005, Engels and Savioli, 2006). Moreover, recent immunology research indicates that chronic parasitic infections can impair protective responses against many unrelated acute bacterial and viral infections, including impaired responses to childhood vaccines (Malhotra et al., 1999, Labeaud et al., 2009) and increased risk of mother-to-child HIV transmission (Gallagher et al., 2005, Secor, 2006). Thus, the beneficial effects of the existing major international programs for HIV prevention and for expanded vaccination coverage are jeopardized by the existence of all-pervasive parasite burden in targeted areas. In sum, it is time to critically re-evaluate the aggregate health impact of chronic helminthic infections, and, in particular, their link to poverty formation.
Section snippets
The policy challenge: decisions based on inaccurate or incomplete information on prevalence
Although most experts are proponents of evidence-based decision-making, until recently, our policies regarding helminth control have often been based on flawed information about infection prevalence, particularly in countries where resources for gathering vital statistics are severely limited (Brooker et al., 2000, van der Werf et al., 2003, Steinmann et al., 2006). Because of misclassification, there has also been continuing debate about how much morbidity can be attributed to any single
Inaccurate estimates of schistosomiasis-related disability
Past efforts to quantify the global burden of disease caused by schistosomiasis (Murray and Lopez, 1996, Michaud et al., 2004) have focused or on official estimates of incident or prevalent cases of active infection, and have neglected the life-time duration of chronic sequelae of infection (Giboda and Bergquist, 1999). In many respects, Schistosoma infection can be seen as an acute communicable disease that transitions into a chronic, non-communicable disease in later life. Thus, Schistotoma
Implications for the current research agenda
Although subjectively, it is difficult to believe that any parasitic infection can be consonant with good health (Guyatt, 2000), in past field surveys it has been difficult to clearly demonstrate objective findings of parasite-associated morbidity, disease, or economic impact (Gryseels, 1989, Tanner, 1989, Guyatt, 2000). In fact, many the previous field surveys that are cited in policymaking were statistically underpowered in terms of sample size for the purpose of measuring numerically small,
The challenge of polyparasitism and poverty
Although the empiric phenomenon of the ‘wormy village’ has long been recognized (Ashford et al., 1993), advances in diagnostic technology are now revealing that in endemic areas overlapping chronic parasitic infection with Schistosoma spp. (Wilson et al., 2006), Plasmodia spp. (McNamara et al., 2006), and filaria (Michael et al., 2001) is substantially more common than previously thought.
Where multiple causative pathogens co-exist within the same person, how do we attribute the
Reassess the total burden of chronic parasitic infections
Why should we pay attention to chronic parasitic infections, if they are mostly ‘minimally’ disabling and non-lethal? The answer comes from the typical context of infection, i.e., that of severe rural poverty. Although our Global Burden of Disease assessments have gone to great lengths to remove location or social context from the disability estimates for all diseases (Murray and Lopez, 1996), it is an inescapable fact that small deficits in performance status have a strong, asymmetric leverage
Implications for control—a new focus on transmission
Although current large-scale drug treatment can significantly reduce schistosome infectious burden (measured as egg output) in participating communities (King et al., 1988), parasite transmission may, nevertheless, continue unaffected because of several different factors (Muchiri et al., 1996, Satayathum et al., 2006). Therefore, alternative and complementary strategies (Table 1) need to be considered.
Schistosome transmission is often a periodic, inhomogeneous process in which any single
Outlook for the next decade
The link between schistosomiasis and poverty now appears evident, though causation is likely to be bidirectional (Fig. 1). Schistosomiasis caused a disabling state of chronic ill-health that impairs human capital, while local poverty fosters schistosomiasis transmission by enforcing exposure to contaminated water, limiting access to health care resources, and reducing resources to decrease transmission. While some may argue that structural social and governmental factors may be the proximate
Acknowledgments
This project was supported in part by National Institutes of Health Research Grant R01 TW008067 funded by the Fogarty International Center. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
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