Parasites and poverty: The case of schistosomiasis

Abstract

Simultaneous and sequential transmission of multiple parasites, and their resultant overlapping chronic infections, are facts of life in many underdeveloped rural areas. These represent significant but often poorly measured health and economic burdens for affected populations. For example, the chronic inflammatory process associated with long-term schistosomiasis contributes to anaemia and undernutrition, which, in turn, can lead to growth stunting, poor school performance, poor work productivity, and continued poverty. To date, most national and international programs aimed at parasite control have not considered the varied economic and ecological factors underlying multi-parasite transmission, but some are beginning to provide a coordinated approach to control. In addition, interest is emerging in new studies for the re-evaluation and recalibration of the health burden of helminthic parasite infection. Their results should highlight the strong potential of integrated parasite control in efforts for poverty reduction.

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.