Model-Based Insights into Multi-Host Transmission and Control of Schistosomiasis

Abstract

Schistosomiasis is a tropical disease of great antiquity that remains endemic in 76 countries, affecting 200 to 300 million people in the developing world. Current control of schistosomiasis is heavily dependent on chemotherapy, primarily through praziquantel, a safe and highly effective drug introduced in the early 1980s. In recent decades, large-scale school- or community-based treatment strategies have been successful in suppressing average infection intensity in many parts of the world, dramatically reducing schistosomiasis-associated morbidities such as hepatosplenomegaly, hepatic fibrosis, or bladder and kidney inflammation for urinary schistosomiasis [1,2]. We expect to see a similar pattern in low-income countries, particularly in sub-Saharan Africa, as praziquantel is made increasingly available [3]. However, increasing evidence shows that chemotherapy-based strategies alone are unlikely to be a sustainable strategy for prevention of schistosome infections in all endemic areas [2,4,5]. This evidence has two important implications. First, although advanced stages of schistosomiasis can be effectively controlled through praziquantel use, there is evidence that light chronic infections due to continuing transmission contribute significantly to chronic morbidities such as growth retardation, anemia, exercise intolerance, poor school performance, and lower work capacity [6]. Second, drug treatment does not change the environmental conditions that foster transmission of the parasite in endemic communities [2,4]. In some environments, the cessation of treatment even for a few years can result in a re-occurrence of high levels of infection, even back to pretreatment levels in extreme cases [7,8]. Hence, there is a pressing need to return to a more comprehensive strategy for suppressing schistosomiasis transmission beyond drug treatment. Linked Research Article This Perspective discusses the following new study published in PLoS Medicine: Riley S, Carabin H, Belisle P, Joseph L, Tallo V, et al. (2008) Multi-host transmission dynamics of Schistosoma japonicum in Samar Province, the Philippines. PLoS Med 5(1): e18. doi:10.1371/journal.pmed.0050018 Obtaining schistosome infection data from thousands of humans and mammalian hosts in the Philippines, Steven Riley and colleagues find that mammalian acquisition, rather than transmission to snails, drives prevalence. In the continuing absence of viable vaccines against the schistosome, there is reason to reconsider earlier approaches targeting the parasite transmission cycle to augment the chemotherapy-based strategy. However, such approaches (e.g., mollusciciding, improved sanitation, and provision of safe water) require significantly greater financial resources than does chemotherapy. To make best use of limited resources, the key is to identify and target control efforts at locally vulnerable stages of the transmission cycle. Although the complete life cycle of the schistosome was described almost a century ago, characterizing local or regional vulnerabilities of the parasite-snail-host interaction still remains a challenging task because of its complex dependence on agricultural and other ecological factors. This is particularly true for Schistosoma japonicum, which, in contrast to other species of schistosome, has a number of nonhuman mammalian hosts. The contribution of these zoonotic carriers to transmission is seldom well characterized [9]. In a new study in PLoS Medicine, Steven Riley and colleagues present an analysis using a mathematical model complemented with statistical approaches to unravel this particular aspect of the transmission cycle in an endemic region in the Philippines [10].