Schistosomiasis Japonica: The DALYs Recaptured

Schistosoma parasites are disease-producing trematodes, or ‘‘blood flukes’’, that infect an estimated 207 million people worldwide [1]. Although schistosomes are common in the developing world, their role in human disease and disability is not widely appreciated, even by tropical medicine and health policy specialists [2]. A disability analysis reported by Finkelstein and colleagues in this edition of PLoS Neglected Tropical Diseases [3] is part of a new research movement that aims to correct this problem. By way of background, it should be recognized that any infection with the major species that affect humans, i.e., Schistosoma haematobium, S. mansoni, or S. japonicum, can cause chronic disability [4]. Schistosome transmission occurs in or near freshwater, and requires the presence of specific intermediate host snails [5]. Schistosomiasis is also strongly associated with poverty—snail-bearing water must be contaminated by human sewage for the snails to acquire their infection. Then, in turn, people must come into contact with the same snail-infested areas in order to become infected. In lessdeveloped communities that have only limited water resources, exposure to infection is often unavoidable. As a rule, over a person’s lifetime, schistosome infection will happen in recurrent waves throughout childhood and adulthood. Where snails prevail and there are no options for safe water, schistosomiasis is a common fact of life [6]. Once human infection is established, schistosomiasis can last for years. It results in chronic immune-mediated inflammation induced by parasite eggs that are trapped in host tissues. The consequence is decades of active tissue damage leading to tissue fibrosis, organ damage, and, potentially, to severe illness or death [7]. How debilitating is it for an average person to have schistosomiasis? This is an important question, and one that has not been well answered in the past. It is particularly relevant that we have an accurate disability estimate because in the Global Burden of Disease (GBD) program’s disability-adjusted life year (DALY) ranking system [8], it is the average health impact of schistosomiasis that determines the clinical and public health commitment to treat or prevent schistosome infection. In past decades, the day-to-day impact of schistosomiasis was often minimized as ‘‘asymptomatic’’, except when certain schistosomiasis-specific pathologies such as advanced hepatosplenomegaly, intestinal bleeding (in the case of S. mansoni and S. japonicum) or bladder cancer, hydronephrosis, or renal failure (S. haematobium) occurred [9]. Past researchers’ focus on these relatively rare clinical outcomes led policymakers of the 1980s and 1990s to conclude that, on average, schistosomiasis was not much of a public health problem [10,11]. Those who have carefully re-examined the GBD program’s DALY rankings [10] believe that the 0.5%–0.6% disability weight assigned to schistosomiasis must reflect this ‘‘minimal average impact’’ viewpoint. In addition, because of difficulties in attributing certain syndromic outcomes to specific infections, in the GBD weighting assessments, outcomes such as anemia, advanced liver disease, and epilepsy were disaggregated away from schistosomiasis, to be included as separate ‘‘diagnoses’’, in a process that numerically devalued the importance of preventing schistosome infection. Accordingly, in health policymaking, schistosomiasis was relegated to be one of the most ‘‘neglected’’ among the neglected tropical diseases (NTDs) [4,12]. In reality, the day-to-day experience of schistosomiasis involves a number of under-acknowledged, insidious, sub-clinical morbidities that affect patients on a daily basis. Many current studies are now focusing on these aspects of disease, including performancerelated impacts such as chronic anemia, malnutrition, cognitive impairment, and physical endurance. However, to dispute the GBD program’s previous underassessments, we need further, evidence-based assessments that will provide more valid overall disability weights that can be used to add up the lifetime health burden of having an ‘‘average’’ infection with schistosomiasis [4].