Human schistosomiasis

Abstract

Human schistosomiasis--or bilharzia--is a parasitic disease caused by trematode flukes of the genus Schistosoma. By conservative estimates, at least 230 million people worldwide are infected with Schistosoma spp. Adult schistosome worms colonise human blood vessels for years, successfully evading the immune system while excreting hundreds to thousands of eggs daily, which must either leave the body in excreta or become trapped in nearby tissues. Trapped eggs induce a distinct immune-mediated granulomatous response that causes local and systemic pathological effects ranging from anaemia, growth stunting, impaired cognition, and decreased physical fitness, to organ-specific effects such as severe hepatosplenism, periportal fibrosis with portal hypertension, and urogenital inflammation and scarring. At present, preventive public health measures in endemic regions consist of treatment once every 1 or 2 years with the isoquinolinone drug, praziquantel, to suppress morbidity. In some locations, elimination of transmission is now the goal; however, more sensitive diagnostics are needed in both the field and clinics, and integrated environmental and health-care management will be needed to ensure elimination.

Figure 1. Lifecycles of Schistosoma mansoni, Schistosoma haematobium, and Schistosoma japonicum

(A) Paired adult worms (larger male enfolding slender female). (B) Eggs (left to right, S haematobium, S mansoni, S japonicum). (C) Ciliated miracidium. (D) Intermediate host snails (left to right, Oncomelania, Biomphalaria, Bulinus). (E) Cercariae.

Figure 2. Global distribution of countries where human schistosomiasis is transmitted

Adapted from Gryseels and colleagues.

Figure 3. Age-prevalence and age-intensity of infection curves for Schistosoma haematobium (A) and Schistosoma mansoni (B)

Data from King and colleagues and DeStigter and colleagues.

Figure 4. Schistosoma mansoni egg-induced granulomas in the liver of an infected mouse

Eggs are roughly 120–180 μm long, 45–70 μm wide.

Figure 5. Effect of schistosomiasis on aerobic capacity in children in Kenya and Canada

Data taken from Bustinduy and colleagues.