Asian Schistosomiasis: Current Status and Prospects for Control Leading to Elimination

Abstract

Schistosomiasis is an infectious disease caused by helminth parasites of the genus Schistosoma. Worldwide, an estimated 250 million people are infected with these parasites with the majority of cases occurring in sub-Saharan Africa. Within Asia, three species of Schistosoma cause disease. Schistosoma japonicum is the most prevalent, followed by S. mekongi and S. malayensis. All three species are zoonotic, which causes concern for their control, as successful elimination not only requires management of the human definitive host, but also the animal reservoir hosts. With regard to Asian schistosomiasis, most of the published research has focused on S. japonicum with comparatively little attention paid to S. mekongi and even less focus on S. malayensis. In this review, we examine the three Asian schistosomes and their current status in their endemic countries: Cambodia, Lao People's Democratic Republic, Myanmar, and Thailand (S. mekongi); Malaysia (S. malayensis); and Indonesia, People's Republic of China, and the Philippines (S. japonicum). Prospects for control that could potentially lead to elimination are highlighted as these can inform researchers and disease control managers in other schistosomiasis-endemic areas, particularly in Africa and the Americas.

Keywords: Asia; Schistosoma japonicum; Schistosoma malayensis; Schistosoma mekongi; control; elimination; epidemiology; schistosomiasis.

Figure 1

(A) Map of Southeast Asia showing the location of endemic areas for schistosomiasis, including a focus in central Myanmar. (B) Map of Myanmar highlighting the state of Rakhine.

Figure 2

Schistosome lifecycle. Adult worms (1) reproduce sexually in the mesenteric veins surrounding the small intestine of the definitive mammalian host. Female worms deposit eggs (2), which are excreted in the feces. Upon contact with freshwater, the eggs hatch miracidia (3), which penetrate a snail intermediate host (4) and undergo asexual reproduction; this includes development of mother and daughter sporocysts, which produce cercariae (5). Cercariae exit the snail and swim around until they penetrate the skin of the mammalian definitive host, potentially causing cercarial dermatitis (I), shed their tail and become schistosomula (6). The schistosomula migrate through the body to the lungs before migrating and maturing to adult worms in the mesenteric veins. Chronic schistosomiasis occurs as the result of an immune reaction to the eggs resulting in granuloma formation in tissues where eggs are lodged. This most commonly occurs in the liver and spleen (II), which can result in hepatosplenomegaly and portal hypertension; in the walls of the intestine (IV) as eggs pass from the blood into the intestine; and less commonly in the brain (III), causing neuroschistosomiasis, characterized by a range of neurological symptoms. (Abbreviation: GIT, gastrointestinal tract).

Figure 3

Water buffalo (carabao) in the Philippines are tethered in rivers, rice fields, and wallows—the same areas where the intermediate snail host for S. japonicum is also found. (Images from the Philippines, captured by C.A.G.).

Figure 4

Washing and recreational uses of waterways are risk factors for contracting schistosomiasis. (Image from the Philippines, captured by C.A.G.).