Schistosomiasis in the People's Republic of China: the era of the Three Gorges Dam

Abstract

The potential impact of the Three Gorges Dam (TGD) on schistosomiasis transmission in China has invoked considerable global concern. The TGD will result in changes in the water level and silt deposition downstream, favoring the reproduction of Oncomelania snails. Combined with blockages of the Yangtze River's tributaries, these changes will increase the schistosomiasis transmission season within the marshlands along the middle and lower reaches of the Yangtze River. The changing schistosome transmission dynamics necessitate a comprehensive strategy to control schistosomiasis. This review discusses aspects of the epidemiology and transmission of Schistosoma japonicum in China and considers the pathology, clinical outcomes, diagnosis, treatment, immunobiology, and genetics of schistosomiasis japonica together with an overview of current progress in vaccine development, all of which will have an impact on future control efforts. The use of synchronous praziquantel (PZQ) chemotherapy for humans and domestic animals is only temporarily effective, as schistosome reinfection occurs rapidly. Drug delivery requires a substantial infrastructure to regularly cover all parts of an area of endemicity. This makes chemotherapy expensive and, as compliance is often low, a less than satisfactory control option. There is increasing disquiet about the possibility that PZQ-resistant schistosomes will develop. Consequently, as mathematical modeling predicts, vaccine strategies represent an essential component in the future control of schistosomiasis in China. With the inclusion of focal mollusciciding, improvements in sanitation, and health education into the control scenario, China's target of reducing the level of schistosome infection to less than 1% by 2015 may be achievable.

FIG. 1.

Life cycle of Schistosoma japonicum.

FIG. 2.

The Three Gorges Dam, Poyang Lake, Dongting Lake, and Hunan, Jiangxi, Hubei, Jiangsu, and Anhui Provinces. (Adapted from reference with permission of the publisher.)

FIG. 3.

Clinical features of acute schistosomiasis. (A) Cercarial dermatitis. (Reprinted from the Centers for Disease Control and Prevention website [http://www.dpd.cdc.gov/dpdx/HTML/ImageLibrary/A-F/CercarialDermatitis/body_CercarialDermatitis_il2.htm].) (B) Pulmonary infiltrates. (Reprinted from the Prince Leopold Institute of Tropical Medicine website [http://www.itg.be/itg/DistanceLearning/LectureNotesVandenEndenE/imagehtml/ppages/CD_1069_051c.htm] with permission of the publisher.)

FIG. 4.

Clinical features of chronic schistosomiasis. (A) A characteristic perioval granuloma formed around a Schistosoma japonicum egg in a mouse liver. (B) An ultrasonogram showing gross hepatic fibrosis (grade 3).

FIG. 5.

Cerebral schistosomiasis. (A) An unenhanced axial computed tomography (CT) scan shows a small, oval, hyperdense lesion (arrow) in the paraventricular zone, dorsal of the right posterior horn. (B) An axial T2-weighted (2437/90/1 [repetition time/echo time/excitations]) magnetic resonance (MR) image shows a hypointense lesion (white arrow) with a small, centrally located area with an intermediate signal (black arrow). (C) A coronal contrast-enhanced T1-weighted (600/15/2) MR image shows an oval lesion with an intermediate signal (arrow) with ringlike and septumlike contrast enhancement. (Reprinted from reference with permission of the publisher. © by American Society of Neuroradiology).

FIG. 6.

Chemotherapy. (A) Praziquantel. (B) Artemisia annua, used in the preparation of artemether and other artemisinin derivatives.

FIG. 7.

Mathematical model of Schistosoma japonicum transmission and control. t_MS1, t_SM1, t_MS2, and t_SM2 are transmission parameters, which are described generally in the text as and . (Adapted from reference with permission of Elsevier.)

FIG. 8.

Mathematical modeling of two Schistosoma japonicum DNA vaccine constructs. The DNA vaccine constructs SjC23-Hsp70 (A) and SjTPI-Hsp70 (B) were modeled alone and as an intervention (in conjunction with annual human mass treatment and an initial water buffalo treatment) and compared to a hypothetical vaccine providing 45% efficacy.