Evaluation of mammalian and intermediate host surveillance methods for detecting schistosomiasis reemergence in southwest China

Abstract

Background: Schistosomiasis has reemerged in China, threatening schistosomiasis elimination efforts. Surveillance methods that can identify locations where schistosomiasis has reemerged are needed to prevent the further spread of infections.

Methods and principal findings: We tested humans, cows, water buffalo and the intermediate host snail, Oncomelania hupensis, for Schistosoma japonicum infection, assessed snail densities and extracted regional surveillance records in areas where schistosomiasis reemerged in Sichuan province. We then evaluated the ability of surveillance methods to identify villages where human infections were present. Human infections were detected in 35 of the 53 villages surveyed (infection prevalence: 0 to 43%), including 17 of 28 villages with no prior evidence of reemergence. Bovine infections were detected in 23 villages (infection prevalence: 0 to 65%) and snail infections in one village. Two common surveillance methods, acute schistosomiasis case reports and surveys for S. japonicum-infected snails, grossly underestimated the number of villages where human infections were present (sensitivity 1% and 3%, respectively). Screening bovines for S. japonicum and surveys for the presence of O. hupensis had modest sensitivity (59% and 69% respectively) and specificity (67% and 44%, respectively). Older adults and bovine owners were at elevated risk of infection. Testing only these high-risk human populations yielded sensitivities of 77% and 71%, respectively.

Conclusions: Human and bovine schistosomiasis were widespread in regions where schistosomiasis had reemerged but acute schistosomiasis and S. japonicum-infected snails were rare and, therefore, poor surveillance targets. Until more efficient, sensitive surveillance strategies are developed, direct, targeted parasitological testing of high-risk human populations should be considered to monitor for schistosomiasis reemergence.

Figure 1. The age distribution of 4,279 residents from 53 villages in regions where schistosomiasis reemerged.

Figure 2. The distribution of S. japonicum infections in humans (top), bovines (middle) and snails (bottom).

The 53 villages surveyed are located in regions of Sichuan, China where schistosomiasis has reemerged. Human and bovine S. japonicum infections were measured in November and December 2007. Snail density and S. japonicum infection status were measured in April 2007. Snail density is the number of O. hupensis per m² of irrigation ditch.

Figure 3. The prevalence of S. japonicum infections by age in bovines in regions where schistosomiasis reemerged.

Bovine infections were measured in November and December 2007 in 44 villages in Sichuan, China. None of the 18 water buffalo aged 4 years and older were infected.