The challenge of effective surveillance in moving from low transmission to elimination of schistosomiasis in China

Abstract

Until recently, intensified efforts in China to suppress the transmission of Schistosoma japonicum relied principally on routine praziquantel treatment, extensive use of molluscicides and health education programs. These efforts, now supplemented by a broader range of control measures, have been quite successful in reducing the prevalence and intensity of human infection to very low levels. However, re-emergent transmission has occurred in formerly endemic areas of several provinces, signalling the need for more locally effective, integrated control strategies. We argue that these low but persistent levels of transmission also require important changes in both the tactics and strategy of disease surveillance to move forward towards elimination. Here we present recent data exemplifying the low transmission environment which suggests that we are reaching limits of detection of current diagnostic techniques used for human infection surveillance in these communities. However, both epidemiological data and theoretical results indicate that (i) transmission in the human population can persist at very low infection intensities even in the presence of routine control activities; (ii) the parasite can be reintroduced into parasite-free environments by very modest external inputs; and (iii) transmission at these low infection intensities exhibits very slow inter-year dynamics. These observations motivate the need for new, sensitive tools to identify low-level infections in mammalian or snail hosts, or the presence of S. japonicum in environmental media. Environmental monitoring offers an alternative, and perhaps more efficient, approach to large-scale surveillance of human infections in low transmission regions.

Fig. 1

Equilibrium states of a deterministic mathematical model of schistosomiasis transmission showing the interaction of the basic reproductive number (R₀) of the system and its non-linear functions describing mating in the definitive host, , the worm establishment limitation in vivo, f, and effects of acquired immunity in humans, H.

Fig. 2

Mean worm burden in humans in a fictitious village expressed as eggs per gram of feces (epg) over a 20 year period (expressed in days) as a result of one infected bovine in residence for 1 month following time (t) = 0.