Although the definition of schistosomiasis elimination was announced by WHO in recent years and the debate of how to prove elimination is still going on, Guangdong (1985), Shanghai (1985), Fujian (1987), Guangxi (1988) and Zhejiang (1995) provinces were announced that schistosomiasis was eliminated successively according to the criteria issued by Chinese government at that time. Then the five provinces transferred to post-elimination surveillance with main tasks to find and eliminate local residual infectious sources and remained snail habitats, and prevent the import of infectious sources and snails from other endemic areas [15–17]. Before our study, except one foci with two new cases reemerged in a farm of Guangdong province in 1992 but rapidly was under controlled [17, 18], no new infections occurred in other provinces [19].
In our study, no local infection in reservoir hosts and intermediate host was detected in the five provinces during 2005–2016, proving the successful consolidation of schistosomiasis elimination. The comprehensive assessment results showed that the staffs have mastered the basic knowledge of schistosomiasis prevention and presented good capacity for schistosomiasis detection. All of these are elementary components of a sensitive and rapid surveillance platform. However, we noticed that there were some samples misdiagnosed and wrong answers for questionnaire answered by staff at county level. Capacity building should be strengthened focusing on schistosomiasis control and diagnosis through continued training and practices to consolidate local achievements on schistosomiasis control.
However, risks of the re-emergence and resurgence of schistosomiasis still existed in the five provinces through our study. Over the past 12 years, a total of 221 imported cases were found in the five provinces, mainly in Zhejiang province, neighbored with Jiangxi and Anhui provinces where the transmission of schistosomiasis is still going on. Studies showed that most of the imported cases in Zhejiang Province are farmers, migrant workers and merchants from schistosomiasis endemic areas, primarily from Anhui, Jiangxi, Hubei [20–22]. In recent years, owing to urbanization and economic development, the number of the floating population from domestic epidemic areas has an increasing trend. At the same time, with the escalation of international trade and entry-exit personnels, the risk of importing cases from abroad infected with S. haematotium or S. mansoni is also increasing [23–25]. In view of this situation, on the one hand, supervision and treatment should be strengthened for imported definite cases to eliminate the transmission potential; on the other hand, health education should be promoted for the floating population from schistosomiasis endemic areas to spread the knowledge of schistosomiasis prevention and control [26].
The surveillance data proved that the area of snail habitats kept a low level during 2005–2016, compared with 95 900.70 hm2 reported by Wu [17]. The remained snail habitats mainly distributed in the places that the ecological environments are complicated or water level is unstable, where molluscaciding approach doesn’t work well. In addition, several articles published also presented the evidence that the rebound or spread of snails habitats were occurred in Shanghai, Fujian, Guangxi and Zhejiang Provinces [17, 27–29]. It is worth noting that the connection of water systems or the transplanting of seedlings and aquatic plants from the snail habitat areas may also lead to the possibility of snail importation and spread [30, 31]. The development of new snail habitats and snails reappeared in former snail habitats in four provinces except Guangdong Province, providing evidence that eliminating oncomelanid snails completely was quiet difficult. Although Guangdong Province kept the achievement with no oncomelanid snails detected, a new challenge for Guangdong Province is the invasion and spread of Biomphalaria Strami [32].
Considering the potential risks of schistosomiasis still existed in five provinces, snail control through environmental modification and surveillance focused on eliminating remaining snails and preventing imported infection sources should be continued and strengthened, to prevent the re-emergence of schistosomiasis, and consolidate the achievements of schistosomiasis elimination. Risk assessment should be conducted timely if there were large water conservancy projects or importing plants or animals from endemic areas etc [33, 34]. Specifically, the monitoring of the environments where snails infested previously or connected with snail habitats should be strengthened through multi ways [10, 35]. Meanwhile, the floating people and livestock from the areas where the transmission of schistosomiasis has not been interrupted or the epidemic situation of schistosomiasis is recovering should be inspected emphatically, and the patients should be treated in time if they are found [36–38].