National surveillance of Oncomelania hupensis in China, 2015-2019

Abstract

To analyze the monitoring data of Oncomelania hupensis in the national schistosomiasis surveillance sites of China from 2015 to 2019, so as to understand the changes of Oncomelania snail status in the schistosomiasis-endemic areas of China and to provide the scientific evidence for Oncomelania snail control. According to the requirements of National Scheme for Schistosomiasis Surveillance in China (2014 Edition), national schistosomiasis surveillance sites were assigned in all schistosomiasis-endemic counties (cities, districts) and the potential endemic counties (cities, districts) in the Three Gorges Reservoir areas, and Oncomelania snail status was monitored according to different epidemic types. In endemic areas, Oncomelania snail survey was performed by means of systematic sampling and environmental sampling, and the occurrence of frames with Oncomelania snails and the prevalence of Schistosoma japonicum infections in Oncomelania snails were calculated, while in potential endemic areas, the risk of imported Oncomelania snails and Oncomelania snails in floating debris were monitored. Oncomelania snail survey was performed covering an area of 116 834.16 hm2 in the national schistosomiasis surveillance of China from 2015 to 2019, with 35 007.62 hm2 Oncomelania snail habitats identified. A total of 6 908 292 frames were surveyed during the 5-year period, and there were 364 555 frames detected with Oncomelania snails, with a 5.28% mean occurrence of frames with Oncomelania snails. Among 997 508 living Oncomelania snails captured, no S. japonicum infections were detected, and loop-mediated isothermal amplification (LAMP) assay detected 18 positive mixed Oncomelania snail samples. During the period from 2015 to 2019, 147.20 hm2 emerging Oncomelania snail habitats were identified, with an overall tendency towards a rise seen in the proportion of emerging Oncomelania snail habitats in plain regions with waterway networks (0.12% to 92.00%), a tendency towards a rise followed by decline seen in marshland and lake regions (0 to 96.72%), and a large fluctuation in hilly regions (0 to 88.49%). A total of 831.10 hm2 re-emerging Oncomelania snail habitats were found in the national schistosomiasis surveillance sites of China from 2015 to 2019, with an overall tendency towards a rise seen in the proportion of re-emerging Oncomelania snail habitats in marshland and lake regions (16.05% to 79.66%), an overall tendency towards a decline seen in hilly regions (19.25% to 81.00%), and a minor fluctuation in plain regions with waterway networks (1.10% to 10.14%). During the 5-year period from 2015 to 2019, a total of 48 656 kg floating debris were captured in 4 surveillance sites in the Three Gorges Reservoir areas, and 2 204 snails were found, with no Oncomelania snails identified. The areas of Oncomelania snail habitats tended to be stable in the national schistosomiasis surveillance sites of China during the period from 2015 to 2019, however, there was a gradual rise in the area of Oncomelania snail habitats year by year, and LAMP assay identified positive Oncomelania snail samples, suggesting Oncomelania snail control is far from optimistic in China.