Genetic diversity and structure of Oncomelania hupensis hupensis in two eco-epidemiological settings as revealed by the mitochondrial COX1 gene sequences

BACKGROUND

Oncomelania hupensis hupensis is the only intermediate host of Schistosoma japonicum, the causative agent of schistosomiasis in China and is therefore of significant medical and veterinary health importance. Although tremendous progress has been achieved, there remains an understudied area of approximately 2.06 billion m² of potential snail habitats. This area could be further increased by annual flooding. Therefore, an understanding of population genetics of snails in these areas may be useful for future monitoring and control activities.

METHODS AND RESULTS

We sampled snails from Hexian (HX), Zongyang (ZY) and Shitai (ST) in Anhui (schistosomiasis transmission control), and from Hengtang (HT), Taicang (TC), Dongsan (DS) and Xisan (XS) in Jiangsu (schistosomiasis transmission interrupted), downstream of Anhui. ST, DS and XS are classified as hilly and mountainous areas, and HX, ZY, TC and HT as lake and marshland areas. The mitochondrial cytochrome c oxidase subunit I gene were sequenced. Out of 115 snails analyzed, 29 haplotypes were identified. We observed 56 (8.72%) polymorphic sites consisting of 51 transitions, four transversions and one multiple mutational change. The overall haplotype and nucleotide diversity were 0.899 and 0.01569, respectively. Snail populations in Anhui had higher genetic diversity than in Jiangsu. 73.32% of total variation was distributed among sites and 26.68% within sites. Snails were significantly separated according to eco-epidemiological settings in both network and phylogenetic analyses.

CONCLUSION

Our results could provide important guidance towards assessing coevolutionary interactions of snails with S. japonicum, as well as for future molluscan host monitoring and control activities.

The Three Gorges Dam: Does it accelerate or delay the progress towards eliminating transmission of schistosomiasis in China?

The Three Gorges Dam, located in the largest endemic area of schistosomiasis in China, is one of the world’s largest hydroelectric projects to date. Some large-scale hydro projects have resulted in schistosomiasis emergence or re-emergence. Therefore, the dam’s potential impact on the transmission of Schistosoma japonicum has raised concerns from medical researchers worldwide. A systematic literature review, coupled with an analysis of data on the water level and snail density in the Yangtze River was conducted to assess the impact of the dam on schistosomiasis transmission after more than 10 years of operation. The dam has significantly altered the water levels in the Yangtze River according to different seasons. These changes directly impact the ecology of the schistosome snail host. Due to the dam, there has been a reduction in the density of Oncomelania snails and/or changes in the distribution of snails. The prevalence of infection with S. japonicum has decreased in the downstream areas of the dam, including in the Dongting and Poyang Lakes. The prevalence of infection with S. japonicum in humans has decreased from 6.80 % in 2002 (before the dam began operating) to 0.50 % in 2012, and the number of people infected with S. japonicum have decreased from 94 208 in 2002 to 59 200 in 2011 in the Poyang Lake region. The presence of the dam does not seem to affect snail breeding or the prevalence of schistosomiasis in the Three Gorges Reservoir. Overall, the Three Gorges Dam has significantly contributed to changes in hydrology after more than 10 years of the dam operating. The changes caused by the dam, together with integrated control of schistosomiasis, might be accelerating the progress towards eliminating the transmission of S. japonicum in the middle and lower reaches of the Yangtze River. Despite the positive effect the dam is having in controlling S. japonicum transmission, continued surveillance is required to monitor the future ecological impacts of the dam over the long term.