Novel strategy for control of schistosomiasis : Decoy snails that can reduce the number of miracidia reaching the intermediate snails

诱螺可作为发展中国家血吸虫病防治的新工具。本研究试图在肯尼亚夸莱血吸虫病血液病流行区主要非易感螺种中寻找诱饵螺候选种。我们研究了1)蜗牛排泄和分泌产生的微生物趋化性（蜗牛调节水：SCW）， 2)蜗牛体内的微生物渗透，3)实验室对非易感蜗牛的诱捕效果评估，以及4)诱捕候选蜗牛在传播地点的现场应用。钉螺为易感钉螺，非易感钉螺为埃及艳后、独色螺、兰氏螺卵、黑脉螺。钉螺取自夸莱市学龄儿童卵。结果：1)SCW具有神奇的趋化作用。结果表明，水蛭SCW引起的奇迹反应最高。miracidia对l.a ovum产生的SCW的反应与B.globosus产生的反应相似。Miracidia对c.f erinea的SCW有轻微的趋化性。另外两个物种没有引起这种反应。2)蜗牛的神奇渗透。PCR证实在球螺中有miracidida渗透，但在其他蜗牛中未发现miracidida。3)实验室候选钉螺诱饵效果评价。我们试图发现当易感螺和非易感螺占据同一空间时，插入的非易感螺是否能保护易感螺免受感染。在另一项实验中，不敏感的蜗牛被放置在通道的中心。然后在一端放上球芽甘蓝，在另一端放上神奇菌。在所有的实验中，我们都没有发现被干预的非易感蜗牛保护了球螺。4)在传播点现场应用诱饵螺候选物。在3个研究点进行了1年的钉螺调查后，通过引进大量从其他孳生点采集到的铁铁螺旋体，将非易感钉螺的种群密度提高到原来的3倍。然后对蜗牛进行了一年多的跟踪调查。结果表明，各试验点的球形芽孢杆菌感染率均略有下降，但差异无统计学意义。讨论：本研究表明，在我们的研究区域，一些蜗牛的繁殖引起了神奇的反应。然而，目前的研究表明，在自然环境中感染的频率可能不会受到非易感蜗牛的存在和密度的影响。少

Decoy snails could be used as a new tool for the control of schistosomiasis in the developing countries. The present study was attempted to find the decoy snail candidate in main species of non-susceptible snails at the endemic area of schistosomiasis hematobia, in Kwale, Kenya. We studied 1) miracidial chemotaxis generated by excretion and secretion from snails (snail-conditioned water : SCW), 2) miracidial penetration in snails, 3) assessment of decoy effect in non-susceptible snails in laboratory, and 4) field application of the decoy snail candidate at the transmission sites. The snails used were susceptible snail Bulinus globosus and non-susceptible snails Cleopatra ferruginea, Bellamya unicolor, Lanistes ovum, Melanoides sp. The miracidia used were from the eggs obtained from the school children in Kwale.Results : 1) Miracidial chemotaxis generated by SCW. SCW from B.globosus elicited the highest miracidial response. The response of miracidia to SCW from L.ovum mimicked those gen … More erated by B.globosus. Miracidia showed slight positive chemotaxis to SCW from C.ferruginea. Other two species did not elicit the response. 2) Miracidial penetration in snails. PCR confirmed miracidial penetration in B. globosus, but did not show the presence of miracidia in other snails. 3) Assessment of decoy effect in candidate snails in laboratory. We attempted to find whether interposed non-susceptible snails could protect the susceptible snails from infection when susceptible and non-susceptible snails both occupied the same space. In other experiment, non-susceptible snails were placed at the center of the channels. Then B.globosus was placed at one end and miracidia were introduced at the other end. In any experiments, we failed to find that interposed non-susceptible snails protected B.globosus. 4) Field application of decoy snail candidate at the transmission sites. After one year snail survey at 3 study sites, the population of non-susceptible snail, C.ferruginea, was raised to the density 3 times bigger than the original by introduction of huge number of C. ferruginea collected from other breeding sites. Then the follow-up snail survey was conducted for one year more. The results showed a slight reduction in the infection rate of B.globosus at all sites, but there was no statistical significance.Discussion : The present study showed that some snails breeding in our study area elicited miracidial response. However, the present study suggests that frequency of infection in a natural environment might not be influenced by the presence and density of non-susceptible snails. Less