池塘养殖异育银鲫“喉孢子虫病”爆发的遗传与生态基础研究

Allogynogenetic gibel carp, Carassius auratus gibelio (Bloch) is an important freshwater cultured species in China. Increasing occurrence of myxosporidiosis, however, have recently caused severe economic loss, potential ecological problems and risk of food security and dramatically prevent from sustainable development of allogynogenetic gibel carp industry, especially in pond aquaculture system. For lack of effective licenced drugs and control strategies, the development of alternative control measures (Ecosystem-based disease management) by elucidating the transmission process and outbreak mechanisms of the myxosporidiosis are urgent. Our previous investigation showed that span-region trade of gibel carp frys and fingerlings is one of important transmission routes of myxosporidiosis. However, epidemiological details of gibel carp myxosporidiois in epizootic region still remains unknown. What biological and environmental factors influence the transmission of the pathogen and occurrence and severity of the myxosporidiosis? How dose the infective sporoplasm of the myxosporeans mitigrate and develop within allogynogenetic gibel carp ? To answer the above questions and fully elucidate the genetic and ecological mechanisms driving the outbreaks of fish myxosporidiosis, the present project will concentrate on the etiological agent of pharyngeal myxosporidiosis of gibel carp (Myxobolus honghuensis) which causes the most serious threat to the industry among all myxosporidiosis and initiate the following research contents: Firstly, we will unvover possible genotype diversity of M. honghuensis to explain if the virulence difference of different genetic variants is responsible for the difference prevalence and mortality rate of gibel carp cultured in different regions or ponds. Secondly, considering the significant correlation of the abundance of infective actinospores in water column with myxosporidiosis, we will investigate the distribution dynamics of M. honghuensis actinospores in water column and potential biological factors (zooplankton, polycultured fish, etc.) affecting their distribution and abundance by combining the QPCR and LM identification during a full cultrec cycle. Thirdly, myxopsores in substrate sludge of ponds are the infective stages to the inveterbrate host. Their distribution dyanmics and abundance are directly linked with the prevalence and intensity of infected inveterbrate and then the releasing amount of actinospores in water column. So, fully understanding their distribution and potential biological factors (benthic inveterbrate, chintinase-secreting bacterial, etc.) will benefit to get deep insight into the transmission process of M. honghuensis in pond ecosystem. In summary, we expect to find possible interfering strategies (drugs, vaccines, biolgocial monitoring or managements, etc.) and exact interfering timing to prevent from the outbreak of pharyngeal myxosporidiosis after the completion of this project.

异育银鲫在我国淡水渔业中占有重要地位，但近年来养殖过程中频发的粘孢子虫病严重阻碍了异育银鲫养殖业的可持续健康发展。我们前期研究发现鱼苗（种）的跨地域引种是异育银鲫粘孢子虫病广泛传播的重要途径，但对流行区的粘孢子虫病的发生、发展规律依然缺乏充分的了解。本项目以当前危害最严重的"喉孢子虫病"为研究对象，着力于其病原(洪湖碘泡虫)可能的基因型多样性、生活史中两宿主感染源（水柱中放射孢子与底泥中粘孢子）的时空分布动态及影响其分布、丰度的生态因子，全面掌握洪湖碘泡虫在池塘生态系统中的传播规律及其在宿主鱼体内的移行发育过程，阐明异育银鲫"喉孢子虫病"爆发的遗传（病原角度）与生态基础，为寻找可切断或阻止洪湖碘泡虫在池塘生态系统中传播、发育成熟的干预手段及准确干预时机，建立异育银鲫"喉孢子虫病"的可持续防控体系奠定基础，也为其它池塘养殖系统寄生性病害的防控提供参考。

鲫是我国当前重要大宗淡水养殖品种之一，年产量近300万吨，然而近年来鲫养殖业发展受到病害的严重威胁，其中最重要的病害之一就是鲫粘孢子虫病。粘孢子虫是一类后生动物寄生虫，种类众多，某些种类寄生引起的粘孢子虫病给世界渔业生产造成了严重影响。不同种粘孢子虫可危害鲫养殖的不同阶段，其中当下危害最严重的是引起鲫喉孢子虫病的洪湖碘泡虫。本项目基于当前对鲫喉孢子虫病发病机制不明及防控对策的缺失，开展了不同地理株洪湖碘泡虫遗传多样性的研究以阐明不同地域病害程度差异的可能遗传基础，系统调查了影响鲫主养区喉孢子虫病爆发的可能生态因素，并藉此建立鲫喉孢子虫病防控综合技术体系。结果表明，我国不同地域及病害程度不一的洪湖碘泡虫未出现显著遗传分化，造成鲫喉孢子虫病在不到二十年内广泛传播的根本原因是无严格检疫的苗种（成鱼）的自由跨境运输。基于建立的洪湖碘泡虫全生活史阶段的定量监测方法，对病害程度不一的池塘的养殖模式（混养品种与比例）、浮游动物与底栖动物群落机构、浮游与底栖微生物群落结构及用药习惯与养殖系统中病原丰度的相关性进行了分析，发现单一精养、不当杀虫剂的使用及不当病死鱼的处置极大促进了苏北鲫主养区池塘中的病原增殖，高丰度的病原也在近年来导致了寸片鱼种就开始大规格发病。为此，我们建立通过养殖模式调整（轮养、休耕及混养草鱼、鲤、鳊、对虾及河蟹等非易感品种）、药物使用（外用药灭活感染源与内服药阻断成熟孢子生成）及SPF苗种培育的鲫喉孢子虫病综合防控技术体系，已在部分养殖户（企业）推广应用，取得了较好的经济与社会效应。同时，本项目建立的基于精准消降感染丰度的病害干预对策对其它池塘养殖品种寄生性病害防控有很好的借鉴意义。遗憾的是通过四年的大规格筛查，未能鉴定洪湖碘泡虫的无脊柱动物宿主及其对应的放射孢子虫，今后需继续加大采样力度，力争建立鲫喉孢子虫病的室内人工感染平台，为药物筛选及干预措施效果的准确评价奠定基础。