Kawato Yasuhiko, Takada Yuzo, Mizuno Kaori, Harakawa Shogo, Yoshihara Yusaku, Nakagawa Yukihiro, Kurobe Tomofumi, Kawakami Hidemasa, Ito Takafumi
Pathology Division, Nansei Field Station, Fisheries Technology Institute, Japan Fisheries Research and Education Agency , Mie, Japan.
Ehime Fisheries Research Center , Ehime, Japan.
Microbiol Spectr. 2023 Sep 22;11(5):e0156723. doi: 10.1128/spectrum.01567-23.
Aquatic animal viruses are considered to be transmitted via environmental water between fish farms. This study aimed to understand the actual transmission risk of red sea bream iridovirus (RSIV) through environmental water among fish farms. An environmental DNA (eDNA) method using iron-based flocculation coupled with large-pore filtration was used to monitor RSIV DNA copies in seawater from fish farms and from an experimental infection model. RSIV dispersion in seawater from a net pen where the disease outbreak occurred was visualized by the inverse distance weighting method using multiple-sampling data sets from a fish farm. The analysis demonstrated that the center of the net pen had a high viral load, and RSIV seemed to be quickly diluted by the tidal current. To evaluate the transmission risk of RSIV in environmental water, the red sea bream (approximately 10 g) was exposed to RSIV-contained seawater (10, 10, 10, 10, and 10 copies/L) for 3 days, which mimicked field exposure. A probit analysis of the challenge test indicated that the inferred infection rates of seawater containing 10 copies/L and 10 copies/L of RSIV were 50% and 0.0001%, respectively. In the surveillance for 3 years at 10 fixed points ( = 306), there were only seven samples in which the viral load exceeded 10 copies/L in seawater. These results suggest that the transmission of RSIV among fish farms via seawater is highly associated with the distance between the net pens, and the environmental water is not always an infection source for the transmission of RSIV between fish farms. IMPORTANCE Our surveillance of viral loads for red sea bream iridovirus (RSIV) by monitoring environmental DNA in fish farms suggested that the viral loads in the seawater were low, except for the net pens where RSIV outbreaks occurred. Furthermore, our experimental infection model indicated that the infection risk of RSIV-contained seawater with less than 10 copies/L was extremely low. The limited risk of environmental water for transmission of RSIV gives an insight that RSIV could be partly transmitted between fish farms due to the movement of equipment and/or humans from the fish farm where the disease outbreaks. Since our data suggest that seawater can function as a potential wall to reduce the transmission of RSIV, biosecurity management, such as disinfection of equipment associated with fish farms could be effective, even in the semi-open system aquaculture that the environmental water can be freely transferred, to reduce the risk of RSIV outbreaks.
水生动物病毒被认为可通过环境水在养鱼场之间传播。本研究旨在了解海水鱼虹彩病毒（RSIV）通过环境水在养鱼场之间的实际传播风险。采用基于铁的絮凝结合大孔过滤的环境DNA（eDNA）方法，监测养鱼场和实验感染模型海水中的RSIV DNA拷贝数。利用养鱼场的多个采样数据集，通过反距离加权法直观展示了疾病爆发网箱海水中RSIV的扩散情况。分析表明，网箱中心病毒载量高，RSIV似乎很快被潮流稀释。为评估RSIV在环境水中的传播风险，将约10克的真鲷暴露于含RSIV的海水（10、10、10、10和10拷贝/升）中3天，模拟野外暴露情况。攻毒试验的概率分析表明，含10拷贝/升和10拷贝/升RSIV的海水推断感染率分别为50%和0.0001%。在10个固定点进行的3年监测（=306次）中，海水中病毒载量超过10拷贝/升的样本仅有7个。这些结果表明，RSIV在养鱼场之间通过海水传播与网箱之间的距离高度相关，环境水并非总是RSIV在养鱼场之间传播的感染源。重要性我们通过监测养鱼场中的环境DNA对海水鱼虹彩病毒（RSIV）病毒载量进行的监测表明，除了发生RSIV爆发的网箱外，海水中的病毒载量较低。此外，我们的实验感染模型表明，RSIV含量低于10拷贝/升的海水感染风险极低。RSIV通过环境水传播的风险有限表明，RSIV可能部分是由于设备和/或人员从疾病爆发的养鱼场移动而在养鱼场之间传播。由于我们的数据表明海水可作为降低RSIV传播的潜在屏障，因此即使在环境水可自由流动的半开放式养殖系统中，对与养鱼场相关设备进行消毒等生物安全管理措施也可能有效，以降低RSIV爆发的风险。
