In August 2018, a series of large fish kills involving only Silver Carp Hypophthalmichthys molitrix occurred on the Mississippi River in northern Louisiana. Clinical signs observed in moribund animals included erratic swimming behavior, such as spiraling and spinning at the surface. A moribund specimen was captured by dip net near the surface at Lake Providence Landing in East Carroll Parish, northern Louisiana, and was submitted for analysis. An aseptic necropsy was performed, and diagnostic procedures, including bacteriology, parasitology, histopathology, virology, and electron microscopy, revealed that a gram-positive coccus was the primary pathogen. Pure cultures of the organism were obtained from the brain, and it was the predominant colony type isolated from the spleen, kidney, and liver. Bacterial sepsis caused by the gram-positive coccus and involving multiple organ systems was diagnosed histologically. Bacterial colonization and necrotic lesions were seen in the spleen, liver, kidney, heart, eye, and brain. Numerous cocci were observed dividing intracellularly in phagocytic cells of the kidney and brain by transmission electron microscopy. The organism was identified as Streptococcus dysgalactiae ssp. dysgalactiae by conventional biochemical methods and subsequently by the API 20 Strep system. The identity of the pathogen was later confirmed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and sequencing of the 16S ribosomal RNA gene. Multilocus sequence analysis clustered this isolate along with two other S. dysgalactiae isolates from fish in a divergent phyletic group that was separate from other S. dysgalactiae ssp. dysgalactiae isolates from terrestrial animals, implying a possible novel clade that is pathogenic for fish.

中文翻译：

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停乳链球菌：杀鱼事件中鲢鱼野生种群的病原体

2018 年 8 月，仅涉及鲢鱼Hypophthalmichthys molitrix的一系列大型鱼类死亡发生在路易斯安那州北部的密西西比河。在垂死的动物身上观察到的临床症状包括不稳定的游泳行为，例如在水面上盘旋和旋转。在路易斯安那州北部东卡罗尔教区的普罗维登斯湖登陆处，一个垂死的标本在地表附近被浸网捕获，并被提交分析。进行了无菌尸检，包括细菌学、寄生虫学、组织病理学、病毒学和电子显微镜在内的诊断程序显示革兰氏阳性球菌是主要病原体。该有机体的纯培养物是从脑中获得的，它是从脾脏、肾脏和肝脏中分离出来的主要菌落类型。由革兰氏阳性球菌引起并累及多个器官系统的细菌性败血症经组织学诊断。在脾脏、肝脏、肾脏、心脏、眼睛和大脑中可见细菌定植和坏死病变。通过透射电子显微镜观察到许多球菌在肾和脑的吞噬细胞中分裂。该生物被鉴定为停乳链球菌ssp。通过传统的生化方法和随后的 API 20 Strep 系统检测泌乳障碍。病原体的身份后来通过基质辅助激光解吸/电离飞行时间质谱法和 16S 核糖体 RNA 基因测序得到证实。多位点序列分析将该分离株与其他两个S聚类在一起。disgalactiae从鱼类中分离出来，属于不同的种系群，与其他S . 无乳症 从陆生动物中分离出的dysgalactiae，暗示了一个可能对鱼类致病的新进化枝。