Aquatic Animal Health Program, Department of Large Animal Clinical Sciences, College of Veterinary Medicine, University of Florida, Gainesville, Florida, USA.
Aquatic Animal Diseases Laboratory, National Institute of Oceanography and Fisheries, Cairo, Egypt.
J Aquat Anim Health. 2023 Dec;35(4):201-210. doi: 10.1002/aah.10197. Epub 2023 Nov 15.
The Largemouth Bass Micropterus salmoides is an important freshwater fish that is native to the southeastern United States and is cultured for conservation, food, and for the sports fishing industry. Francisella orientalis is a globally distributed bacterial pathogen of warmwater fish species and is associated with granulomatous inflammation and high mortalities. Outbreaks of piscine francisellosis in the United States have been reported in only a few fish species. This study describes three case presentations of francisellosis in Largemouth Bass from a public display system in north-central Florida. Additionally, laboratory-controlled immersion challenges using an F. orientalis isolate from tilapia Oreochromis spp. evaluate susceptibility of Largemouth Bass fingerlings to F. orientalis infection and mortality through this exposure route.
Necropsy, histologic examination, immunohistochemistry, bacterial recovery and culture, and quantitative polymerase chain reaction were used as diagnostic tools to evaluate both the affected display fish and the immersion-challenged fingerlings.
Although the display fish and immersion-challenged fingerlings presented with nonspecific clinical signs, gross and histological changes were indicative of granulomatous disease. Immunohistochemical and molecular testing methods confirmed F. orientalis infection in affected fish.
The three case presentations described here mark the first reporting of naturally occurring piscine francisellosis in Largemouth Bass that were held in a public display exhibit. Additionally, causality was proven in the Largemouth Bass fingerlings through the immersion challenges. These findings demonstrate susceptibility through immersion-based exposure and assert that francisellosis should be considered among the list of differential diagnoses for Largemouth Bass with granulomatous disease.
大口黑鲈 Micropterus salmoides 是一种重要的淡水鱼类，原产于美国东南部，被用于保护、食品和运动钓鱼产业。东方弗朗西斯菌是一种分布广泛的温水鱼类病原菌，与肉芽肿性炎症和高死亡率有关。在美国，只有少数几种鱼类报告了鱼类弗朗西斯菌病的爆发。本研究描述了佛罗里达州中北部公共展示系统中大口黑鲈三种弗朗西斯菌病的病例介绍。此外，使用来自罗非鱼 Oreochromis spp. 的东方弗朗西斯菌分离株进行实验室控制的浸泡挑战，评估了大口黑鲈鱼苗通过这种暴露途径感染东方弗朗西斯菌的易感性和死亡率。
剖检、组织学检查、免疫组织化学、细菌回收和培养以及定量聚合酶链反应被用作诊断工具，用于评估受影响的展示鱼和浸泡挑战的鱼苗。
尽管展示鱼和浸泡挑战的鱼苗表现出非特异性临床症状，但大体和组织学变化表明存在肉芽肿性疾病。免疫组织化学和分子检测方法证实了受感染鱼类存在东方弗朗西斯菌感染。
这里描述的三个病例报告标志着首次报道在公共展示中饲养的大口黑鲈自然发生的鱼类弗朗西斯菌病。此外，通过浸泡挑战在大口黑鲈鱼苗中证明了因果关系。这些发现表明通过浸泡暴露具有易感性，并断言弗朗西斯菌病应该被列入具有肉芽肿性疾病的大口黑鲈的鉴别诊断清单中。
