Diagnosis of piscine francisellosis in Largemouth Bass from a public display exhibit in north‐central Florida, USA

Abstract

ObjectiveThe 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.MethodsNecropsy, 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.ResultAlthough 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.ConclusionThe 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.

Pathogenic bacteria prevalence in cultured Nile tilapia in Southwest Mexico: A real‐time PCR analysis

“…In this multi-year study (2018, 2019, 2020 and 2022), samples of cultured tilapia were analysed to detect fish pathogens using real-time PCR assays. Newly emerging or re-emergent bacterial fish diseases, such as francisellosis in the freshwater fish Micropterus salmoides (Sheehy et al, 2023) and the cichlid Herichthys cyanoguttatus (Chang et al, 2021), streptococcosis in tilapia (Cao et al, 2022;Zhang, 2021) and aeromoniasis in tilapia (Azzam-Sayuti et al, 2021) and spleens were the most frequent clinical signs, which were also observed in apparently healthy tilapias cultured in dams from Sinaloa state, located in north-western Mexico (Soto-Rodriguez et al, 2013).…”