<i>Mycobacterium gordonae</i> (Actinomycetota: Mycobacteriales: Mycobacteriaceae) Infecting Captive‐Reared Laurel Dace, <i>Chrosomus saylori</i> (Cypriniformes: Leuciscidae) in North America

A captive-reared cohort of Laurel Dace, Chrosomus saylori (Skelton 2001) (Cypriniformes: Leuciscidae), a federally-endangered species endemic to several streams in eastern Tennessee's Cumberland Plateau, exhibited signs of morbidity and trickling mortalities in a recirculating aquaculture system. Ziehl-Neelsen (ZN) staining revealed acid-fast rod-shaped bacteria within necrotic and intracellular lesions affecting the eye, swim bladder, spleen, and ovary. Histopathology showed granulomatous inflammation characterised by macrophages, multinucleated giant cells, and eosinophilic exudates. The eye, swim bladder, and ovary were particularly impacted: the choroid plexus of the eye displayed haemorrhages and dense cellular infiltrates; the swim bladder exhibited granulomatous inflammation with extensive fibrosis; the ovary contained caseous granulomas; and the choroid plexus of the eye displayed haemorrhages and dense cellular infiltrates. PCR amplification of the hsp65 gene (316 bp) from formalin-fixed and paraffin-embedded lesioned tissues was identical (100% similar; 424 bp fragment) to that of the type strain of Mycobacterium gordonae Bojalil et al. 1962 (AF547840). This is the first report of an infection by M. gordonae in Laurel Dace; the first record of this bacterium causing a lesion in the swim bladder, eye, or ovary; and the first record of M. gordonae from a freshwater fish in North America.