Morphology, phylogenetics and pathology of "red sore disease" (coinfection by Epistylis cf. wuhanensis and Aeromonas hydrophila) on sportfishes from reservoirs in the South-Eastern United States.

The aetiological agents of red sore disease (RSD) reportedly comprise a taxonomically ambiguous stalked ciliate (a species of Epistylis) and Aeromonas hydrophila. The taxonomic identity of each pathogen remains provisional: using supra-specific morphological features for the ciliate and culture-based methods that cannot delineate bacterial strain. On 7 and 9 November 2017 and 28 May 2020, biologists and anglers reported a local epizootic (Hiwassee and Chattahoochee river basins; Georgia) wherein some moribund fish presented RSD-like lesions. The ciliates were assigned to Epistylis by morphology. The ciliate is regarded as Epistylis cf wuhanensis, as nucleotide sequences from its small subunit ribosomal DNA were identical to those of Epistylis wuhanensis. The bacterium was identified as Aeromonas hydrophila by phenotypic markers and nucleotide sequences from the DNA gyrase subunit B; our sequences comprised 3 strains and phylogenetically were recovered sister to strains of Eurasian origin. Histological sections of lesions revealed effacement or partial deterioration of the epithelium covering scales, scale loss, haemorrhaging, necrosis, oedema, and extensive inflammatory infiltrate in the dermis. This is the first nucleotide sequence information for the symbionts implicated in RSD.