First evidence of nocardiosis in farmed red drum (Sciaenops ocellatus, Linnaeus) caused by Nocardia seriolae in the Gulf of Mexico

Between August and December 2013, the offshore cages of a commercial marine farm culturing red drum Sciaenops ocellatus in Campeche Bay were affected by an outbreak of an ulcerative granulomatous disease, reaching 70% cumulative mortality. Thirty-one adults displaying open ulcers on the skin were submitted to our laboratory for diagnosis. Multiple white-yellowish nodules (0.1-0.5 cm in diameter) were present in all internal organs, where the kidney and the spleen were the most severely affected. Histopathological observations of these organs evinced typical granulomatous formations. Gram and Ziehl-Neelsen stains on tissue imprints, bacterial swabs and tissue sections revealed Gram-positive, acid-fast, branching beaded long rod filamentous bacteria. Tissue samples resulted positive for nocardiosis when a Nocardia genus specific nested PCR was used. Definite identification at the species level and taxonomic positioning of the fastidious pathogen was achieved through a specific Nocardia seriolae PCR and by sequencing the gyrB gene of pure isolates. After application of antibiotics during fry production, a posterior follow-up monitoring (from 2014 to 2017) detected mild but recurrent outbreaks of the bacteria with no seasonality pattern. To the extent of our current knowledge, this is the first report of piscine nocardiosis (in a new host -red drum) in Mexico.