First confirmed case of systemic granulomatosis caused by Nocardia seriolae in farmed snubnose pompano Trachinotus blochii in the Philippines

Ulcerative granulomatous disease in farmed snubnose pompano Trachinotus blochii (Lacepede, 1801) in the Philippines was documented in 2019 by the Bureau of Fisheries and Aquatic Resources and has since become an annual problem for fish farmers, with outbreaks occurring between September and October. Three apparently healthy adults and 5 diseased adults displaying ulcerated skin were subjected to diagnosis. During postmortem examination, multiple nodules were present in all kidneys and the spleen in the 5 diseased fish. Histopathology revealed severe granulomatous formations at different stages. Bacterial pathogens were isolated from diseased fish using modified Lowenstein-Jensen medium, producing wrinkled, cream-colored colonies. Staining revealed Gram-positive, acid-fast, branching, beaded, long rod filamentous bacteria. Identification at the species level and taxonomic positioning of the isolate was achieved through a specific Nocardia seriolae PCR and by sequencing the 16S rRNA gene using pure cultures and tissue sample DNA. PCR amplification of 13 virulence genes resulted in the detection of mce, mce1A, gapA, fbpA, nirD, nirB, narI, narJ, narH, and katG, which are important in N. seriolae invasion, infection, and intracellular survival in macrophages. Two tetracycline resistance genes, namely tetB and tetC, were also found, which might contribute to oxytetracycline resistance, rendering the farm's prophylactic strategy of using oxytetracycline ineffective. To our knowledge, this is the first confirmed report of nocardiosis in farmed snubnose pompano in the Philippines.