Nontuberculous Mycobacteriosis in Farmed Thread-sail Filefish Stephanolepis cirrhifer

In 2009 and 2010, unusually high mortality events were recorded among cultured populations of thread-sail filefish Stephanolepis cirrhifer in Ehime Prefecture, Japan. Diseased fish exhibited abdominal distention and many white nodules filled with thick, pale-yellow material scattered on the surface of the serosae of internal organs and mesentery. Histopathologically, the disease was characterized by variable sized granulomatous lesions with central necrotic core surrounded by thin irregular arrangement of epithelioid cells and the outermost thin rim of connective tissue. The central part of granuloma showed colliquative necrosis with abundant cellular debris and some clusters of long-rods that were positive with Ziehl-Neelsen stain. From the results of microbiological examinations and simplified identification with DNA-DNA hybridization, two representative isolates collected in 2009 and 2010 were classified into rapidly growing “nontuberculous mycobacteria (NTM)” that closely related to Mycobacterium chelonae. A pathogenicity test using one of the isolates successfully reproduced the granulomatous lesions closely resembled to those in the spontaneous cases, suggesting that the rapidly growing NTM is pathogenic to the thread-sail filefish, and is a causative agent of the spontaneous case. This is the first report of the disease caused by a rapidly growing NTM in maricultured fish in Japan.