Cutaneous appearance of mycobacteriosis caused by Mycobacterium marinum, affecting gilthead seabream (Sparus aurata) cultured in recirculating aquaculture systems

Mycobacteriosis is a chronic and often lethal disease in fish, affecting a wide range of cultured and wild species worldwide. Several species of the genus Mycobacterium have been isolated from fish, with M. marinum being the most significant due to its wide host spectrum, economic impact in aquaculture, and zoonotic potential. The damage caused by mycobacteriosis to intensive fish farming and the ornamental trade has been extensively reported, and there is currently no effective treatment other than depopulation and facility disinfection. In this study, we describe a case of cutaneous granulomatous mycobacteriosis in gilthead seabream (Sparus aurata) reared in two onshore recirculating aquaculture systems (RAS) in Israel. External clinical signs of disease (cutaneous hyperplastic nodules) in the two RAS were similar in their appearance but their prevalence differed: in RAS-A, the prevalence was relatively low (1%), whereas in RAS-B, prevalence varied with time (up to 33%). Histopathological findings in both RAS showed the presence of granulomatous lesions limited to the hypodermis in all specimens, and less frequently in the coelomic parenchymas. Most of the cutaneous nodules consisted of multiple discrete to coalescent mature granulomas and pre-granulomatous lesions. In all analyzed specimens, acid-fast bacilli, consistent to the genus Mycobacterium, were positive by Ziehl-Neelsen staining. Bacteria were isolated on selective media (solid and liquid), and identified as M. marinum by molecular and mass spectrometry (MALDI-TOF) analyses. The spectra of M. marinum isolates in this study were compared to M. marinum isolates from clinical cases in humans. Mycobacterium marinum isolated from both RAS showed similarity to those in four human cases, diagnosed by the National Mycobacterium Reference Centre of Israel, which underline the zoo-notic potential of these isolates.