Infection and histopathological consequences in Siamese fighting fish (Betta splendens) due to exposure to a pathogenic Mycobacterium chelonae via different routes

Mycobacteriosis, a chronic progressive disease in fish caused by Mycobacterium spp., is typically characterized by granulomatous lesions that illustrate its harmful pathogenesis. This study was initiated to understand the progression of mycobacteriosis in Siamese fighting fish (Betta splendens) through various transmission modes. Experimental infections were conducted using a pathogenic isolate of Mycobacterium chelonae originally derived from a naturally infected betta fish. The bacteria were administered by five different routes at three doses (approximately 105, 106, 107 CFU/fish or CFU/mL), including intraperitoneal (IP) and intramuscular (IM) injections, oral administration, immersion, and immersion with skin trauma. Acute infections, typified by short survival, non-specific gross signs, and severe organ necrosis, were frequently observed in the IP and IM methods. The median lethal dose (LD50) for these methods was 4.16 x 105 CFU/fish and 1.66 x 106 CFU/fish, respectively, within seven days following infection. Conversely, oral administration and immersion with or without skin trauma resulted in negligible fish mortality but triggered progressive chronic disease. Chronically infected fish survived the 28-day trial but exhibited macroscopic pathology such as emaciation, aesthetic deterioration, along with an increased presence of melanomacrophage centers (MMCs) in lymphoid tissues and/or systemic granulomas, as noted in histopathology. The results indicate that the course of M. chelonae infection is strongly correlated with the exposure routes, with injuries on the fish body and/or via the digestive tract being the most probable natural pathways. This study expands the understanding of the pathogenesis of Mycobacterium species in betta fish and emphasizes the need to mitigate risk factors associated with exposure routes to effectively control the disease.