LAUSR

Mycobacteriosis has been recognized as an infectious disease caused by Mycobacterium fortuitum in aquaculture. In Indonesia, mycobacteriosis outbreak has been detected in West Java, Central Java, and East Java. However, no studies have yet described the pathogenesis of mycobacteriosis in gourami. Therefore, this study was designed to detect the portal of entry and tissue distribution of M. fortuitum in gourami through immersion challenge. The immersion route was selected for the infection method as it is capable of describing the occurrence of natural disease as an infection model. This study was conducted in two steps. First, the fish was immersed in M. fortuitum concentrations of 104–108 CFU mL−1 to determine the lethal dose of 50 (LD50). Second, the fish was immersed in LD50 to examine the portal of entry and tissue distribution of M. fortuitum in gourami and to determine the nonspecific immune response of fish after infection. Results showed that the LD50 of M. fortuitum through immersion challenge in gourami was 107 CFU mL−1. The portal of entry of M. fortuitum was the skin and gills, after which it spread through blood circulation to internal organs such as the liver and kidney. Finally, it was observed that the bacteria were released through the intestine. These findings indicate the M. fortuitum infection outbreak in fish was chronic and systemic and distributed in tissues. The infected fish responded to the infection by increasing the total leukocyte count and phagocytic activity after challenge with M. fortuitum.