Mycobacterial infection in Siamese fighting fish (Betta Splendens) : pathogen characteristics, pathogenicity, pathogenesis, and control strategies

Abstract

This study conducted a comprehensive investigation into mycobacteriosis in Siamese fighting fish (Betta splendens), a disease that poses major challenges to the sustainability of the sector. In the first phase, five different Mycobacterium species (i.e., M. chelonae, M. cosmeticum, M. farcinogenes, M. mucogenicum, and M. senegalense) were meticulously characterized. Bacterial speciation was determined through phenotypic and biochemical characteristics. Concurrently, antibiotic resistance of the bacteria with MAR indices ranging from 0.22 to 0.61, indicated potential challenges in treatment strategies. Furthermore, disinfectant susceptibility revealed that ethanol, formalin, chlorine, and povidone-iodine were effective in killing the bacteria, while potassium permanganate was less effective. Intraperitoneal injection confirmed that all five isolates were pathogenic to betta fish, with M. chelonae exhibiting the highest virulence. In the second phase, the infection progression was explored by exposing B. splendens to the pathogenic M. chelonae isolate using various exposure methods, including injection (IP and IM), oral administration, and immersion with or without skin trauma. Results showed that pathogenicity was largely dependent on the transmission routes, with IP and IM methods causing particularly severe infections. It is also likely that infection occurs naturally primarily through an injured body surface and/or the digestive tract. All routes led to chronic conditions, suggesting that controlling mycobacteriosis requires a rigorous reduction of risk factors associated with these routes. In the third phase, an innovative approach using ozone nanobubbles (NB-O3) to disinfect water was introduced to mitigate the risk of mycobacteriosis in betta fish. After a 60 min incubation in NB-O3 treated water, M. chelonae concentration was significantly reduced by 99.92%. Direct treatment of farm water with NB-O3 for 10 min resulted in over 90% reduction in total bacterial count. Application of NB-O3 to water contaminated with M. chelonae effectively reduced the risk of infection and improved the fish's survivability. Thus, NB-O3 represents a promising strategy for disease control, offering a new avenue for enhancing betta fish health. Taken together, these multifaceted studies provide essential insights into the pathogen characteristics, pathogenicity, pathogenesis, and preventive measures against the disease. This work offers a valuable perspective on mycobacteriosis and contributes significantly to the sustainable management of the betta fish industry.