First report of molecular characterization, pathological analysis, and antibacterial susceptibility of Yersinia ruckeri RTACP-14 a isolated from farmed rainbow trout, Oncorhynchus mykiss (Walbaum 1792) in the Indian Himalayan Region.

内容摘要

The present study represents the first detailed report of Yersinia ruckeri infection in farmed rainbow trout (Oncorhynchus mykiss) from the Indian Himalayan Region, where intensification of aquaculture has elevated the pathogen load and transmission dynamics, causing occurrence of enteric redmouth disease (ERM). The study examined infections by Y. ruckeri, RTACP-14 A (PX411334), in farmed rainbow trout from the Indian Himalayas. Affected fish displayed clinical signs such as hemorrhagic mouth, deep ulceration at caudal peduncle region, fin degeneration, lesions, black pigmentation, and erratic swimming behaviour. The isolated Y. ruckeri RTACP-14 A strain was identified through biochemical tests and confirmed by 16 S rRNA sequencing showing the closest evolutionary relationship (100) to Y. ruckeri strain BY-1Y. Virulence analysis revealed β-hemolysis, low hydrophobicity, and substantial biofilm formation under glucose-rich conditions. The cumulative mortality increased from 12.5% at day 1 to 83% by day 32 during a controlled experimental infection trial conducted in rainbow trout. Antibiotic susceptibility test showed that Y. ruckeri RTACP-14 A was sensitive to most antibiotics, with imipenem, fosfomycin, and ciprofloxacin exhibiting the largest inhibition zones (> 40 mm), while bacitracin was the only antibiotic to which resistance was observed. The lowest minimum inhibitory concentrations (MICs) were recorded for oxytetracycline (0.125 µg mL- 1), florfenicol (1 µg mL- 1), and tobramycin (1 µg mL- 1). Histopathological analysis revealed gill hyperplasia, intestinal villus necrosis, kidney damage, liver congestion, and muscle degradation, with distinct patterns distinguishing acute from chronic infections. To mitigate outbreaks of enteric red mouth disease caused by Y. ruckeri, trout farmers should adopt best management practices, including maintaining optimal water quality, minimizing handling stress, implementing strict biosecurity measures, and using antimicrobials judiciously within a regulated framework. These measures are essential for safeguarding fish health and welfare, minimizing economic losses, and ensuring the sustainability of trout farming in the region.