Alteration in hematological and serum biochemical parameters of rainbow trout, Oncorhynchus mykiss, during citrobacteriosis outbreak

The aquaculture sector is crucial for global food security, yet it faces considerable problems from infectious diseases. Oncorhynchus mykiss, being one of the most vital cultured freshwater fish species in India, faces a significant challenge from an emerging bacterial disease called citrobacteriosis. The disease poses a significant threat to rainbow trout aquaculture since it can cause major financial losses through mortalities, slowed development rates, and enhanced sensitivity to other diseases. It affects the economics and sustainability of rainbow trout farming as well as the approaches to disease management. The present study investigates the hematological and serum biochemical alterations in rainbow trout, Oncorhynchus mykiss, during a natural outbreak of citrobacteriosis caused by Citrobacter gillenii and Citrobacter freundii . The results indicate significant (P < 0.05) reductions in red blood cells (RBC), hemoglobin (Hb), and hematocrit (Hct) content, along with notable changes in metabolic parameters, such as elevated glucose and blood urea nitrogen (BUN) levels during the outbreak of disease. These changes suggest metabolic shifts, hepatic damage, and renal dysfunction. Biochemical and molecular characterization (16S rRNA gene sequencing) was carried out for the proper identification. The pathogens identified through PCR were submitted to the GenBank under the accession numbers PQ686306 for C. gillenii LKRT11 and PQ686305 for C. freundii LKRT01. Principal Component Analysis (PCA) distinguished diseased fish from healthy fish, thus validating the importance of these biomarkers for fish health diagnostics. Blood profiling acts as a non-invasive, low-cost diagnostic initial marker used for the early identification of physiological changes occurring in fish, while PCR offers pathogen-specific detection. Therefore, combining the use of PCR for proper identification of the pathogen in addition to the hematological and serum biochemical analysis would be useful for early detection of the physiological and metabolic shift in the fish body during the outbreak of disease. • First report of Citrobacter gillenii and C. freundii as pathogen causing citrobacteriosis in O. mykiss from the cold Himalayan conditions. • Principal Component Analysis clearly distinguishes diseased and healthy fish, supporting blood profiling as early disease detection tool in aquaculture. • PCR analysis confirmed C. gillenii and C. freundii as the causative pathogens.