Hematological and biochemical alterations in rainbow trout Oncorhynchus mykiss during a natural Aeromonas sp. outbreak in high-altitude Ladakh, trans-Himalayas.

An outbreak of bacterial septicemia associated with Aeromonas sp. was investigated in intensive high-altitude aquaculture of rainbow trout, Oncorhynchus mykiss, at the Drass fish farm, Ladakh, during the seasonal transition of May-June 2025. This observational study documents physiological associations rather than experimental causality. A total of 72 fish were sampled: 36 severely diseased and 36 asymptomatic but exposed specimens from adjacent raceways of the same facility. Phenotypic, biochemical, and 16S rRNA gene sequencing of isolate SFB099 identified the agent as Aeromonas sp. with sequence similarity to A. salmonicida; however, definitive species-level confirmation was limited by the reliance on a single molecular marker. Diseased fish exhibited severe hematological disturbances, including pronounced leukocytosis and shifts in the systemic erythrocyte index consistent with an anaemic trend. Serum analysis showed possible stress-related hyperglycemia, hypoalbuminemia, and elevated creatinine, blood urea nitrogen, alanine aminotransferase, aspartate aminotransferase, lactate dehydrogenase, and alkaline phosphatase. These alterations suggest heightened physiological stress, altered metabolic pathways, and selective ionoregulatory imbalances: hyponatremia, hypochloremia, and elevated potassium. Conclusions about definitive tissue damage are limited by the lack of histopathological validation. Statistical analyses used Spearman's rank correlation, the Shapiro-Wilk test, and the Wilcoxon rank-sum test. Multivariate analysis via principal component analysis PCA explained a moderate 44.7% cumulative variance across PC1 and PC2, reflecting noticeable group-wise clustering alongside high individual heterogeneity. Water parameters measured concurrently showed structural variations characteristic of early summer glacial melt. This study provides localized physiological data on salmonids reared under extreme trans-Himalayan culture constraints for spontaneous field-outbreak investigations.