Modulation of immune and pathological responses in Nile tilapia by single and co-infections with Lactococcus garvieae and Aeromonas hydrophila

Nile tilapia (Oreochromis niloticus), the linchpin of the aquaculture industry in India, is increasingly threatened by bacterial co-infections, particularly during summer months. Lactococcus garvieae and Aeromonas hydrophila are two significant pathogens affecting tilapia survival and productivity. This study aimed to investigate immune gene expression profiles, serum biochemical responses, and histopathological changes in Nile tilapia exposed to single and co-infections with L. garvieae and A. hydrophila, to understand the pathogen-pathogen and pathogen-host interactions influencing disease severity. Pathogens were isolated from diseased tilapia collected from a culture farm in Kerala, India. Experimental infection (single and co-infection) trials were performed in healthy tilapia with these isolated pathogens. Spleen, kidney, liver, and brain samples were collected on days 1, 3, and 7 post-infections. Immune-related genes (IL-8, CC-chemokine, TLR7, MHCIIα, NF-κB) were quantified, serum biochemical parameters measured, and histopathology examined. Pro-inflammatory cytokines (IL-8, CC-chemokine) and immune-related genes (TLR7, MHCIIα, NF-κB) were differentially expressed across tissues in infected fish. Histopathological examinations of co-infected organs revealed comparatively minimal inflammation in the spleen, fewer inflammatory infiltrates in the kidney, mild congestion in the liver, and mild gliosis in the brain than in those subjected to single infections. Significant differences in serum biochemical parameters were observed between single and co-infection groups. This study is the first to demonstrate an antagonistic interaction between a Gram-positive and a Gram-negative bacterium in cultured Nile tilapia, revealing how co-infection can modulate disease severity. By integrating immune response profiling, serum biochemical analysis, and tissue pathology, our findings provide actionable insights for developing targeted, effective disease management strategies in tilapia aquaculture.

Graphical Abstract