Alterations in the epidermis and mucus viscosity of the carp, Cirrhinus mrigala, experimentally infected with Edwardsiella tarda

This study investigated effects of a bacterial pathogen, Edwardsiella tarda on the epidermis and mucus viscosity of an Indian major carp, Cirrhinus mrigala. The fish were divided into three groups: a control group (no treatment), a vehicle control group (fish injected with 50 μl of phosphate-buffered saline (PBS) at day 0), and an infected group (fish injected with 50 μl of PBS containing a sublethal dose of 2.2 × 10⁶ CFU/fish, which is 10 % of the 96-h LD₅₀, at day 0). Alterations in the surface architecture, histology, proliferating cell nuclear antigen (PCNA) and inducible nitric oxide synthase (iNOS) expression, specific activity of lactate dehydrogenase (LDH) and succinate dehydrogenase (SDH) were studied at 2d, 4d, 6d and 8d post-infection. Microscopic examination showed hypertrophy of the epidermal epithelial cells, accompanied by disrupted and disorganized microridges, as well as exfoliation. Mucous goblet cells (MGCs) density increased significantly at an early stage of infection. Club cells exhibited degenerative changes, including vacuolization, confluence with neighbouring cells at intervals and simultaneous discharge of their contents onto the surface. A significant increase in iNOS-positive cells was detected. PCNA expression was significantly lower in infected fish, indicating reduced cell proliferation. Cutaneous mucus showed non-Newtonian behavior, with higher viscosity at low shear rates which decreased significantly in infected fish, indicating thinning and shedding under stress. E. tarda infection also caused a significant increase in LDH activity and a decrease in SDH activity. This study will provide deep insight into the host defence mechanisms and serve as a knowledge base for the establishment of early warning systems to control disease outbreaks in farmed fish.