Effects of formalin treatment on epithelial structure and mucous cell densities in rainbow trout, Oncorhynchus mykiss (Walbaum), skin.

Formalin bath treatments of rainbow trout are widely used for control of ectoparasitic infections in fish farms and the effects of this chemical on parasite infections have been studied. In addition, tolerance of the host to various concentrations of formalin are also known. However, effects of formaldehyde on the skin epithelium integrity and mucous cell density still need elucidation. The present study revealed that exposure of rainbow trout fry 2 months post-hatching to various concentrations of formalin affected the mucous cell dynamics of the fish. Limited exposure (50 ppm, 1 h) stimulated mucous cell proliferation as indicated by the slight increase in densities of Alcian blue positive cells in tail fin epidermis. In contrast, high concentrations (200-300 ppm, 1 h) or lengthy exposure (24 h) at lower concentrations caused a decrease in mucous cell density. Scanning electron microscopy revealed effects on the epithelial structure of fish fins. Blebbing of epithelial cell membranes was the first sign of injury. The dilated openings of mucous cells indicated increased release of mucus. Highly irregular organization of the cells followed, with regional differences occurring in different parts of fins. The implications of formalin bathing for susceptibility or resistance of fish to subsequent pathogen exposure are discussed.