High Mortality in Zebrafish (Danio rerio).

A group of 100 adult zebrafish were housed in a new system at a stocking density of 20 fish per tank. Four weeks after arrival, 15 fish presented with petechial hemorrhages and ulceration on the surfaces of the skin. Samples of the fish were collected for histopathology, fungal culture, and bacterial culture and sensitivity. Water samples were analyzed for pH, ammonia, nitrite, and submitted for bacterial and fungal culture. Histologically, the epidermis had multiple areas of ulceration and mononuclear cell infiltrate. Gram-positive bacteria were observed beneath the surface of the skin and surrounding the outer aspect of the spinal cord. Both Aeromonas hydrophila and A. sobria were isolated from the affected fish, and a diagnosis of motile aeromonad septicemia (MAS) was made. Water from the tanks had a nitrite level of 1-5 ppm, a toxic concentration that indicated poor water quality. Because the housing system had been seeded with Nitrobacter spp. and Nitrosomonas spp. only 2 weeks prior to the arrival of the fish, a lack of colonizing nitrifying bacteria was deemed to be the cause of the high nitrite level, which, along with over-crowding, stressed the fish and increased their susceptibility to MAS. No further cases of septicemia were observed once the nitrite level and stocking density were reduced.