Pathologic alterations in adult rainbow trout, Oncorhynchus mykiss, exposed to dietary 2,3,7,8-tetrachlorodibenzo-p-dioxin

Adult female rainbow trout (Oncorhynchus mykiss) fed [³H]2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) impregnated diet at 0, 1.8, 18 or 90 ng/kg food for up to 320 days were evaluated by clinical pathology, gross pathology and histopathology procedures. Gross pathological changes were limited to a slight increase in the incidence of lesions of the caudal fins in the treated groups. Mixed mononuclear inflammatory infiltrates were present in multiple organs of fish from control and treatment groups, but did not appear to be treatment related. Lesions associated with exposure to TCDD were observed histologically in liver and spleen after 100 and 250+ days of exposure. The livers of TCDD-treated fish contained less hepatocellular glycogen, more mitotic figures, greater anisokaryosis, anisocytosis, nuclear chromatin clumping and margination. Prominent nucleoli were directly proportional to TCDD dose. Hepatocellular changes in fish exposed to TCDD also included single cell necrosis and clear cytoplasmic vacuoles consistent with lipid. Some fish from all TCD exposed groups had lower lymphoid density compared to controls at all time intervals. Fish exposed to the highest TCDD dose had decreased peripheral leukocyte counts after 50 and 100 days. The lowest observable adverse effect level (LOAEL) for these effects was 5.69 ngTCDD/kg in diet and 0.90 ng TCDD/kg liver.