Histopathology of Japanese medaka (Oryzias latipes) chronically exposed toa complex environmental mixture

Japanese medaka (Oryzias latipes) were used to evaluate the carcinogenicity of a complex groundwater that contained 5 U.S. Environmental Protection Ag ency priority pollutant heavy metals and 13 chlorinated aliphatic hydrocarb ons. A test protocol that used 10 mg/L diethylnitrosamine (DEM prior to gro undwater exposure was designed to assess both initiation and promotion. The fish were exposed continuously for 9 mo with 0, 1, 5, or 25% groundwater, by volume, with either West Branch of Canal Creek water (Aberdeen Proving G round-Edgewood Area, Aberdeen Proving Ground, MD) or dechlorinated tap wate r as the diluent, while concurrent controls were run in the laboratory. Inc idental findings included various neoplasms in the nares, ovary, skeletal m uscle, skin, swim bladder testis, thymus, and thyroid. Factors evaluated du ring statistical analyses of Fish neoplasm prevalence included diluent type , groundwater percentage, fish gender and DEN initiation. Liver neoplasm pr evalence was higher in DEN-initiated fish and was frequently higher in male s. Concentrations of up to 25% groundwater, by volume, showed no evidence o f being a complete carcinogen and showed no consistent, conclusive evidence of being a promoter.