Pathology and tumor development through time in guppies dosed with diethylnitrosamine (DEN)

Within the last decade unusually high frequencies of neoplasms have been reported in feral fish populations from a variety of locations. At many of these locations organic carcinogens have been noted as a potential cause. We sought to identify toxic effects including neoplasia in fish exposed to an organic carcinogen, and to quantify these effects through time. We exposed guppies ( Poecilia reticulata ) to multiple doses of DEN, an organic carcinogen. Fish were then subsampled and examined for liver histopathology at 2‐month intervals over 12 months. Necrotic zones, macrophage centers, bile duct proliferations, enlarged lipid deposits, neoplastic foci, cholangiocarcinomas and hepatoblastomas were quantified by frequency of occurrence and the percentage of liver area involved. DEN toxicity resulted in necrotic zones that peaked in frequency at the first sample period (2 months). Lipid deposits increased, then plateaued in guppies, indicating a more chronic toxic effect. Similarly, macrophage centers increased through the sampling period. Bile duct proliferation appeared to be of two types: a reversible toxic response which peaked at 4 months and then declined and a less frequent irreversible proliferation which continued to develop into cholangiocarcinoma. Neoplastic foci of mixed hepatocytes and cholangiocytes increased in livers of exposed guppies from the second month, developing into hepatoblastomas, which occurred in almost 100% of exposed guppies by the twelfth month. The irreversible bile duct proliferations and the neoplastic foci had cellular densities different from corresponding control tissue and similar to cellular densities of cholangiocarcinomas and hepatoblastomas, respectively.