Hepatotoxicity of the drinking water disinfection by-product, dichloroacetic acid, in the medaka small fish model

Recent studies have shown that dichloroacetic acid (DCA), a by-product of chlorination of public water supplies, is carcinogenic to both rats and mice. However, conflicting data have left the mechanism of DCA carcinogenicity, vital to assessment of human health risk, unclear. Elucidation of this mechanism in another animal model at a different phyletic level than rodents would advance the risk assessment process for government agencies concerned with regulation and provision of safe drinking water. The Japanese medaka ( Oryzias latipes), a well characterized small fish model, is being used increasingly for carcinogenicity testing because of its low cost, ease of maintenance and carcinogen sensitivity. In this study, 6-week-old medaka were exposed to diethylnitrosamine (DEN, a known initiator), followed by continuous exposure to 0.5 or 2.0 g/l DCA in the ambient water, over a 4 week period. At both exposure concentrations, changes in the liver included marked hepatocellular cytoplasmic vacuolation, cytomegaly, karyomegaly, nuclear atypia and multifocal areas of hepatocellular necrosis and loss as early as week two of DCA exposure. The majority of the hepatocellular cytoplasmic vacuoles were shown by periodic acid Schiff (PAS) staining to contain large amounts of glycogen. These elevated glycogen levels may reflect a disruption in the enzyme pathways for glycolysis. The total cellular changes seen in this short-term exposure regimen are compatible with preneoplastic changes seen in rats and mice exposed to DCA. The results of this study strengthen the role of the Japanese medaka as a suitable species in carcinogenicity testing as well as its implementation in the risk assessment process for DCA across several phyletic levels.