No chronic oral toxicity of a low dose of microcystin-LR, a cyanobacterialhepatotoxin, in female BALB/c mice

Chronic oral toxicity of a low dose of microcystin-LR (MCLR) was examined i n female BALB/c mice for 18 months. Six-week-old female mice received 20 mu g/L of the toxin in drinking water, which is about 200-fold higher than th e level in contaminated drinking water. Control mice received water alone. Mortality, clinical signs, body weights, and food and water consumptions we re recorded during the study. Examinations on hematology, serum biochemistr y, necropsy, organ weights, and histopathology were performed at months, 3, 6, 12, and 18. The immunohistochemical distribution of MCLR was examined i n the liver at these time points. Mean cumulative MCLR intake after 18 mont hs was estimated at 35.5 mu g per mouse. The present test indicates that ad ministration of a low dose of MCLR in drinking water resulted in neither ch ronic toxicity nor accumulation of the toxin in the liver. Based on previou s epidemiological studies and the present chronic toxicity test, we recomme nd 0.01 mu g/L as an maximum acceptable level for microcystins in drinking water, applying a safely factor of more than 1000. (C) 1999 by John Wiley & Sons, Inc.