One-year dietary toxicity study with methidathion in beagle dogs

The purpose of this study was to determine the chronic toxicity of methidathion, an organophosphate insecticide, in dogs. Groups of beagle dogs, four/sex/dose, were fed methidathion at constant dietary concentrations of 0, 0.5, 2, 4, 40, or 140 ppm for 1 year. The equivalent daily dosages were approximately 0, 0.02, 0.07, 0.15, 1.4, and 4.7 mg/kg. There were no deaths or adverse clinical signs associated with the treatment. Weekly body weights and weight gains were not affected. Mean daily food consumption was reduced in male dogs given the 140-ppm diet. Major treatment-related effects were cholestasis, chronic inflammation in the liver, and cholinesterase (ChE) inhibition. The liver effects were indicated by gross and microscopic pathologic findings as well as moderate increases in serum bile acids and enzyme activities (alanine aminotransferase, aspartate aminotransferase, sorbitol dehydrogenase, and alkaline phosphatase) in all dogs receiving greater than or equal to 40 ppm. RBC ChE was inhibited in males at greater than or equal to 40 ppm and in females and 140 ppm. Brain ChE was inhibited in both sexes at 140 ppm; the magnitude of inhibition relative to control was slightly greater with the cerebellar fraction than with the cerebral fraction. Serum ChE was not affected at any dose level. In conclusion, liver was the target organ in beagle dogs given greater than or equal to 40 ppm (equivalent to 1.4 mg/kg/day) methidathion in diet for 1 year. The no-observable-effect level was 4 ppm (0.15 mg/kg/day) for both liver cholestasis and ChE inhibition.