Clinical toxicosis and erythrocyte cholinesterase inhibition of trichlorfon combined with mebendazole in horses.

: Thirty adult horses were used to compare the toxicity and cholinesterase (ChE) inhibition of various dosages of a combination anthelmintic, trichlorfon + mebendazole. Single oral doses of up to 5 times the effective dosage (39.7 mg of trichlorfon and 8.8 mg of mebendazole/kg of body weight) did not result in deaths. Horses given a placebo and horses treated at the recommended dosage rate showed few or no side effects, whereas horses given higher dosages showed dosage-related increases in the severity of clinical signs of organophosphate toxicosis. Dosage-related inhibition of erythrocyte ChE activity was also detected. A 2nd treatment of some of the horses at the recommended dosage 35 days after the initial treatment resulted in minimal or no side effects, despite the fact that erythrocyte ChE activity was still depressed before the repeat treatment. Changes indicating organ toxicosis were not detected in any of the clinical pathologic determinations in any of the dosage groups.