Critical test and safety evaluations of an oral paste preparation of mebendazole and trichlorfon in horses.

Critical tests were done on 24 naturally parasitized horses to compare the antiparasitic activity of an oral paste preparation of mebendazole and trichlorfon with that of the marketed powder formulation. Each formulation was administered at the recommended dosages of 8.8 mg of mebendazole and 40 mg of trichlorfon/kg of body weight. Efficacy of the paste formulation ranged from 97.7% to 100% against 2nd- and 3rd-stage Gasterophilus spp, adult Strongylus vulgaris, S edentatus, Parascaris equorum, small strongyles; and larval and adult forms of Oxyuris equi. Adverse effects were generally limited to slight softening of the feces. Mild and transient restlessness or sweating were also observed in 2 of 12 horses treated with the paste formulation. The toxic effects of the paste, administered at 2.2 times the therapeutic dose, were examined in 6 horses and compared with the effects of a nonmedicated paste, administered in similar volumes to 6 other horses. Drug-related changes were not detected in clinical chemical analyses, hematologic values, or liver function tests. Transient clinical signs of organophosphate toxicosis (primarily the passage of loose feces) and prolonged inhibition of erythrocyte cholinesterase activity were evident within 1 hour after drug treatment. These effects were similar to those reported for the 2.2 X dose of marketed powder formulation.