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

This research article describes testing done on horses to compare the efficacy and safety of an oral paste that contains the antiparasitic drugs mebendazole and trichlorfon, to a marketed powder formulation of the same drugs. The study found the paste to be highly effective against various equine parasites and to have generally mild side effects.

Methods

• The research was carried out on 24 horses that were naturally parasitized.
• Both the new oral paste and existing powder formulations contained 8.8 mg of mebendazole and 40 mg of trichlorfon per kilogram of body weight, and were administered at these recommended dosages.
• Separate tests were also performed to evaluate the toxic effects of the paste at 2.2 times the recommended therapeutic dose, compared with similarly dosed nonmedicated paste to serve as controls.

Findings

• The oral paste formulation was found to be highly effective against 2nd- and 3rd-stage Gasterophilus species, adult Strongylus vulgaris, S edentatus, Parascaris equorum, small strongyles, and both larval and adult forms of Oxyuris equi. The effectiveness ranged from 97.7% to 100%.
• Adverse effects in horses treated with the paste formulation were generally minor, including mild fecal softening and transient restlessness or sweating in two out of 12 horses.
• Critical evaluations of horses administered the paste at 2.2 times the therapeutic dose did not show any drug-related changes in clinical chemical analyses, hematologic values, or liver function tests.
• However, signs of organophosphate toxicosis (associated with overdose of trichlorfon), such as loose feces and extended inhibition of erythrocyte cholinesterase activity, were observed within an hour of drug treatment.

Conclusion

• The results of this study demonstrated that the paste preparation of mebendazole and trichlorfon is effective as an antiparasitic treatment for horses and that the side effects are relatively mild and comparable to those of the marketed powder formulation, including when administered at higher than the recommended dose.