Evaluation of the Efficacy of Oral Paste Formulations of Ivermectin against Gastrointestinal Parasites in Horses

Abstract

The anthelmintic efficacy of oral paste formulations of ivermectin against gastrointestinal parasites in horses was evaluated, using fifteen active racehorses in which nematodes had been detected in their gastrointestinal tracts by fecal examination. A single dose of ivermectin was given orally to twelve horses of the treatment groups at a dosage rate of 200 μg/kg in order to obtain and identify gastrointestinal parasites in their feces for 3 days after treatment. Small strongyles, strongyles, and Parascaris equorum were found passed in feces up to post-treatment day (PTD) 2, but they were no longer observed on PTD 3. Horse bot fly Gasterophilus spp. larvae were found on PTD 2 and 3. The percentages of small strongyles found on PTD 1 were 75-100% (88% on average), and the rest were found (12% on average) on PTD 2. The nematodes were 16 species from 7 genera, of which 11 species were identified as small strongyles. In the fecal examination conducted on PTD 14, all horses of the treatment groups showed negative for nematode eggs. No abnormalities were observed in general clinical signs of the treated horses for 14 days after treatment. Three non-treated horses of the control groups still showed positive for nematode eggs on PTD 14. These results confirm that when ivermectin was given orally to horses at a dosage rate of 200 μg/kg, small strongyles, strongyles, and P. equorum, were excreted in feces up to PTD 2, and Gasterophilus spp. larvae on and after PTD 2, demonstrating the potent anthelmintic efficacy of ivermectin against these parasites.