Efficacy of moxidectin and other anthelmintics against small strongyles in horses.

This study investigates the effectiveness of various anthelmintic treatments (moxidectin, ivermectin, oxibendazole, and morantel) in controlling small strongyle worm infestations in horses. The results demonstrated that moxidectin produced the longest-lasting reduction in worm egg count in horse feces, indicating superior efficacy in dealing with worms that were resistant to other treatments.

Methods and Procedures

• The researchers selected a farm with horses known to have a small strongyle population resistant to oxibendazole, one type of anthelmintic.
• Horses were sorted into treatment groups based on their initial pre-treatment fecal egg counts.
• Horses were then treated with the prescribed anthelmintic and monitored over a course of 109 days. Fecal samples were collected from the horses throughout this period.
• Using these samples, the scientists conducted fecal egg counts and cultures to evaluate the presence and quantity of small and large strongyles. This data helped track the efficacy of each anthelmintic over time.

Findings

• Moxidectin, given at a dose of 0.4 mg/kg, suppressed fecal egg counts for 109 days after treatment in most horses. This result was notably more efficacious than ivermectin (effective for 40 days), morantel (effective for 13 days), and oxibendazole (effective for less than 13 days).
• Fecal cultures revealed that most of the fecal egg count was attributable to small strongyles. The parasite Strongylus vulgaris was also found in some samples but only in low quantities.
• Resistance of small strongyles to oxibendazole was confirmed in this study, and there was also an observed less-than-expected efficacy of morantel.

Conclusions

• The research concluded that moxidectin was highly effective in reducing fecal egg counts for a period of 12 to 16 weeks in the majority of horses.
• Evidence suggested that these horses were infested with small strongyles known to be resistant to both oxibendazole and perhaps morantel, common anthelmintics used to treat worm infestations in horses.
• Compared to ivermectin, moxidectin’s duration of fecal egg count reduction was double. Furthermore, it was more than four times that of morantel and oxibendazole, thereby demonstrating the superior efficacy of moxidectin against small strongyles.