Shortened strongyle-type egg reappearance periods in naturally infected horses treated with moxidectin and failure of a larvicidal dose of fenbendazole to reduce fecal egg counts.

The research investigates the effectiveness of two deworming drugs, moxidectin and fenbendazole, in reducing the fecal egg counts in horses. Despite common usage, it was found that fenbendazole failed to significantly decrease the counts, while moxidectin was effective for only a short period, indicating possible resistance of strongyle worms to these drugs.

Investigation of Two Deworming Drugs

• In this research, the focal point was on the application of two deworming drugs witnessed in common practice that are used against encysted small strongyle larvae.
• These two drugs, moxidectin (MOX) and fenbendazole (FBZ), are popular for their effectiveness against these parasites. MOX is given in a single dose of 0.4 mg/kg and FBZ is administered as a larvicidal dose (10mg/kg for 5 days).
• The research main aim was to study how effective these drugs are in reducing the count of strongyle-type eggs in the feces of naturally infected horses.

Results of Drug Application

• In the research, 15 yearlings naturally infected were observed after deworming. Initially, a larvicidal dose of FBZ was given to 6 yearlings, and two weeks later, all 15 received MOX.
• The feces of these horses were collected at weekly interval for pre- and post-treatment egg counts.
• Upon comparison, the counts of the FBZ-treated horses did not show significant difference pre- and post-treatment. This implies that FBZ did not make considerable impact on the reduction of egg count in the feces of the treated horses.
• Similarly, at days 0 and 42 of MOX treatment, the mean fecal egg counts of the yearlings that had not received the FBZ treatment did not significantly differ from those of the FBZ-treated yearlings.

Implications of the Findings

• The study observed that MOX was effective in reducing fecal egg counts to 0 EPG for 21 days, though by day 35, all but 2 of the yearlings had some eggs present. By day 42, the mean reduction had fallen from 100% to 67%.
• These results suggest that strongyle worms may be developing resistance to MOX which would shorten the period for egg reappearance after MOX treatment. This calls for further research and potential alternative medication strategies.
• The lack of effectiveness of FBZ in reducing egg count underpins the fact that it’s use for small strongyle control is not supported. The research suggests it might be due to parasites developing resistance to it or it could be inherently ineffective.