Effects of Ivermectin and Moxidectin on Fecal Egg Count and Egg Reappearance Rate in Horses

Parasite resistance to some commonly used anthelmintics is increasing and egg reappearance period (ERP) appears to be decreasing. The objective of this project was to evaluate the efficacy of ivermectin (IVE) and moxidectin (MOX) on fecal egg counts (FECs) and ERP in horses. Fecal samples (n = 46) were collected and evaluated for parasite eggs using the Modified McMaster Fecal Egg Count technique. Eggs per gram (EPG) of feces were recorded. Horses were randomly allocated based on prestudy FEC (low, <200 EPG; moderate, 200-500 EPG; high, >500 EPG), age (young, ≤15 years; old, ≥16 years), and housing (stall or pasture). Treatments included control (CON, no treatment, n = 10), IVE (n = 10), or MOX (n = 10). Fecal samples were collected and evaluated every 2 weeks for 12 weeks after treatment. Statistical analysis was performed using PROC MIXED of SAS. Fixed effects included treatment, age, and location with week as a repeated measure. Fecal egg count reduction tests were 100% for IVE and MOX, indicating that both anthelmintics were effective. However, parasite eggs began to appear in IVE horses in week 6 and MOX horses in week 8. Currently reported ERP for IVE and MOX are 6 to 8 weeks and 10 to 12 weeks, respectively, suggesting decreased efficacy of MOX. In pastured horses, MOX was more effective in reducing FEC than IVE (1.84 vs. 6.43 EPG, respectively; P = .01). Data suggest that anthelmintic use improved internal parasite control and that MOX may have greater efficacy than IVE; however, the shorter ERP for MOX may indicate that MOX efficacy may be decreasing.