The reappearance of strongyle eggs in the faeces of horses after treatment with moxidectin.

The reappearance of strongyle eggs in the faeces of horses treated with moxidectin (0.4 mg/kg of body weight) was compared with that in the faeces of horses treated with ivermectin (0.2 mg/kg of body weight). The study was performed from December 1995 till June 1996. Horses were infected naturally in the preceding grazing period. Two groups of 24 horses each were treated with moxidectin and ivermectin respectively at week 0. No side effects were seen after treatment. Horses were housed from week -1 till week 17. From week 17 onwards the horses were on pasture. Faecal samples were taken from each horse at week -1, week 0 and weekly from week 3 to week 25. The ivermectin treated horses had to be retreated in week 17 to prevent pasture contamination. For this reason this group was withdrawn from the trial in week 17. Individual faecal egg counts and group faecal cultures and larval differentiation were performed. In the ivermectin treated group strongly eggs were seen for the first time after treatment in week 8. A steady rise in the mean number of eggs per gram faeces (EPG) was seen from week 8 till week 15. After week 15 a plateau was reached. In the moxidectin treated group mean egg counts remained very low throughout the study. A plateau was reached in week 19, with egg counts varying from 10 to 30 EPG between week 19 and week 25. The difference between the egg output after moxidectin- and ivermectin treatments can be explained by a higher efficacy of moxidectin against mucosal stages or by a longer residual effect of moxidectin than ivermectin.