Comparative studies of early season moxidectin treatment and conventional ivermectin/ benzimidazole treatments in the control of cyathostomes in horses

Abstract

Moxidectin administered in January or February at a single dose was tested for efficacy in horses on two farms for 12 and 11 months, respectively. Horses were infected with cyathostomes naturally in the previous grazing period. Forty horses of farm 1 and 20 horses of farm 2 were used in controlled tests to evaluate the efficacy of moxidectin 2% gel formulation at the dosage 0.4 mg moxidectin per kg of live weight, ivermectin commercial paste formulation at the dosage 0.2 mg ivermectin per kg of live weight, mebendazole and fenbendazole commercial paste formulation at the dosage both 7.5 mg mebendazole and fenbendazole per kg of live weight, all applied orally. Three control groups of 10 horses each (farm 1) were treated twice a year with ivermectin and benzimidazoles, respectively. Individual faecal egg counts, faecal cultures and larval differentiation were performed. Moxidectin had more prolonged and greater suppressive effects on the post-treatment reappearance and magnitude of strongyle egg counts than did ivermectin or benzimidazoles. In the moxidectin treated group (M1) strongyle eggs were seen for the first time in April and a slight increase in the mean count of eggs per gram of faeces (EPG) was observed during the rest of the season. Litter larval counts significantly reflected levels of exposure during the tested season. Twenty animals of farm 2 were allocated into two groups of ten horses each based on pre-treatment eggs per gram (EPG) counts (moxidectin treated group and control group). In the moxidectin treated group mean egg counts remained very low throughout the study. A plateau was reached by autumn, with egg counts ranging from 74 to 145 EPG. The faecal egg counts of moxidectin treated group (M2) were significantly higher in March, April, May and June.