Time required by different anthelmintics to reach expected efficacy levels in horses infected by strongyles

Abstract

The aim of this study was to determine the time required by different anthelmintic agents to reduce strongyle egg shedding in horses. Fifty horses were divided into five homogenous groups based on faecal egg counts (FECs). Treatment groups received either ivermectin; moxidectin; fenbendazole; piperazine; or no treatment (control group). Faecal examinations were performed 4, 8, 12, 18, 24, 36 and 48 h after the anthelmintic treatment. After this period, faecal samples were taken every 24 h over the next 12 days and finally on alternate days (48-h intervals) for another 14 days until the end of the experiment (28 days post-treatment). The faecal egg count reduction (FECR) was calculated based on the post-treatment mean FECs in the controls and treated animals. Eggs were absent from the faecal examinations beginning at 72 h and 4 days, respectively, following treatment with moxidectin or ivermectin. Piperazine showed an FECR greater than 95% from 48 h up to 9 days post-treatment, with the highest FECR value recorded at 7 days post-treatment (98.1%). However, the FECR was lower than 90% in the last two samplings (26 and 28 days post-treatment). The febendazole group presented the lowest efficacy with FECR below 90% in all samplings. The faecal cultures showed that at the beginning of the trial, all of the groups presented with mixed infections and were predominantly composed of cyathostomins (92.8%), followed by Strongylus vulgaris (5.6%) and Triodontophorus serratus (1.6%). Only cyathostomin larvae were identified following treatment with fenbendazole or piperazine. In conclusion, horses in the present study had a segment of the cyathostomin population with resistance to fenbendazole and piperazine. The strongyle population was susceptible to macrocyclic lactones, with cessation in egg shedding three and four days after treatment with moxidectin and ivermectin, respectively.