Evaluation of parasiticidal activity of fenbendazole, ivermectin, oxibendazole, and pyrantel pamoate in horse foals with emphasis on ascarids (Parascaris equorum) in field studies on five farms in Central Kentucky in 2007

Horse foals on five farms in Central Kentucky were used in field studies in 2007 evaluating activity of paste formulations of four compounds (fenbendazole--FBZ, ivermectin--IVM, oxibendazole--OBZ, and pyrantel pamoate--PRT) against internal parasites with emphasis on ascarids (Parascaris equorum). It has been well established the last few years that there is widespread resistance of P. equorum to ivermectin. The main purpose of the present research was to obtain current data on ascaridicidal activity of FBZ, OBZ, and PRT; also, to acquire further information on ascarid resistance to ivermectin. Additionally, data were documented on drug activity on small strongyles. Detection of ascarid and strongyle eggs in feces of foals was by a qualitative method (presence or absence) or a quantitative method (eggs per gram of feces). Strongyle eggs all were assumed to be from small strongyles. This is based on fecal cultures from horses on one farm and historic records from horses in this area on excellent deworming programs. A girth tape was used to estimate the body weight of each foal so that the appropriate dose rate of each drug could be given. Many of the foals were used in more than one cycle of treatments. Efficacy of the drugs, administered intraorally, was determined by calculating the average percentage reduction (% red.) of the number of foals passing eggs after vs. before treatment: (1) FBZ at 10 mg/kg was tested on four farms; 76 foals were examined, 50 with ascarid eggs (84% red.) and 62 with strongyle eggs (0% red.); (2) IVM at 200 μg/kg was tested on three farms; 58 foals were examined, 18 with ascarid eggs (0% red.) and 48 with strongyle eggs (100% red.); (3) OBZ at 10 mg/kg was tested on three farms; 181 foals were examined, 78 with ascarid eggs (94% red.) and 79 with strongyle eggs (0% red.); (4) PRT was tested on two farms, one farm at 1x (6.6 mg base/kg); 42 were foals examined, 16 with ascarid eggs (0% red.) and 33 with strongyle eggs (12% red.) and one farm at 2x (13.2 mg base/kg); 18 foals were examined, 13 with ascarid eggs (23% red.) and 15 with strongyle eggs (27% red.).