Benzimidazole resistance in horses in western Canada.

This research study investigates a second reported case of benzimidazole resistance in recently weaned horses in western Canada. The insensitivity to anthelmintic treatment was examined using oxfendazole-trichlorfon, a benzimidazole compound, and was evidenced by its failure in curbing strongylid fecal egg output in these horses.

Research Methodology

The study encompassed 14 healthy, recently weaned male horses aged between three to nine months. These horses grazed a common pasture and were fed alfalfa cubes, alfalfa-grass hay, whole oats and nutritional supplementary items. The horses’ body weight was estimated using a girth tape, and fecal tests were performed before and after treatments.

Treatment and Results

• Horses were initially treated with oxfendazole-trichlorfon and fecal egg samples were tested every two weeks.
• The treatment would be repeated if the mean total fecal egg count exceeded 200 eggs per gram (epg).
• After the initial treatment, the total fecal egg output dropped from 571 epg to 125 epg.
• All horses continued to shed strongyle eggs and testing indicated a potential lack of effectiveness of the treatment.
• A separate oxfendazole treatment without trichlorfon was then administered and reduced the fecal egg count further.
• Finally, the horses were treated with ivermectin, which proved more than 99% effective in reducing fecal output of strongyle and ascarid eggs.

Analysis and Insights

• Anthelmintic resistance was indicated by less than 70% reduction in fecal egg output post-treatment.
• The study concluded that an oxfendazole-resistant strain of small strongyles were present despite having not used oxfendazole on that farm before.
• The researchers speculated that benzimidazole cross-resistance might be the cause of the observed resistance as benzimidazole had been used in other treatments on the farm.
• The origin of the resistant nematodes could not be traced as the horses came from various sources, and some could have hosted a benzimidazole-resistant strain before their arrival.
• Veterinarians are advised to be aware of potential resistance development and should routinely perform fecal examinations after treatment to ensure optimal parasite control.