Traditional deworming methods dictate that all horses on a farm be dewormed with moxidectin or ivermectin at specific intervals, usually every other month or quarterly. Though significantly easier on farm managers, this method increases anthelmintic resistance two to three times more rapidly than other deworming programs, reports The Horse.
Dr. Thomas Geruden, with Zoetis in Belgium, said these results are not surprising as the worm population is continuously pressured for selection of survivable genes in the calendar-based deworming routine.
Geurden and other researchers in Belgium, along with scientists at the Gluck Equine Research Center in Lexington, Ky., studied two farms that utilized the standard deworming protocol on their Belgian draft horses.
The deworming schedule should have corresponded with the timeline of worm egg development, but drug resistance has caused the worm eggs to appear twice as fast as expected.
Fecal egg counts on every horse on the farm were performed every two weeks from April to September for three years. Horses that had more than 250 eggs per gram of manure were given pyrantel embonate, an alternative dewormer. All other horses were treated with standard anthelmintics in the spring and fall, reducing their deworming by half or two-thirds.
The researchers used the fecal egg counts, both before and after treatment, to create a model of worm life cycles that could predict drug resistance over the next 40 years. The team found that the alternative dewormer maintained low egg counts in all horses and slowed drug resistance in worms by 200 to 300 percent.
The scientists also found that the horses considered “high shedders” of worms were almost always under 5 years old. This knowledge might allow horse owners and caretakers to target which horses may need fecal egg counts run more often – and which may need more-frequent treatments.
The data suggests that fecal samples should be gathered for egg counts between 42 and 56 days after the spring moxidectin/ivermectin treatment, with a second sample taken between days 98 and 112. These numbers will help determine future egg counts and reduce deworming frequency.
The ability to model the rate of anthelmintic resistance using different deworming protocols has been helpful when discussing deworming with both horse owners and vets, the researchers found. A deworming plan specifically tailored to a farm may be more expensive initially, but the less-frequent, targeted deworming will pay off financially in the long run as dewormer resistance slows.
Read more at The Horse.
