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Double Down: Traditional Deworming Increases Drug Resistance At Alarming Rate

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.

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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.

Bots: Where Fecal Egg Counts Fail

A fecal egg count test is a common horse health practice designed to combat the overuse of equine dewormers and consequent drug resistance. Though the test is helpful in determining which horses need to be treated for parasites, there is one parasite that escapes being found on the screening: bots.

Fecal egg counts don't show the bot load a horse is carrying because bot eggs are not found in horse manure. Bot flies lay eggs on a horse's body, often on the lower legs and chest. As the horse licks the eggs, they hatch and he swallows the larvae, where they enter his digestive tract. The larvae, not eggs, leave via manure and the process begins again.

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Bot fly eggs look like small yellow grains of rice that stick to horse's hair. To remove them, horse owners can use a bot knife, which has a serrated blade, or a fiberglass block, which grabs and removes the eggs.

If bot fly eggs are removed diligently throughout the year, a horse owner may be able to skip deworming for bots – the horse will be fine if he ingests a few. If the horse isn't groomed regularly, however, it's worthwhile to give him a single dose of moxidectin or ivermectin after the first frost.

Study: Dewormer Use Drastically Reduced On Farms Using Fecal Egg Counts

Scientists have been touting the need for horse owners to use fecal egg counts before blanket deworming the horses in their car for years. However, until recently, no studies had been conducted to see if farms that used this method saw a true reduction in deworming.

Drs. Liselore Roelfstra, Marion Quartier and Kurt Pfister studied five farms in France and Switzerland that had switched to an evidence-based deworming protocol in 2014 to determine the long-term reduction rate of anthelmintic treatments.

The study used 90 horses and three ponies ranging in age from 3 to 32. All of the horses were housed at riding stables that used paddocks and pastures. There was no routine manure collection in the fields on three of the farms; the other two farms removed manure from fields at least once a week.

Since 2014, each horse had a fecal egg count performed twice a year, in the spring and fall. The horse only received a dose of dewormer if the analysis showed that he carried a worm burden of 200 eggs per gram or more. In total, 757 fecal egg counts were taken; only 34.7 of them had an egg count over 200, which resulted in the horse being given a dewormer.

This meant that 263 doses of dewormer were not given, which would have been routinely administered in the past. This shows an overall reduction in anthelmintic treatments of 65.3 percent.

The scientists conclude that conducting fecal egg counts is feasible on farms with multiple horse owners, and on farms with a transient equine population. Buy-in of the farm owner or manager was paramount, but all the horse owners were prepared to pay the cost of the fecal egg count test. An additional positive outcome was that the horses with high worm loads were able to be treated with the specific product needed to eliminate the parasites the horse is hosting.

The overall reduction in dewormer use shows the potential of fecal egg counts to slow the development of drug resistance.

Read the study here.

Easy-To-Implement Ways To Reduce Parasite Resistance

Currently, equine internal parasites can resist all classes of dewormers on the market. This is especially concerning as internal parasites can cause so much harm—and there are no new dewormers on the horizon. However, there are some things horse owners can do to keep horses healthy naturally, reports The Horse.

Veterinarians originally recommended that horses be dewormed every two months, as that was when parasitologists began seeing worm eggs returning. Now, strategic deworming and an integrated approach to parasite management is preferred. This includes only deworming the horses that need it and not blanket deworming all horses on the farm.

Other ways to prevent worm burdens include:

Quarantining new horses—this includes not turning them out on fields other horses will eventually use. The point is to keep the horse and the worms it os carrying separate long enough for the eggs to pass through his system.
Feeding off the ground to prevent ingestion of larvae
Ensuring feed and water sources are not contaminated with manure
Removing manure piles before eggs hatch
Composting manure at temperatures above 104 degrees F to destroy eggs and larvae
Keeping grass taller than 3 inches to minimize larvae ingestion
Dragging fields on hot days to expose larvae in manure to temperatures they cannot survive
Implementing pasture rotation, which disrupts the parasite life cycle
Stocking pastures with an appropriate number of horses (one horse per acre at minimum) to reduce grazing around manure
Renovating pastures for better forage options
Including other species in the pastures with horses so different plants get eaten