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Strategic De-Worming of Horses

As a horse owner you are likely familiar with the full spectrum of de-worming products available. Most horse owners use a schedule for using those products in their horses. The most widely used approach to parasite control has been a rotational treatment program (developed in the 1960s), which employs a rotation between products in 2-3 month intervals for all horses living in a common area, or within a group.

Fast- forward 50 years, and we are seeing an alarming rate of parasite resistance to the drugs currently in use today. This is of particular concarn since there are essentially no new classes of anti-parasite drugs available.

A new program for de-worming our horses has been recommended by veterinary colleges and the American Association of Equine Practitioners. The goal is to only treat the horses in need, thereby slowing down the rate at which these parasites are developing resistance to our drugs. This new program has been called strategic de-worming.

Strategic de-worming identifies horses carrying high parasite loads, and treats them aggressively, while decreasing the number of treatments for horses that continually harbor low parasite numbers. it is thought that 80% of all the parasites in horses may be harbored by only 20% of the animals. Since most horses can clear parasite infections via their immune system, many horses within your herd will likely not need to be medicated aggressively.

Strategic de-worming can easily be initiated in your horses by both quantitatively and qualitatively analyzing a manure sample on all horses. This will identify both the type of parasite, and the parasite load in individual animals. Since resistance tends to be a farm issue, and not an individual animal issue, it is also vital to know which de-wormers are still effective at eliminating parasites on your farm. An appropriate de-worming schedule can then be recommended by your veterinarian.

The most common intestinal parasite in our horses today is the small strongyle; unfortunately, this parasite has also developed the strongest resistance patterns to our de-wormers, with entire drug classes now considered ineffective. Small strongyles are ingested in the larval form. Following ingestion, they quickly migrate into the intestines where they hibernate in the gut lining. Your horse may have several thousand, to more than several millions of these encysted larvae lining the gut. When these parasites emerge simultaneously they can cause major disease in your horse. This includes colic, severe and chronic diarrhea, weight loss, limb swelling and low serum protein levels.

The adult worms, with few exceptions, are the stages killed by de-wormer treatments and cause very little damage to the horse. In order to optimize horse health, it is necessary to prevent new infections. Consequently, we aim to kill adult worms without treatments, but it is actually the prevention of egg shedding that does the most for horse health and overall worm control because by doing this we reduce the numbers of infective larvae on pasture and subsequent infections in the grazing horse.

Other intestinal parasites that we need to be aware of in our horses include: large strongyles, roundworms, pinworms, bots, and tapeworms. Horses under the age of 3 years tend to be more susceptible to these parasites than our adult horses.

We would be more than happy to talk with you more about this de-worming strategy, and help you set up a manure testing and de-worming plan for your horses. Our recommendation is to analyze manure samples on all horses on the property in spring and fall, followed by de-worming with the appropriate drugs and intervals on selected animals only.