Parasite article by Dr. Kellon


Worming Strategies

–         Dr. Eleanor Kellon, VMD, Staff Veterinary Specialist for Uckele Health & Nutrition

One of the most confusing aspects of horse ownership can be sorting out deworming plans.  It’s important to remember that all horses are at risk for intestinal parasites.  Regular deworming is easy, affordable, and your best investment in making sure your horse gets the most from his diet.  The process of effectively deworming your horse involves considering your horse’s age, the health of your horse, and risk of exposure to parasites.

The very young and very old are at highest risk and should be dewormed at 4 to 6 week intervals, while healthy adult through middle-aged horses may have a higher level of resistance to intestinal parasites.  Individual adults, however, may have less resistance to parasites than the average and can significantly contribute to contamination of the environment and exposure to other horses.  To detect horses with poor resistance that will need more frequent deworming, retest fecals 8 to 12 weeks after deworming.

Monitoring by fecal egg counts twice yearly in adults is reasonable, but fecals do not detect immature or tissue forms of parasites and are poor at detecting tapeworms.  Egg concentrations in manure can also vary throughout the pile and over the course of a day.

Because of limitations of fecal egg testing, a minimal deworming frequency for tapeworms and other parasites is usually twice a year – spring and fall or beginning and end of grazing season.

Roundworms, a significant parasite of young horses and the very old, are developing resistance to Ivermectin.  Recheck fecals in two weeks if using Ivermectin.

Strongyles and other parasites are showing widespread resistance to many classes of dewormers other than Ivermectin or Moxidectin. If using these other drugs, recheck fecals in two weeks to make sure they have been effective.


Exposure is a big part of risk. Low levels of parasite infestation do not necessarily have any obvious health consequences, but they do keep parasites alive and well in the environment. Because no dewormer gets each and every life stage of parasites, when you deworm your horse you will always be missing some immature worms. These are the ones that eventually mature and begin laying eggs during the deworming interval. The bottom line here is that no horse will ever be 100% parasite free, even if it’s the only horse on the property, because he will be a source of parasites himself.

Exposure can come from other horses too, of course. The more horses your horse is turned out with and the more crowded the living conditions are, the higher the risk is of being exposed to parasites from another horse. Rotating horses through small paddocks or round pens on crowded facilities is also a high-risk situation. If you take your horse away from home for shows or trail rides, always be picky about how you let him graze (if at all) in high traffic areas, and never feed directly on the ground. A few mouthfuls in the wrong place can result in a heavy parasite infestation.

Most at risk

The very young (under 1 year), the very old, and debilitated horses have lowered resistance to parasites. Because of this, even the most effective dewormers often will not reduce egg counts to zero as they will in healthy adults. These horses are constantly reinfesting themselves and can build up large parasite burdens very easily.

On the flip side, healthy horses that are not very young or old may have a very vigorous immunity to parasites. The immune system cells lining their intestinal tract can manage to keep parasite infestations very low or even to none at all, especially if they live in uncrowded pasture conditions where they never have to eat close to fecal droppings.

Once you’ve sorted out these basic monitoring plans according to your horses’ age, health, risk of exposure and resistance to parasites, managing the issue is far less complicated than living with the consequences.

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