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The evolution of deworming strategies in the horse

Old recommendations of regular treatments aren’t the best course of action anymore

The fecal egg count test measures the number and type of parasite eggs a horse is passing in its manure which helps better target 
the use of deworming products.
Reading Time: 4 minutes

Many horse owners are very committed to the regular and timely deworming of their horses. Oral paste dewormers have become a major staple in stables, tack shops, feed outlets and veterinary pharmacies and readily available to the horse owner.

Although this availability of paste and gel dewormers does seem ideal, traditional practices of deworming horses at regular intervals have contributed to a shotgun approach to deworming horses that may no longer be a reasonable nor sustainable practice in equine parasite control programs.

In the mid-’80s something very significant happened in the horse world — the introduction of a highly effective chemical dewormer called ivermectin. Ivermectin could be safely and easily administered orally by the horse owners themselves. Prior to this time a veterinary visit was required to administer the older classes of dewormers which were caustic to the esophagus and thus were only suitable to be delivered by nasogastric entubation. Other anthelmintic manufacturers soon followed suit with paste and gel formulations rapidly growing in popularity and becoming widely used by horse owners.

Something significant also happened in the world of the equine internal parasites at the same time as well. When chemical dewormers were first introduced, the most common and troublesome internal parasite of the horse was the large Strongyles (Strongylus vulgaris) and bloodworm.

The larval stage of this parasite set up housing in the blood vessels of the horse’s body. Due to the parasite’s predilection for the vascular sites of the horse’s intestinal tract, bloodworm infestation was a significant cause of colic in horses, often with life-threatening consequences. It was the population of egg-laying adults in the intestinal tract that ivermectin was originally targeted to remove. At the time it was determined that egg reappearance in the feces occurred approximately two months after an ivermectin deworming and thus the original recommendations for deworming horses every two months was set in motion.

This deworming strategy was highly effective, reducing the incidence of disease related to the large Strongyles bloodworm so much so that the dreaded colic associated with infestation is currently a very rare occurrence. However, like a game of chess, the parasites have themselves responded and rallied with their own adaptive strategies. With selection against the large Strongyles other species of internal parasites such as the small Strongyles or cyathostomins, roundworms and tapeworms began to fill the ecological niche previously held by the bloodworm.

In addition there have been individual parasites that have inadvertently survived the deworming process. These individuals then pass their adaptive genetic information for resistance to the deworming products along their genetic lines. And so the populations of problematic species of internal parasites in the horse as well as their vulnerabilities to deworming products have shifted over the last 30 years.

Thus original recommendations for deworming horses at the regular two-month intervals is outdated and blanket recommendations for deworming horses is no longer applicable in nearly every population of horses.

Therefore recommendations for employing deworming products in parasite control programs must also evolve in response to the adaptive strategies of the internal parasite populations and the new information regarding host-parasite dynamics.

Currently the best measure for targeted decision-making comes from the information derived from the fecal egg count (FEC) test which determines the level of shedding of parasite eggs in an individual horse’s manure. Testing involves collecting a manure sample from the horse and submission of the sample to a veterinary laboratory which can then calculate the number and type of parasite eggs per gram of feces.

The numbers generated from each horse will generally place the horse in one of three categories. Low shedders pass no more than 200 eggs per gram of feces; moderate shedders pass 200 to 500 eggs per gram; and high shedders pass 500 or more eggs per gram. It is interesting to note that once a mature horse has been tested several times to determine its shedding status, the individual’s classification is unlikely to change as the horse will consistently repeat the same level of shedding.

This information can then be used to determine:

  • Whether or not deworming is even warranted;
  • The optimal timing for deworming; and
  • The type of deworming product to be used.

In order to determine the effectiveness of the deworming product used it will be necessary to perform and compare the results of a second fecal egg count 10 to 14 days after deworming. Effective products reduce the parasite egg output in the feces by 90 to 95 per cent.

This testing procedure is known as the fecal egg count reduction test (FECRT) and is used to determine if the deworming products are working. Introducing this step can reduce drug resistance, increase product effectiveness and minimize exposure to chemicals by only treating horses with moderate to high parasite burdens. Ineffective deworming products not only amplify the intensity of the resistance in parasite populations they are economically draining and create a false sense of proper parasite management.

It is now recommended that deworming treatments include the individual horse’s parasite burden and needs as determined by the FEC test. Horses with low shedding rates may not require deworming. Treatment programs are no longer intended to totally annihilate parasite populations from the horse as this is not only impractical it is also based on the flawed assumption that parasite infestations are entirely harmful. Within acceptable parameters parasite infestations seem to provide the horse with a favourable “street immunity.”

Further advice regarding targeted use of deworming products can be obtained from your veterinarian who will be familiar with the FEC testing procedure/s and the horse’s environment.

About the author


Carol Shwetz is a veterinarian focusing on equine practice in Millarville, Alberta.



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