Parasite Control and Resistance

Dewormers have been utilized indiscriminately to control horse parasites for over 40 years. There are three classes of dewomers (anthelmintic classes) for nematode control in horses: benzimidazoles (fenbendazole, oxibendazole), tetrahydropyrimidines (pyrantel) and macrocyclic lactones (ivermectin, moxidectin). Resistance is a genetic adaptation that worms pass to succeeding generations; once they develop resistance to a particular drug, those worm populations are not likely to revert back to susceptibility. Important changes in the parasitic fauna of horses have occurred such that Strongylus vulgaris and other large strongyles are now rare, and cyathostomins (small strongyles) are now the major parasite of concern in adult horses, while Parascaris equorum remains the most important parasite infecting foals and weanlings.

  1. Anthelmintic resistance is highly prevalent in cyathostomins and Parascaris equorum (Ascarids- round worms), and this must be factored into treatment decisions (Kaplan and Nielsen, 2010).
  2. Adult horses vary greatly in their innate susceptibility to infection with cyathostomins and their level of strongyle egg shedding and thus, require individualized attention to their parasite control needs.
  3. Horses less than about 3 years of age require special attention as they are more susceptible to parasite infection, and are more at risk for developing disease.

The goal of deworming your horse should never be to eradicate any parasite. Not only is this impossible, but the inevitable result is accelerated development of parasite drug resistance. Instead, the goals are to minimize the risk of parasitic disease; control parasite egg shedding; maintain effective drug control; and avoid further development of anthelmintic (drugs used to treat infections with parasitic worms) resistance as much as possible.

For adult horses, the guidelines state that one or two yearly treatments are sufficient to prevent large strongyles infection. Cyathostomin (small strongyle) parasites can be treated based on strategically performed fecal egg counts. Tapeworm treatments should be included annually in most regions.

In foals, it is not recommended to base treatments on egg counts. Instead, foals should receive about four anthelmintic treatments their first year. The guideline provides information about the timing and choice of anthelmintic drug.

One of the most important concepts is to work with your equine veterinarian to perform fecal egg counts on your horse. As there are no new drugs are on the horizon for deworming horses, so it is important to be circumspect and contact your veterinarian to develop a deworming protocol for your horse. In all age groups it is highly recommended to perform routine screenings of anthelmintic efficacy with the fecal egg count reduction test.


Kaplan, R.M. and Nielsen, M.K.. (2010). An evidence based-approach to equine parasite control: It ain’t the 60s anymore. Equine Veterinary Education 22(6) 306- 316.

Images used under creative commons license – commercial use (9/15/2015) NIAID (Flickr)

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