Cascade to Columbia Veterinary Services

2090 Vantage Hwy
Ellensburg, WA 98926



Equine Internal Parasite Management & Deworming Guide


Internal parasites are still a major health concern in horses.  Preventable illness and death due to these "worms" is still being seen by our veterinarians.  Also, in recent years the concern has become parasites that escape traditional deworming methods?encysted small strongyles and tapeworms. 

At the same time, concern has arisen with potential development of resistance to traditional dewormers in large animal species, including horses.  In some cases, horses are being "over-dewormed."

For these reasons, newer recommendations focus on "targeted deworming," based on fecal egg counts of parasites in individual horses.  We now know that with different equine immune systems, parasite susceptibility varies from horse to horse.  Twenty percent of horses in a herd account for 80% of the parasites shed onto the ground.  The goal is to identify equine "shedders" (often young horses or those with stressed immune systems) through fecal testing.  Testing also allows you to determine how effective your deworming medication is on your horses' parasites, and how often treatment is needed.  In the long run, this can save you time and money and helps slow down the onset of resistance to deworming medications. 


Specific Recommendations


Since fecal testing will NOT identify encysted small strongyle larvae and is limited in detecting tapeworms, these parasites should be targeted 1-2 times annually.

  1. Tapeworms (acquired through grazing or consuming hay) use Praziquantel, available in combination  with ivermectin (Zimectrin Gold), at least once yearly (at least  one treatment Oct 1-Nov 1, after the first frost).  Zimectrin Gold is also effective against virtually all equine internal parasites, including roundworms, tapeworms, strongyles, and bots.
  2. Encysted small strongyles can cause a deadly condition in winter called larval cyathostominosis characterized by diarrhea and weight loss.  Therefore we   recommend targeting encysted small strongyles in late fall/early winter (Jan 1) with the most effective treatment, moxidectin, (Quest, or Quest Plus). Moxidectin is also effective against roundworms, strongyles, and bots. We no longer recommend fenbendazole (Panacur Powerpack ), as more recent studies have shown moxidectin to be a superior treatment.

Fecal testing identifies strongyle, roundworms, pinworms, and other parasite eggs.  We use the most current laboratory procedures in detecting these parasites (sugar-centrifugation/flotation).

  1. To obtain a pretreatment baseline egg count, perform a fecal around 12-16 weeks after the last deworming.  A very low number of parasite eggs are acceptable, but at a certain threshold we will recommend deworming.  The veterinarian will also take into account your horse's age, environment, and herd size in this recommendation.
  2. To assess the effectiveness of a recent deworming, perform a post-treatment     fecal testing  2 -3 weeks after a deworming treatment.  This may help identify any potential deworming resistance on your farm.
  3. After we determine the frequency of deworming needed for a specific horse, follow-up pre-treatment fecal testing annually is recommended.
  4. To collect a sample, collect several tablespoons of fresh manure and bring it to the clinic double ziplock bagged within 24 hrs.  Refrigeration is advised if sample in not brought in immediately.

If you choose not to have fecal testing done, we recommend deworming your horse every 3-4 months with a moxidectin or ivermectin product, taking into account the guidelines in (#1) above.  Keep in mind that a fecal egg count test is less expensive than the cost of two deworming treatments! (Fecal: $26.75, additional horses $20.25 each when submitted at the same time).  A common scenario is that testing reveals that less deworming treatments are necessary for your horse, and that saves time and money.

Specific risk category groups:

     1. Young horses 

Horses less than 5 years of age have higher parasite rates.  Yearlings and weanlings are most susceptible.  All foals should receive their first deworming by 2 months of age and may be dewormed with an ivermectin or pyrantel product.

     2. Pregnant mares

Most commercially available dewormers are labeled safe to use in pregnant mares.  We recommend the spring deworming treatment be given 4-6 weeks prior to foaling, as this will help to protect the foal.

     3. Newly acquired horses 

They can have a big impact on parasite levels in your pasture and herd.  Prevent problems by quarantining until fecal egg testing and deworming can be completed.

Other parasite management recommendations:

     1. Pasture management (also contact Kittitas Co Conservation District) 

Manure should be collected and/or dispersed frequently.  Grazing more than 1 horse per 2 acres results in higher parasite levels.

Interesting Fact: moxidectin, the ingredient in Quest Deworming, is non-toxic to "fecal fauna", such as dung beetles, which are beneficial to pastures.  Other dewormers can have a negative effect on these beneficial organisms.    

     2. Separate pastures

Graze yearlings and weanlings on separate pastures from older horses.

     3. Understanding dosing

Under dosing deworming medications is a frequent problem that allows resistance to develop  more rapidly.  If in doubt of your horse's weight, ask us for estimation, or use a weight tape.