Parasite control is not as straight forward as one would imagine.  This is the precise reason you have many different “opinions” as to what the appropriate parasite control method should be. Reaching a balance between parasite load, anthelmintic resistance, and general knowledge is our goal. There is no way to completely eliminate parasites from your horse, but we can help you manage a program which will ensure a long healthy life for your animal and/or herd.

Performance Equine Vets Parasite Control Guidelines

Traditional parasite control programs involve rotating treatment with anthelmintics at regular intervals. This approach is based on concepts and strategies developed more than 40 years ago. Frequent anthelmintic treatments are not needed to keep adult horses healthy. What is needed are properly timed treatments with effective anthelmintics administered at appropriate times of the year.

What are fecal egg counts used for?

  1. Clinical Diagnosis?– For example if your horse is colicing and parasites are a suspected culprit, a fecal egg count can be done to support that diagnosis and to determine the exact type of parasite causing the problem, and therefore help the clinician target a treatment protocol for the patient. However, a negative fecal egg count does not mean your horse does not have parasites causing the problem. High specificity (can be as high as 98%), moderate sensitivity (can be as low as 72%), Negative Predictive Value (can be as low as 66%- these are horses that have a negative fecal egg count and this is the percent that are actually truly negative). This means fecal egg counts may not be valuable in clinical diagnosis.


  2. Fecal Egg Count Reduction Tests– To help determine if a dewormer used is efficacious and if it is working to reduce the parasite load. This is the most important use of the fecal egg count in the horse. This is however done on the herd level, this is not effective for use in the individual horse. We like to have 6 horses if possible for this test. The anthelmintic in question is administered and a fecal sample is collected 14 days following treatment.

% Efficacy = (Pre-trt EPG- Post-trt EPG)/ Pre-trt EPG X 100 = % Efficacy of De-Wormer used

Anthelmintic       Expected Efficacy if No Resistance     Susceptible     Suspected Resistance     Resistance

Fenbendazole:                            99%                                           >95%                     90-95%                     <90%

Pyrantel:                                  94-99%                                         >90%                     85-90%                    <85%

Ivermectin/Moxidectin:         99.9%                                          >98%                    95-98%                     <95%

*Strongyle resistance to ivermectin or moxidectin has not been diagnosed in the US to date. Therefore, any FECRT result that yields <95% reduction for these drugs should be repeated before concluding there is resistance.

It is recommended to include at least six horses in a FECRT on each farm. Further, it is recommended to always recruit the horses with the highest possible pre-treatment egg count for the FECRT. Horses should not have received anthelmintic treatment at least 9 weeks prior to the FECRT after benzimidazoles (fenbendazole/oxibendazole) or pyrantel was used, after 12 weeks if an ivermectin was used, (preferably 16 weeks, if moxidectin was used). FEC are quite variable, so if testing is done with few horses there is potential for variability, which could lead to an incorrect inference. Therefore, borderline results should be interpreted with care, and the test should be repeated before any conclusion is made.


  1. Determination of Egg Shedding Levels– This is individual to the horse and can be quite variable from one horse to another. A healthy pastured horse with a low egg shedding potential will tend to always have a low FEC, while a healthy pastured horse with a high egg shedding potential will tend to always have a high FEC, this can change based on a horse’s immunological status, but seems to be consistent. 20% of the horses shed 80% of the eggs in a given herd. Determining egg shedding levels is not useful for young horses < 3yrs of age.

Low Contaminators 0-200 EPG 50-70% of adult population

Moderate Contaminators 200-500 EPG 10-20% of adult population

High Contaminators >500 EPG 20-30% of adult population

  • There are no direct linear correlations with egg counts and worm burdens.
  • Egg counts do NOT reflect larval worm burdens. In the case of strongyle type worms, it is most often the larval worm burden that inflicts disease in our horses.
  • Should be done in the Spring prior to deworming. Can be repeated in Fall before deworming.

  • After Moxidectin (ERP = 10-12 weeks): Wait ≥ 16 weeks to collect a fecal

  • After Ivermectin (ERP = 6-8 weeks): Wait ≥ 12 weeks to collect a fecal.

  • After benzimidazoles (fenbendazole/oxibendazole) or pyrantel (ERP = 4-5 wks): Wait ≥ 9 weeks to collect a fecal.

So what does this mean?

1-3 year old horses: Should be dewormed 3X year (Fecal Egg Count not needed)

  • April 15 Ivermectin or Power Pac
  • September 1 Ivermectin (when temps <90)
  • November 15 Equimax

>4yr old horses: 1-2 yearly treatments, unless identified as a high shedder by Fecal Egg Count

1. Every two years evaluate Efficacy of Dewormers by doing Fecal Egg Count Reduction test in Spring

2. Yearly evaluate high shedders by doing a Fecal Egg Count test in Spring prior to administration of Dewormer, this can be repeated if desired in Fall. These high shedders will need additional dewormings.

  • May 15 Ivermectin or Moxidectin or Panacur Power Pac (either PP or Moxidectin should be used at least every other year to remove encysted small strongyles)
  • November 15 Equimax

Moxidectin should be used with discretion since it is easier to overdose a horse and cause a toxic reaction. It should be used with caution on geriatric horses or very lean horses.


Samples should be stored in airtight and leak-proof plastic ziploc bags, baggies can be turned inside out and used as a glove to pick up samples, re-inverted and sealed pressing the air out of the bag. Please mark each bag with the name of the patient using a sharpie.

Collected manure should be as fresh as possible. Samples less than 12 hours old are acceptable, but should be refrigerated immediately after collection. Fecal samples that are or have been frozen are not acceptable, as this will damage the eggs and decrease the recovery rate.

**All fecals done at Performance Equine Vets, utilize a centrifugation, concentration method. This method is superior to the standard flotation method and provides a detection limit of 1 EPG.

Environmental Control

  • Infection of horses could be prevented if all feces were promptly removed from the pasture. Equine strongyle parasites begin life as an egg in a manure pile, which then must develop to an infective larvae in the feces, get out onto the pasture, and then be ingested by a horse.

  • Proper composting of manure and soiled bedding will generate relatively high internal temperatures, and strongyle larvae in manure are virtually eradicated by exposure to temperatures over 104 ºF for a minimum of two weeks. Eggs hatch and develop into infective larvae under conditions of moderate temperature and moisture. Cold slows the rate of development or stops it altogether, and excessive heat kills eggs and larvae. Non-composted horse manure should never be spread on pastures as this will increase the level of parasite contamination.

    We are always available for a consultation.

    If you have any questions and would like to speak with a Veterinarian, please call us at 803-641-0644

    * Performance Equine Vets Parasite Control Recommendations are derived from the AAEP’s Parasite Control Subcommittee of the Infectious Disease Committee’s comprehensive set of recommendations for helping veterinarians develop improved strategies and programs for parasite control in horses of all ages.