How to find out if your horse's dewormer is working

Fecal Egg Count
Reduction Testing for Horses

Calculating the fecal egg count reduction test (FECRT) results for your herd is a multi-step process, so this blog contains a few tables, formulas, calculations and examples. But if you are in a hurry, here's a summary.

Fecal egg count reduction tests are used to gauge parasite resistance to dewormers on your property. Resistance occurs when the same amount of the same dewormer you used to use, does not control the same parasites at the same time of year as it used to (try saying that fast three times.)

Finding out if the parasites you are trying to control have developed resistance to the dewormer you are using, consists of plugging in the results of a pretreatment fecal egg count test (FEC) and a post-treatment fecal egg count test into a mathematical formula (see Formula A).

The steps to perform a fecal egg count reduction test (FECRT) are:

What's the difference between a Fecal Egg Count Test and a Fecal Egg Count Reduction Test

A fecal egg count test is a diagnostic test performed on a sample of your horse’s manure to identify the type and number of parasite eggs, lurking inside your horse. Fecal egg count tests primarily identify small strongyles (redworms) and ascarids (roundworms). Fecal egg count test results are reported in eggs per gram (EPG). The higher the EPG, the more parasite eggs were detected. The more parasite eggs detected the greater your horse’s parasite burden. Equine fecal egg count test are available here.

A fecal egg count reduction test consists of comparing the results of a fecal egg count test conducted before deworming your horse and a second fecal egg count test performed after deworming your horse. Both fecal egg count test results are then inputted into a formula (see Formula 1) to calculate the fecal egg count reduction result. Equine fecal egg count reduction tests are available here.

Why perform a fecal egg count reduction test?

Anthelmintic (dewormer) resistance has emerged as a serious threat to horses worldwide. Some parasitic resistance has been reported for all classes of equine dewormers on the US market today (i.e., benzimidazoles, macrocyclic lactones, and tetrahydropyridines). With no new dewormers being developed, the current dewormers must remain effective for as long as possible.

Conducting a fecal egg count reduction test at the beginning of Spring and then again in late Fall, allows you to consistently monitor parasitic resistance on your property by indicating how effective your dewormer is at controlling your horse’s parasites.

" The Fecal Egg Count Reduction Test (FECRT) is the only method currently available for detecting resistance in parasites of horses."
- American Association of Equine Practitioners, Parasite Control Guideline -


Each of your horses is unique, but the parasites are the same across your property. To properly interpret fecal egg count reduction results, there should be at least four (preferably six) horses with pretreatment fecal egg counts of 100 EPG or greater. Fewer horses can be used; however, the confidence in the fecal egg count reduction test results diminishes the fewer the number of horses treated.

Discovering your horse’s egg shedding potential.

Before implementing a deworming strategy based on fecal egg count reduction testing, you first need to determine your horse’s parasite egg shedding potential. Your horse’s egg shedding potential (i.e., high, moderate or low) is an indication of how many parasite eggs your horse is passing in their manure.

You can discover your horse’s egg shedding potential by checking your horse’s pretreatment fecal egg count test result (e.g., 200 EPG small strongyles) and then comparing it to the center column of Figure A. Classifying your horse into a HIGH, MODERATE or LOW shedder category allows you to take control of your horse’s healthcare and decide if you need to deworm your horse. Recent research indicates not deworming low shedders or a most treating them once a year is healthier for your horse and reduces parasitic resistance. Interestingly, 50-75% of horses are low egg shedders (see the last column of Figure A).

Classification based on egg shedding Egg Count (per gram of feces) % of horse population


    0 - 200

  50% to 75%


    200 - 500

  5% to 15%



  10% to 30%

Figure A- Suggested guidelines for classifying horses into different levels of strongyle egg shedding and the expected percentage of the horse population belonging to each group (Kaplan and Nielsen, 2010).

How and when do I perform a FECRT?

Fecal egg count reduction tests should be performed twice a year, beginning with conducting a fecal egg count test in early Spring.

NOTE: Make sure any prior deworming treatment does not affect your test results. Dewormers can affect egg counts several weeks after a treatment (see Figure B). For example, if you treated your horse with ivermectin, you should wait at least 8 weeks before performing your next pretreatment FEC test.

Based on your pretreatment FEC test results, decide which horses to treat. After treating your horse(s), wait 14 days before conducting a second FEC on the treated horse(s).

Input your test results of the first (pretreatment) FEC test and the results of your second (post-treatment) FEC test into the FECRT formula (see Formula A). The result will be your FECRT. The results for the FECRT help you determine how effective your dewormer is at controlling the targeted parasites.

Fecal Egg Count Reduction Test Formula:

Formula A

For example, you performed a Zero Egg Count FEC test on your horse in the Spring before any deworming treatment. The Zero Egg Count FEC test results revealed that your horse had 800 EPG of redworms (small strongyles). You chose to treat your horse with the proper dose of ivermectin oral paste for equines. Fourteen days later you performed another Zero Egg Count FEC on your horse and the results were 10 EPG of redworms (small strongyles). Plugging your two FEC test results into the FECRT formula (see Formula B) resulted in a FECRT result of 0.99 or 99%.

Formula B

So what does a FECRT tell me about resistance?

Parasitic resistance is measured using a horse’s Egg Reappearance Period (ERP). The ERP is defined as the time interval between the last effective anthelmintic (dewormer) treatment and the resumption of significant strongyle egg shedding. Egg reappearance periods that are shorter than anticipated (see second column of Figure B) are an indication that resistance is developing to that dewormer. Figure B lists the egg reappearance period for equine dewormers when they were first developed and the current ERP period for when the drug is fully effective.

Dewormer Usual ERP when drug is effective ERP when drug was first introduced


  4 to 5 weeks

  6 weeks


  4 to 5 weeks

  5 to 6 weeks


  6 to 8 weeks

  9 to 13 weeks


  10 to 12 weeks

  16 to 22 weeks

Figure B- Small strongyle egg reappearance periods (ERP) for equine dewormers
- AAEP Parasite Control Guidelines

Efficacy means the ability to produce a desired or intended result. Each equine dewormer has an expected efficacy (e.g., 99%), if the parasite(s) being tested for show no resistance to the dewormer. In other words, efficacy is how effective the dewormer is at controlling the targeted parasite(s) in a horse at that time. Using the previous example, you would compare your FECRT result of 99% to column 2 of Figure C which shows the efficacy of the ivermectin used to deworm your horse is greater than the minimum of the 98% efficacy required for ivermectin. Therefore the parasites you are targeting are NOT showing resistance to the ivermectin. If your FECRT test results were 95% or less, then that would indicate the parasites you are treating for have developed some resistance to the ivermectin.

Dewormer No Resistance Suspected Resistance Resistant



90% - 95%




85% - 90%




95% - 98%




95% - 98%


Figure C- Efficacy of equine anthelmintics against small strongyles

Fecal egg count reduction tests are only part of a comprehensive sustainable equine parasite control program. There are three basic elements to consider when creating a sustainable, comprehensive parasite control program for your horse – testing, treatment, and environment. Each of these elements plays a critical role in helping to identify, eliminate and manage your horse’s parasite or worm burden.

About Zero Egg Count

Zero Egg Count is an Equine Healthcare company offering diagnostic fecal count test kits and laboratory services. Our inexpensive test kits use a non-invasive collection process that is quick and easy to follow. The test results provide critical information about the general health and effectiveness of your horse's deworming program.

Read next

Leave a comment

Please note, comments must be approved before they are published