How many parasites are lurking in your pasture?

Can you find the 192,902,500 Parasite Eggs?

Do you see a lush grass field in the photo above?

It looks like a delightful place for your horse to leisurely graze, enjoying each mouthful of grass (provided your horse does not have founder, laminitis, or some other ailment that limits their exposure to grass.)

Don't be fooled!

That lovely pasture of green potentially represents a parasite minefield filled with millions of harmful larvae and eggs just waiting for your horse to consume them.

Where do the parasites come from?

Parasites have been around since the beginning of time. Perhaps even the dinosaurs had parasite problems. Horses are born with some sort of internal parasites making parasitism one of the most, if not the most, common equine disease. However, most horses also develop some level of immunity to these dangerous intruders.

Parasite eggs are introduced into a pasture each time a horse passes manure on to the pasture (scientists call this process contaminating the environment). Once outside a horse, the parasite eggs hatch into larvae (little bitty worm-like thingies) that scooch around in the grass near the manure pile. Grazing horses accidentally consume these larvae, and depending on the type of larvae, the parasites burrow through the horse, causing damage to the horse’s organs and tissue. The parasite larvae eventually take up residence inside a horse, and like a bad tenant in a nice rental property, wreak havoc on the place by attaching themselves to a horse’s intestinal walls to steal your horse’s vital nutrients.

So how do I know if my horse has parasites?

You can’t “see” inside your horse, and even if you could, parasite eggs are too tiny to be detected without a microscope. Therefore, you need to examine the parasites that come out of your horse (more specifically, the parasite eggs).

The best way to find out which and how many parasites are lurking inside your horse is to conduct a fecal egg count test on a sample of your horse’s manure. Identifying the type and number of parasites present in your horse’s manure sample tells you how many parasite eggs your horse is shedding (contaminating) on to your pasture.

Equine fecal egg count tests are easily performed by using a mail-in test kit like Zero Egg Count’s Equine Parasite Kit. Zero Egg Count’s Equine Parasite Kit test results tell you the type and number of parasites your horse is shedding on to your pasture by counting the number of parasite eggs in a sample of your horse’s manure.

Eggs Per Gram

Just to make things a little more complicated, fecal egg count testing results are reported in a number like 20 or 314 or 1500 EPG. EPG stands for "eggs per gram." EPG is an industry-standard that tells you how many parasite eggs (strongyles and ascarids eggs to be specific) have been counted in your horse’s manure sample. The higher the EPG number, the more parasite eggs were found in your horse’s manure sample, and the more parasite eggs your horse will likely shed onto your pasture.

This information can then be used to classify your horse as a High, Medium, or Low shedder. Additionally, EPG test results can be used to compare your current test results to previous test results (as long as the same fecal egg count technique is used).

See Table 1 for the number of parasite eggs associated with each classification.

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%

Table 1- 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).

Different Testing Methods = Different Test Results

To calculate the actual number of parasite eggs your horse is shedding per day, you need to know which testing method was used.


Because each testing method uses a specific amount of manure (called a sample size) when testing your horse's manure for parasite eggs. You may have sent a ball of poop to be tested, however the lab only used a small portion of that poop ball to test (a.k.a. the sample size).

But why, you ask again?

The sample size provides the grams tested, which we need to plug into the Pasture Contamination Calculation formula below to find out how many parasite eggs your horse is shedding per day.

Did you say sensitive?

The sample size also plays a role in how sensitive the test is. Sensitivity means how many eggs can be accurately detected when performing the test. For example, Zero Egg Count uses our True Count® testing method, which is based on the Modified Wisconsin Sugar Floatation Testing technique for detecting parasite eggs.

Hold on – the Modified Wisconsin- what did you call it?

The Modified Wisconsin Sugar Floatation Test (let’s just call it the MWSFT going forward) uses a 3-gram sample size of manure (that’s triple the size used by some other testing techniques). It is sensitive to 1 EPG, which means our lab technicians count every single egg they see (hence the True Count© thing). Besides that, one of our labs is located in Wisconsin – coincidence, you say (actually our lab’s location doesn’t really have anything to do with the method, but I thought it was worth mentioning.)

So. what’s the big deal you ask?

The more poop sampled, the more precise the EPG results.

Most fecal egg count testing methods (like the one your vet probably offers) only use 1 gram of manure and are only sensitive down to 25 EPG or 50 EPG. These methods also use dilution and multiplication techniques, meaning they don’t count all the parasite eggs, just the eggs that appear on a specific part of a microscope’s slide. Then they multiply that number by 25 or 50 to estimate the EPG.

For example, if your horse's fecal egg count was 100 EPG (which would be considered a LOW shedder), these other techniques would report 100 X 25 or 250 as the total EPG, which changes your horse's contamination category to a MODERATE shedder. If the 100 EPG result was multiplied by 50, then your horse's EPG would be reported as 500 EPG, which is considered a HIGH shedder!

Regardless of the testing technique, the resulting EPG count only represents the parasite eggs detected in that particular manure sample at that time of testing. To find out what your horse is shedding daily you need to do some additional math.

The Pasture Contamination Calculation below converts your horse’s fecal egg count EPG testing results into a daily (and annual) pasture egg shedding number.



To make it even easier, following is a Parasite Contamination Calculator.  All you need to do is input how many grams of your horse's manure was used to test for parasites by the lab (a.k.a. the sample size). HINT - your lab can provide this information or you can use a number between 1 and 3. BTW, use 3 grams for Zero Egg Count's fecal egg count test) and your horse's EPG test results.

The example calculation below shows a horse with a fecal egg test result of only 20 EPG would potentially shed 55,115,000 parasites eggs annually, onto your pasture.

Step by Step Guide to the Pasture Contamination Calculation

Unit of Measure or Calculation Description

(Step 1)  3 grams

Zero Egg Count's laboratory uses 3 grams of a horse’s manure sample to calculate the total number of parasite eggs per gram (EPG).

(Step 2)  453 grams = 1 pound

There are 453 grams in a pound.

(Step 3)  453 grams/3 grams sample size = 151 grams

Divide 453 grams by the testing sample size of 3 grams.

(Step 4)  20 eggs per gram (EPG)

Let’s say your horse’s Zero Egg Count equine parasite test result was 20 EPG - meaning your horse is shedding 20 parasite eggs per gram of manure.

(Step 5)  20 EPG x 151 grams = 3020 eggs per pound of manure.

To calculate the number of parasites shed in 1 lb of horse manure, convert the eggs per gram into eggs per pound, by multiplying your horse’s 20 EPG by 150 grams to get the number of parasite eggs per pound your horse is shedding on to your pasture.

(Step 6)  50 pounds

A horse produces an average of 50 pounds of manure/day (don’t take my word for it or go ahead and look it up).

(Step 7)  3020 eggs per pound x 50 pounds = 151,000 eggs per pound

Multiply the eggs per pound in step 6 by 50 to find the number of parasite eggs your horse is shedding per day.

(Step 8)  151,000 eggs per pound x 365 days = 55,115,000 egg per year

Multiply the result of step 7 (150,000 eggs per pound) by 365 (days in a year), and the result is the number of parasites eggs your horse sheds per year on to your pasture.

(Step 9)  What about your other horses?

Do the same calculation for any other horses that share the pasture and add the results together.

192,902,500 Parasite Eggs Shed per Year – you've got to be kidding!

The above calculation shows a horse with a test result of 20 EGP would shed 55,115,000 parasites annually, onto your pasture. Now do the same calculation for your other horses and add them up to discover what your herd is shedding onto your pasture. If you owned a second horse with a 50 EPG test result and your horses share a pasture, you would do the same calculation for your second horse, which would yield an additional 137,787,500 parasite eggs shed per year). Adding the two test results together would yield a whopping total contamination rate of 192,902,500 parasite eggs per year shed on to the pasture - that's from just two low shedding horses!

"Many times, larval build-up can reach as high as
10,000 larvae per square meters of grass collected.
- Dr. Donald H. Bliss, Veterinary Parasitologist

So what can I do about it?

Pasture management is a critical part of a comprehensive parasite control program for your horse.

After all, it only takes a day or two to test and treat your horse. Pasture management is what we need to do for the remaining 363 days of the year.  

Pasture management is all about preventing reinfection. Specifically. preventing your horse from ingesting the parasites in the first place. However, that is easier said than done.

Pasture Management for Parasite Control

For most of us, pasture management is a real estate problem (too many horses and not enough pasture). Since most of us (or maybe it’s just me) can’t’ go out and buy another 100 acres of pasture next to where we are keeping our horses, following are some more economical recommendations to follow:

Practice good pasture hygiene - remove and dispose of your horse’s manure at least twice per week.

Rotate pastures - move horses between pastures to naturally break the parasitic life cycles and allow pastures to rest long enough to reduce the parasite population.

Reduce pasture load - fewer horses per acre means reduced fecal contamination.

Elevated feeders - lift grain and hay off the ground where parasites thrive.

Harrow pastures – this is a tricking one. If your horse lives in a climate that is warm (30oC or 86oF) and dry for 4 to 6 weeks at a time, breaking up the manure piles to expose eggs and larvae to the elements may be helpful otherwise it just further spreads the manure around pasture.

About Zero Egg Count

Zero Egg Count is an Equine Healthcare company offering diagnostic fecal egg count test kits and laboratory services. Our inexpensive mail-in test kits allow horse owners to pick the time they want to test. No waiting on the vet or driving to the vet's office. Just scoop, seal and send--- it's that easy. Our easy to understand test results describe which parasites were detected and what to do about it.

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