Horses and Worms – Strongyle Egg Shedding Rate (Part 1)

By John Byrd, DVM


The articles that I write are most often answers to questions Horsemen’s Laboratory has received over the 25 years we have been helping horse owners evaluate their horse’s worm control program. This is the first part of a two-part series discussing several of these questions.

We have reviewed the results of over 70,000 samples that Horsemen’s Laboratory has examined. We have put this information into graphs, so our clients and others can evaluate it.  The overall average is 35% positive and 65% negative. Over 95% of the positive samples are due to strongyle eggs. We are unable to determine whether these are eggs from large strongyles or small strongyles because their eggs all look the same. However, since the introduction of ivermectin dewormer in 1981, most large strongyles have been killed off, therefore through research it has been determined that over 90% of strongyle eggs passed by horses are from small strongyles. Therefore, all of the data in this article is in reference to small strongyles.



Does temperature have an effect on results when sending samples through the mail?

Does the time of year or season have an effect on results?

What percentage of horses are positive?

What percentage are low, medium, or high shedders (discussed in part 2)?


Here is a summary of the data that we collected over the last 25 years broken down according to the last 3 years (2015, 2016 and 2017) versus the years (1992 – 2014). We have also included the actual graphs and numbers for those of you who enjoy analytics.


What we found:

  • 1992 – 2014 — 32% of samples were positive which means 68% of samples were negative (no eggs found on counting chamber).
  • 2015 — 37% positive, 63% negative, 2016 — 40% positive, 60% negative, 2017 — 39% positive, 61% negative.


Changes in Deworming Recommendations:

You may ask why does it appear there were less positive and more negative samples in the early years? We believe it is because up until 2012, Horsemen’s Laboratory recommended to our clients to deworm any horses that were positive and re-test them in 2 – 3 weeks causing more samples to be negative. This likely caused many of the low shedders to remain negative for more than 3 months causing more samples to be negative. After 2012, Horsemen’s Laboratory changed our recommendation about deworming. Only deworm horses with an egg count of 200 or more strongyle eggs/gm or if any other worm eggs are found in the stool sample. We also recommend deworm horses that are low shedders and negative once a year generally in late fall or early winter with a combination of ivermectin and prasiquantel (Zimecterin Gold or Equimax).


One other possible trend:

The graphs of the results tend to indicate a trend. That trend is that there appears to be an increase in the number of eggs in the samples during February and March, and again in September and October. This trend appears only slight and is somewhat inconsistent. This likely trend is in response to the changing season in the different parts of the county. January February and March it is still cool in the southern U.S. and starting to warm in the central U.S.  It is believed as the southern part of the U.S. begins to get hotter the strongyle activity slows down due to the heat. At the same time the temperature in the central and northern U.S. is getting warm enough for the strongyle activity, and it begins to increase. The reverse is likely to be occurring in September and October.


What effect did temperature have on results sending samples through the mail?

To find the answer to this question the staff at Horsemen’s Laboratory compared the results in the months of July, and August with the results from the months of January and February for the years of 2015, 2016, and 2017.

  January February July August
2015 38% Pos. 33% Pos. 35% Pos. 39% Pos.
2016 38% Pos. 44% Pos. 40% Pos. 38% Pos.
2017 35% Pos. 43% Pos. 36% Pos. 36% Pos.
Average 37% Pos. 40% Pos. 37% Pos. 37.7% Pos.


In conclusion, it does not appear the temperature has an effect on the results.  However, I do believe as I mentioned above the time of year may have some influence on the results. For those of you that are interested in an expanded view of the positive versus negative results be sure to look at the charts and graphs below. Remember in part 2, we will discuss the egg shedding rate throughout the years of 2015, 2016 and 2017.

After studying the data collected from the results of over 70,000 fecal egg counts it appears there is very little variations in the percentage of horses that are positive for worm eggs in their stool throughout the year. The results also indicate that the temperature when the samples were mailed did not affect the results substantially. Therefore, Horsemen’s Laboratory recommend horses be tested any time without concern that the weather or time of year are going to affect the results. So, if it has been over 3 months since your horses were tested or dewormed it is time to order kits to test your horses. You can either go online at or call 1-800-544-0599 to place your order now.

Shedding rate chart page 1

Shedding rate chart  page 2