Disease Du Jour: Fecal Egg Counts for Horses 
In this episode, Dr. Kathryn Duncan discusses the importance of fecal egg counts for effective parasite management in horses.
Horses in a herd can be selectively dewormed based on fecal egg counts.
Selectively deworming horses in a herd based on fecal egg counts and allowing grass to grow taller are two effective parasite control strategies. | Getty Images

In this episode, Kathryn Duncan, DVM, PhD, DACVM (Parasitology), joined us to talk about fecal egg counts for horses. During the conversation, she explained why fecal egg counts are important and offered suggestions for practitioners who want to offer the service to clients.  

What Is a Fecal Egg Count? 

Duncan started the conversation by explaining what a fecal egg count is. “We’re taking a small portion of the horse’s fecal sample, performing a fecal float, and then counting or quantifying the number of eggs in that sample,” she said. Then, with a small amount of math, you can determine the eggs per gram in the horse’s feces.  

“That information is really critical if we’re going to formulate a successful parasite plan,” Duncan said. 

She said in the general U.S. horse population, practitioners will usually see small strongyle eggs.  

Why Should Vets Perform Fecal Egg Counts? 

Duncan said fecal egg counts help determine which horses contribute the greatest contamination of parasite eggs to pasture, allowing the veterinarian to create a selective deworming program.  

“There’s been some research that’s shown just 15-30%, a pretty small proportion, of horses in a herd are actually contributing 80% of all the eggs that are out on pasture,” she said. Selecting only a few horses in the herd to deworm allows for refugia, which means a population of larva on pasture is in refuge from the compound.  

“We want some worms to stay susceptible to the anthelmintics we have so they can mate with worms that contain resistant genes,” Duncan explained. “Refugia is allowing us to prolong the efficacy of the dewormers we have.” 

How to Determine Which Horses to Deworm 

Veterinarians can use fecal egg counts to classify horses as low, moderate, or high egg shedders, Duncan explained. Low shedders have fewer than 200 eggs per gram, moderate shedders have 200-500 eggs per gram, and high shedders have more than 500 eggs per gram. She said its best to classify horses using more than one fecal egg count performed at different points throughout the year; initial classification might take six to 12 months.  

Duncan said all horses should be dewormed at least one or two times per year. Practitioners can then use fecal egg count information to recommend additional treatments for horses that are moderate or high contaminators. She said those horses can benefit from deworming three times per year or four if they’re showing signs of parasitic disease.  

“Nature actually can help us in this pursuit of strategically deworming,” Duncan continued. She said deworming in summer is often unnecessary in very hot climates, because nature is killing most of the larvae in the environment. In very cold climates, deworming is likely unnecessary during winter.  

Duncan recommended consulting the AAEP internal parasite guidelines. She noted recommendations might change as more research is released.  

What is Required to Perform Fecal Egg Counts? 

Duncan then discussed the equipment necessary to perform fecal egg counts. She recommended solo practitioners organize the logistics to send samples to a reference lab. She said a university lab is ideal because it can become a great point of contact if you have questions about the results or want the most up-to-date information about deworming protocols.  

For multi-vet practices with support staff, running fecal egg counts internally can make sense. “Once you do it a handful of times, honestly, it’s going to be one of the easiest things that you have to do in that laboratory setting,” Duncan said.  

She described multiple methods for running fecal egg counts, all of which require a microscope. The most common method, and the one she prefers, is the modified McMaster test. This method requires a small scale to weigh the feces, paper cups, sodium nitrate solution, tongue depressors for handling the feces, cheesecloth for straining out debris, a plastic pipette, and the McMaster slides.  

Duncan recommended consulting the AAEP guidelines for descriptions of the different deworming methods and step-by-step instructions for performing them.  

Fecal Egg Count Timing 

Duncan stressed the importance of knowing when to perform fecal egg counts. She recommended sampling the horse no more than seven days prior to deworming, which will likely occur during the spring and fall. Then, sample the horses 10 to 14 days after deworming.  

“So, you have a pre-treatment fecal egg count and a post-treatment fecal egg count. Then you can determine the percent reduction in the egg count, and it will help you to determine the efficacy of the dewormer,” Duncan said.  

Final Thoughts 

Duncan concluded by saying we can’t begin to hope for successful parasite management for horses without the help of equine veterinarians in the field and horse owners advocating for fecal egg counts and the judicious use of anthelmintics.  

“We are now at a point where we’re dependent on maximizing the remaining usefulness of the dewormers that we have until new drug classes can be released,” she said.  

She said in addition to fecal egg counts and selective deworming, pasture management is another effective parasite control strategy. Removing manure from fields and composting manure before spreading it on pasture can help prevent parasites from spreading. Rotating pastures, co-grazing with ruminants, elevating feed off the ground, and allowing grass to grow taller so the horse is eating at a higher level than the parasites are other useful strategies.  

About Dr. Kathryn Duncan 

Kathryn Duncan, DVM, PhD, DACVM (Parasitology), is an assistant professor in the Veterinary Pathobiology department at Oklahoma State University’s College of Veterinary Medicine and co-director of the National Center for Veterinary Parasitology. She teaches parasitology in the veterinary curriculum and maintains a research lab that focuses on parasitic diseases of domestic animals. She graduated from University of Tennessee’s College of Veterinary Medicine, completed a residency in veterinary parasitology through the National Center for Veterinary Parasitology (NCVP), and obtained a PhD in biomedical sciences at Oklahoma State University. In 2022, Duncan became a diplomate of the American College of Veterinary Microbiologists, board-certified in parasitology, and her most recent publications discuss ticks, tick-borne diseases, and gastrointestinal parasites of domestic animals.

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