Case 3: Worms!


If you have browsed through a horse publication in the last few months, you have likely seen more than one article on deworming.  Why all the fuss?  The concern is that parasites resistant to available dewormers are being found in horse populations around the country.  Contributing to this resistance is the overuse of dewormers — using dewormers more often than they are needed (just like the overuse of antibiotics leads to antibiotic resistance).  There is a finite number of deworming drugs available and no new products are due to hit the market any time soon.  A world without effective dewormers is not a pleasant thought – we must make some changes so we can make the drugs we have last.

What can you do?  If we change the way we think about deworming we can slow the development of resistant parasite populations.  This means only deworming your horse only when he needs it and not deworming on a set rotation of every 1, 2, or 3 months or when the farrier comes or whatever you use to set your deworming calendar.

It is said that 20% of the horses in a herd are responsible for 80% of the parasite load.  Yet, many of us continue to deworm all horses according to a set schedule, some up to 7 or more times a year!   As the veterinarians at North Florida Equine continue to examine more of our horse population we are finding that many horses need to be treated only twice a year.  The frequency of treatment depends on how well your horse’s immune system protects him from parasites in the environment and also on the degree of environmental contamination.

So how do you know if your horse needs deworming?  You usually cannot tell just by looking.  In fact, you might be surprised to learn which of your horses carry the most worms.  A fecal exam is all it takes!  Armed with the information from that test, you and your veterinarian can develop a targeted program.

Now for our case:

Let’s take a look at the small herd of Martin horses.  They were seen by North Florida Equine for vaccinations this spring.

Biscuit Belle Gracie Raindrop Skittles Star

Belle, Biscuit, Gracie, Raindrop, Skittles, and Corby.

A fresh fecal sample was collected from each member of the herd and examined for parasite eggs.
The parasites of greatest concern in adult horses are small strongyles (also called Cyanthostomes).

Strongyles (also called Cyanthostomes).
 A small strongyle egg as seen under the microscope.

The number of strongyle eggs seen in a gram of feces was quantified and each horse put in a category according to the number of eggs found.

Low Shedders
(0 to 250 eggs/gram):

Biscuit Gracie Raindrop Skittles Star

Biscuit, Gracie, Raindrop, Skittles, and Corby

Moderate Shedders
(250-500 eggs/gram):


High Shedders
(over 500 eggs/gram):


Even though all of the horses had been exposed to the exact same environment, only Belle developed a high parasite load.   She sure didn’t look any different from the rest of her herd.  It is important to remember that it can be impossible to tell which horse has a higher worm burden just by simple observation.

Armed with the information from the fecal analysis, Belle’s owner can target her for more intensive deworming.  As a high shedder Belle is also responsible for putting high numbers of parasite eggs out into the environment.  The strategic deworming program developed for Belle will decrease the level of contamination in the environment, a benefit for the rest of the herd.  The information gained from the fecal analyses also spares the rest of the herd from administration of drugs that they don’t need.


North Florida Equine went one step further with Belle.  Approximately 2 weeks after she was given her prescribed dewormer a Fecal Egg Count Reduction Test was performed.  This is a comparison of fecal egg counts done before and after deworming to determine how well a product is working.  This is a test run on all High Shedders by North Florida Equine to ensure that resistance has not already developed in the parasite population.

The result: Belle went from 1400 eggs/gram down to 25 eggs/gram.  A 98% reduction and evidence that the dewormer is doing a fine job!