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Dr. John Byrd, DVM

How do you know my horse has worms? How do you know you got rid of the worms my horse had? These two questions were commonly asked when I dewormed horses as a general equine veterinarian. My standard response was that the drug companies tell us they work.

When the daily dewormers came on the market many of my clients requested that I get these dewormers for their horses. I often told them I thought it was unnecessary because most of the horses were already being dewormed every 2 months. In addition, 90% of the horses I cared for in southern California were kept in clean box stalls 22 hours a day except when they were being ridden or exercised. I did not see how these horses could have many worms, if any at all. Therefore, I decided to seek the answers to these common questions for myself by doing fecal egg counts before I dewormed my horses.

What I found was that less than 1 out of 20 horses had any eggs in the stool sample to indicate they were infected with adult worms. After consulting with several experts in equine parasitology and recognizing that no one was performing routine fecal egg counts for horse owners, I started Horsemen’s Laboratory to fill this void. I felt owners should have the opportunity to know whether or not their horses had worms rather than just treating them blindly.

For several years now researchers have known that many of the worms that horses have were becoming resistant to the dewormers that were being used to treat them. There also appears to be a link between the over use of these dewormers and the developing resistance. What appears to be occurring is that the dewormers kill most of the worms that are sensitive, leaving only the resistant few to mate with each other. These worms then create more worms that are resistant and soon we have large populations of worms that are now resistant to the dewormer. Performing fecal egg counts can help owners identify if their horse has resistant worms and can also indicate which dewormers an owner should be using.

Stool samples will also indicate which horses are passing the most eggs and therefore identifying the worst contaminators of the pasture. Horses can then be dewormed according to how rapidly they are spreading eggs that will become infective larva on their pasture and in their environment. Research shows that 20% of the horses in a pasture are responsible for 80% of the eggs in a pasture.

Regular stool samples will give horse owners the peace of mind that they have a worm control program that is working to protect their horse from the effects of these parasites. There is no other way of evaluating whether or not you have an effective worm control program. By testing a stool sample before automatically deworming, you may also be helping to slow the development of resistance to dewormers by only deworming when it is necessary. In the long run this may reduce the cost of their worm control program and give owners the confidence that it is truly effective.

HORSEMEN’S LABORATORY has made it affordable and convenient to have fecal egg counts done on your horses. Visit or call (800) 544-0599 and take advantage of this great service to help protect your horse from worms.

You can find more informative articles in our section on Health & Education.

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