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Strategic Deworming
Parasite Control

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Sarah L Rhoades, DVM

Providing quality veterinary services for horses in the greater Franklin County, Missouri area.

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Lets Address these points

Parasite control is a big, complicated and highly concerning subject. And rightfully so! Parasites can cause severe, even fatal, gastrointestinal disturbances if left unchecked and out of balance.

However, a lot of the former recommendations are based on information over 50 years old! We know a lot more now, and further, the actual landscape of species of concern have actually changed in that time.

I go into a lot more detail on why this is on my blog.

But for the time being, let me summarize these key points.

Why strategic parasite control?
  • It's about preventing disease in healthy horses. Treating sick or thin horses is another subject.

  • It reduces the amount of parasite resistance, on your farm, making your herd safer.

  • It allows you to target the few, individual horses that are actually exposing the rest of the herd to high parasite numbers.

  • It allows you to monitor, objectively, how effective your parasite control program is and watch for problems with resistance.

  • Most of your horses will end up being treated with de-wormer much less than before.

My horse is fat! Myth #1
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This chunker here has a high Fecal Egg Count at almost every test - he's what we refer to as a 'permissive shedder.' A good parasite control program isn't about treating sick, thin, poor doing horses - those horses have active problems needing specific treatment. A good parasite control program is about preventing disease in apparently healthy horses. Approximately 80% of your pasture's parasite load comes from 20% of the horses in your herd. 1 in 5 horses are responsible for 4/5s of the parasites on your pasture. In the case of permissive shedders, like this guy, treating him is about reducing the exposure of the entire herd to parasites while using minimal amounts of anthelminthics (de-wormers).

Diagnostic Testing is Expensive! Myth #3
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Fecal Egg Counts allow us to identify the small proportion of horses that are shedding the vast majority of parasites on our pastures. They also allow us to monitor your parasite control program to ensure it remains effective. They are relatively cheap, approximately the cost of a Coggin's Test and easy to perform - a fresh stool sample is all that's required.

Rotating de-wormers prevents parasite resistance! Myth #2
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You're probably familiar with the concept of resistance - when you give a de-wormer, only the strong survive, the parasites with resistant traits survive to reproduce creating more parasites with resistant traits. When you switch de-wormers, that previous resistance doesn't just go away, it's still there, but now you're creating resistance to a second drug. Rotating de-wormers not only fails to prevent parasite resistance, it facilitates it!!!

Equine parasites of modern concern
I just feel Safer! Myth #4

50 year ago, when the original recommendations were made to deworm every other month and rotate de-wormers the parasite species of most significant concern was the Large Strongyle, also known as the 'bloodworm.' This parasite invaded the blood vessels that supplied the host's large intestine, eventually occluding the vessel enough to kill the tissue and cause a massive, almost always fatal colic. Most of us probably have some horror story our elders told us that comes to mind - and it was very real at that time!


However, thanks to commercial de-wormers, this event is very rare today. The diseases that prompted previous recommendations are rare, and we understand them better: they take very few parasites to cause where resistance is not a problem, and they take a long time to develop - this makes interrupting that disease process before it becomes a problem pretty easy and why I advocate for twice yearly deworming for all horses regardless of shedding status.

The diseases of concern today, cyathostomes (small strongyles) and parascaris equorum (round worm) exist in large numbers, our treatments don't kill the entire population, and resistance has become a huge problem. And that's an important distinction - it's not a goal of removing and killing all of these parasites, it's about preventing disease in apparently healthy horses and controlling the numbers of these parasites using as little anthelminthic drugs as possible

What's more, we see this problem on a farm by farm basis - that means you have a great deal of control over your farm's parasite resistance landscape. That means your management decisions directly effect you and your horses. While we can't undo resistance problems, good management and minimizing de-worming use does dilute the problem down on your farm.


Targeting our treatment to the apparently healthy individuals in the herd that are spreading a vast majority of these parasites is absolutely key. (And of course non-anthelminthic means of control such as pasture and manure management!)

Further, recent numbers out of New Zealand comparing regular, timed, de-worming versus those of off Fecal Egg Counts show limiting de-worming does not result in greater gastrointestinal disturbance events (or any increased risk of adverse health events for that matter), and it did not result in greater parasite numbers on Fecal Egg Counts despite a vast difference in the number of anthelminthic treatments between groups. Check out the British Equine Veterinary Association's announcement here!

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