Deworming Strategies

For the last 50 years, most horse owners have been told to deworm their horses every 2 months with rotating dewormers. While this has been effective in decreasing infection from what was considered the major parasite of horses 50 years ago, S. vulgaris or the large strongyle, it is now a moot point. S. vulgaris is no longer considered a major player in the parasitic arena in correctly managed herds. Currently, small strongyles called cyanthosomes, Parascaris equorum or equine roundworms, and A. perfoliata or equine tapeworms, are considered the main parasitic threats to equine health1. Due to the extensive use of rotating dewormers, there is emerging drug resistance in many of these parasites, meaning current dewormers aren’t as effective.  If horse owners continue to deworm every 2 months, we may soon run out of effective dewormers.

            Small strongyles infect almost every horse with access to grass, so it’s important to know how heavy an individual’s parasite burden is. Each horse has some innate resistance to parasites, but it varies from horse to horse. Small strongyles only cause disease when present in large numbers in the horse’s GI tract1. This means that your horse could have a small parasite burden and be perfectly fine. It has been found that in most herds, 15-30% of the horses are responsible for up to 80% of the parasite egg burden1.  The main goal of deworming is to target these horses and get their parasite burden down to a manageable level.

            So, what should you do? The only way to determine how heavy your horse’s worm burden is is to perform a Fecal Egg Count. This is a test that quantifies how heavy your horse’s infection is, and establishes a baseline to determine treatment. Low shedders have less than 200 eggs per gram of feces. These horses don’t generally require treatment. Moderate shedders have 200-500 EPG, and high shedders have over 500 EPG. These two groups should be treated based on the type of worm present and the type of dewormer those worms are susceptible to. Ideally, the test should be performed prior to deworming, or at least 6 weeks after deworming.

            There are several other things you can do to help decrease the egg burden on your farm, such as rotating pastures, removing manure frequently from pastures (at least twice weekly), harrowing/dragging and mowing pastures to expose larvae and eggs to elements and predators, and avoiding equine overcrowding. Florida has different recommendations for deworming due to its climate. The best time to treat horses is during the winter. Below is a chart put together by AED to help aid in deworming your horse with the correct products at the correct time of year based on their Fecal Egg Count test results. If you have any questions, don’t hesitate to ask during our next visit!

Written by: Morgan Bosch, DVM


1. AAEP Internal Parasite Control Guidelines.

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