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Taking the Mystery out of Deworming

Updated: Feb 25, 2022


To Deworm or Not to Deworm?


This Newsletter will explore the current research on internal parasite control for horses, allowing you to have the tools to make the most informed decision for your equine companion. When it comes to deworming the vast array of opinions and information out there can be incredibly daunting to both new and experienced horse owners. What was once the norm may no longer be the most effective. The emphasis on management practices is becoming more beneficial and consulting with your veterinarian can help determine which techniques will work best for you and your horse.


What Worms?


With the overuse of dewormers in the past, there has been a shift in which internal parasites are of most concern in horses. Strongylus Vulgaris was once the major parasite of concern as adult worms reside within a major artery leading to loss of adequate bloodflow to the gut and consequently colic, ischemia, and potentially rupture of the intestine. Due to vigilant deworming, this parasite is now rarely encountered and instead the main concern for adult horses is a group of smaller parasites known as cyathostomes or small strongyles. Though much smaller, these worms in high enough burdens can lead to muscle wasting, emaciation, and intermittent diarrhea with potential to even be fatal. Like most equine parasites, they are ingested by horses out on the pasture in their larval forms and eggs are passed through the feces.


In younger horses, the parasite known as Parascaris Equorum, or "Roundworm" is the biggest concern. This worm is unique as it is transmitted indoors by having a sticky coating on its eggs which allows them to adhere to stall walls, water buckets, etc. They are also then licked up by foals exploring their surroundings. High burdens can lead to stunted growth, emaciation and general ill thrift. These eggs are also highly resistant and can live for months to years in the environment allowing them to pass from one generation of foals to to the next. Unfortunately both Cyathostomes and Parascaris Equorum are already known to be resistant to several classes of drugs used to deworm horses. Consulting with your veterinarian to design an appropriate control program for your farm is one of the most effective ways to ensure your horses are kept healthy. Other parasites that are of lesser concern but can be easily treated with most programs include bots, pinworms, threaded worms, and tapeworms.


How to control


Fortunately, despite the increasing concerns around drug resistance there are still several strategies, both using medications and management changes, that can be effective to keep worm burdens low and your horses healthy.


One of the most important things to keep in mid when designing a control program is the strategic or timed use of deworming medications rather than frequent use. The more often a horse is exposed to a dewormer the quicker resistance can develop through the few worms that are able to withstand each round of deworming. Horses do not need to be completely free of worms to be healthy. Lighter loads can be tolerated very well and help reduce resistance to dewormers. Therefore, timing of dosages along with careful management is the best way to ensure long-term efficiency. One way this can be done is through the use of regular fecal egg counts so that horses who are high shedders of the parasite eggs can be identified and so that the best medication can be selected for the specific parasite being shed. By treating only the high shedders pasture contamination is reduced while keeping the number of horses exposed to dewormers relatively low to decrease resistance buildup of the parasites.


This reduction of resistance is important because currently there is only one class of deworming drug that is still effective against cyathostomes and a single different class that is still effective against Parascaris Equorum. In order to keep the efficacy of these drugs in the future, it is essential to avoid over-deworming now.


Other ways to help with parasite control on farm is to reduce the stocking density within pastures, not feeding on the ground where possible and to keep manure piles out of pastures. It is also best not to harrow paddocks unless they will be kept empty for at least one month under hot, dry temperatures to kill the eggs brought to the surface. For Parascaris Equorum more dramatic measures are needed to rid the barn of the eggs including using a steamer while scrubbing all surfaces.


Take Home Message


Proper parasite management is important for your horses and any future horses health. There are many different approaches that can be taken to design a control program specific to your horses' needs. For more information or if you would like to obtain specific information pertaining to your horses needs. Contact us today!


Written by Dr. Emily Zakrajsek

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