South Shore Equine Clinic & Diagnostic Center
Equine Veterinary Services South Shore Boston Massachusetts

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Strategic Deworming:
Where we are today and what is best for your horse

New research has shown that deworming on a rotation every 6-8 weeks is not a proper way to care for horses anymore.  Because of anthelminthic resistance we are recommending a new system of  fecal testing, deworming and protecting the environment  to prevent horses from becoming infected with parasites in the first place.  For optimal  impact, you must give the right dewormer at the effective dosage at the appropriate time of year.  The purpose of rotating between different drug classes is due to the fact that some parasites will survive treatments with a particular dewormer. If you use the same drug class in successive treatments, then the surviving parasites can reproduce with new generations resistant to that particular drug class. Furthermore, timing of deworming is very important. For example, the small strongyle larvae are able to migrate and hide in the wall of the large intestine and are not affected by many of our common anthelminthics. These larvae begin to emerge as the days get longer (March-April). We often see an increase in fecal egg counts (FEC) at this time. By deworming for small strongyles at the proper time in your area of the country, we can thwart extensive egg laying that would contaminate spring pastures and perpetuate infection the rest of the year.  Many of our current dewormers will kill only the adults. It is ideal to deworm with a product that will treat both the adults as well as the larvae twice annually. It is also imperative to ensure you are giving the proper dosage to your horse, which is based on their body weight. Your horse’s bodyweight should be measured and recorded by your veterinarian at his/her annual physical exam.  We provide all clients who enroll in the Wellness Plan a weight tape which gives an estimation of your horse’s bodyweight  by girth size. Lastly, it is important to be sure that your horse gets and swallows all of the deworming medication. Ask for assistance if your horse resists oral administration.

To determine the effectiveness of the dewormer that you are using, as well as your overall deworming program, it is essential to perform serial fecal egg counts (FECs). FEC testing determines the concentration of parasite eggs in manure. Initially, a FEC should be done 10 to 14 days after deworming to establish the effectiveness of the product used (you should expect a FEC near zero). A second FEC  should be done at the egg reappearance period (ERP). The ERP is a predictable interval where the FEC remains low after an effective deworming agent is administered and it differs slightly depending on the deworming product used. The normal ERP is 4 weeks for benzimidazole and pyrantel products, 6-8 weeks for ivermectin, and 12 weeks for moxidectin.  The second FEC helps to determine which horse  may have a high parasite load (encysted larvae) and/or if your farm or paddock has a parasite problem (re-infection). By identifying the “problem” you can provide targeted treatment of the individual horse and/or environment. 

Cleaning manure out of stalls daily and paddocks 2-3 times weekly is essential to controlling parasite contamination of your horse’s environment. Rotating paddocks periodically in the hot, dry months allows larval stages to emerge and die off without finding hosts. If you spread your manure over actively grazed areas, it is best to compost it prior to spreading it, as the heat generated during the process kills the parasite eggs.  Additionally, you always want to deworm new horses and check their FEC prior to turning them out with the herd to minimize contamination by an unknown host.

Research has found that once a farm and its horses have been cleared of parasites, many horses only need to be dewormed twice yearly.  This approach to strategic deworming and parasite control will minimize the potential of developing resistance and is better for the environment. It also stops us from giving unnecessary medications to our equine friends and, long term, minimizes costs to you, the horse owner.

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