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Selective Deworming Program, Wisconsin Equine Clinic & Hospital



Untitled1The doctors at Wisconsin Equine Clinic and Hospital are excited to work with our Wellness Program clients in implementing a new selective deworming program for your horses.

This program will feature a deworming program individualized to your horse’s needs.

How is strategic deworming different than what I have done in the past?

In the past, we have relied on deworming all the horses on a farm at the same time with a rotation of dewormers administered on a set time interval. This interval program came into use over 40 years ago based upon recommendations to control large strongyles, which are the most damaging of the internal parasites. Indeed, this program was very effective in its control of the large strongyles largely because they have a very long life cycle (6 to 9 months). In fact, large strongyles are now very uncommon in our horse populations. Small strongyles now account for virtually 100% of the strongyle egg output from grazing horses and have a significantly different life cycle than the large strongyles. It has been known for years that 20% of the horses account for 80% of the eggs shed onto pastures. The other 80% of the horses appear to have strong immunity and shed only a few parasites. In some studies, up to 50% of horses on some farms consistently have negative egg counts. In a strategic deworming program, quantitative fecal egg counts are used to identify the horses that are responsible for most of the parasites passed onto the pastures. These horses with poor immunity are dewormed appropriately to control the level of infection while horses with minimal parasitism are given dewormer at much less frequent intervals.

Why should I switch my horses to a strategic deworming program?

The old interval program results in a large population of horses that are minimally parasitized receiving multiple dewormers. This increases the selection pressure for the development of resistant parasites. Over the years since interval deworming programs were introduced, small strongyle drug resistance has increased to 97.7% for fenbendazole, 53.5% for oxibendazole, and 40.5% for pyrantel pamoate. Recently reports of resistance of small strongyles to ivermectin have surfaced. This is particularly frightening since these reports of drug resistance include all available dewormer products, and currently new deworming compounds for use in horses have not been identified. Obviously, we need to change our approach to parasite control to preserve the usefulness of our dewormers.

How do I know if my herd has resistant parasites?

The only way to evaluate for drug resistance is by the fecal egg count reduction test. A quantitative fecal egg count is performed before the administration of each type of dewormer and then repeated 14 days after the dewormer has been administered. Effective dewormers should reduce egg output by at least 90%. If this decrease in egg production is not seen, then resistance to that dewormer is present in your horses and that particular dewormer should no longer be used in your herd.

How do I implement this new program in my herd?

To start a strategic deworming program, quantitative fecal egg counts must be performed on all of the horses greater than 3 years to determine if they are high (>500 eggs per gram), medium (200-500 eggs per gram), or low egg shedders (< 200 egg per gram). This initial fecal egg count must be done at least 2 months after the last dewormer was administered to be accurate (3 months if Quest was the last product used). Egg shedding by individual horses is determined by immune status, so once a horse is assigned to a category, they generally remain in that category for life. Horses between 1 and 3 years of age are still developing immunity to parasites and may change category as they mature. These young horses are assigned to the high shedder category because they are more susceptible to the effects of parasites. The frequency of dewormer administration is dictated by the egg shedding classification.
Egg counts to monitor resistance are recommended before the administration of each new type or dewormer and 2 weeks after treatment. After the first year on this program, twice yearly (Spring and Fall) fecal egg counts should be sufficient to monitor parasite control.

How do I properly collect fecals?

Label a Ziploc bag with your name, the horse’s name, and the date the sample was obtained. Collect 2 fecal balls from the center of a fresh manure pile and place in the bag. Remove excess air and seal the bag. Keep refrigerated until the sample arrives at the lab. Remember, larger samples are not necessary to determine the fecal egg count. Old samples that consist of bedding mixed with manure or that have been allowed to dry out will not provide valid results.

Contact us with questions or concerns:
Wisconsin Equine 39151 Delafield Rd. | Oconomowoc, WI 53066| | 262-569-1550

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