Targeted deworming can help maintain herd health

Treating horses that are ‘heavy shedders’ can reduce total parasitic infestations on the farm by up to 80 per cent

desktop microscope

Most horse owners are keenly aware of the importance deworming plays in the health of their horses. Veterinarians, pharmaceutical companies and extension programs have done an exceptional job at promoting frequent scheduled dewormings. Yet recent science about the biology of equine parasites has found that using more deworming compound does not necessarily result in a better parasite management program.

Equine parasitologists are advocating the fecal egg count as a diagnostic tool to better refine standard protocols. The fecal egg count (FEC) is a measure of the number of parasite eggs per gram of feces. Parasite eggs in the feces serve as a source for environmental contamination and thus translate into transmission potential to other horses in the herd. It is interesting to note that time after time individual horses will have a consistent FEC and occupy one of three categories. A low shedder will have less than 150 eggs per gram (EPG), a moderate shedder 150 to 500 EPG, and high shedders will have values over 500 EPG.

Knowing which horses harbour the vast majority of internal parasites is powerful information. It can enable the horse owner to selectively deworm those individuals that are reinfecting the premises and avoid unnecessary deworming of horses not shedding eggs.

It is estimated that 20 per cent of the horses will be “heavy shedders” and are responsible for 80 per cent of the total egg contamination on the farm. The fecel egg count test is a surveillance tool which identifies those horses that shed the highest number of eggs into the environment. Selectively deworming these horses benefits the entire group of horses.

The goal of parasite control is not to eliminate all intestinal parasites in horses, as this goal is as unrealistic as it is impossible. Rather the goal is to manage the risk of parasitism by identifying those horses that are shedding the majority of parasite eggs into the environment. This test provides the horse owner with much-needed information on an individual horse’s parasite status. This information allows the owner to act accordingly making an informed decision with regard to the timing and the class of dewormer that would be a best fit.

Serial fecal egg counts will reveal an even bigger picture. When a horse with a high fecal egg count is treated with an appropriate deworming product, a second FEC performed 10 to 14 days later should be close to zero.

If the count is above zero, evidence exists that the worms may have developed resistance to that particular drug and a dewormer from a different drug class should be used. By performing fecal egg counts and conducting followup tests you begin to draw a more accurate picture of parasite management on your property.

At times horse farms are large enough to purchase the necessary equipment and educate their caretakers in the fecal egg count procedure, yet for most horse owners it is necessary to involve their veterinarian in their parasite management.

Deworming policies can vary greatly between horse farms, and so parasite management will vary according to stocking rate, age of the horse population, and living and climatic conditions. Fecal egg counts will be instrumental in guiding appropriate recommendations. The parasite challenge for two horses on 10 acres is considerably different than that for 20 horses on 20 acres.

Start by submitting a fecal sample to your local veterinarian, one or two manure balls in a plastic bag or container labelled with the horse’s name, your name and date. The sample can be refrigerated, not frozen, and submitted within 24 hours.

The optimal time to begin sampling would be in the spring prior to giving any dewormer. Egg counts must be done at an appropriate interval after the administration of the last dewormer. Your veterinary can recommend the appropriate interval based on the last drug that was administered.

Although the economic benefits of fecal egg counts and targeted/selective deworming may be slow to reveal themselves, using the dewormers on an as-needed basis is inherently good practice preventing unnecessary medication of horses and preserving long-term sustainable use of our present deworming compounds.

About the author


Carol Shwetz is a veterinarian focusing on equine practice in Millarville, Alberta.

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