DOES YOUR HORSE REALLY NEED DEWORMING? Although most horses have intestinal parasites, not all of them need to be dewormed. The goal of deworming your horse is not to completely eliminate internal parasites but to reduce the amount of eggs shed into the environment.
OLD CONCEPT: In the past, many horse owners (and veterinarians) have managed worms using an “Interval Deworming Program” in which all horses were treated with a different drug every 6-8 weeks. This means that each horse was dewormed as many as 8 times a year! In recent years, veterinarians have realized that this system has many disadvantages and are now shying away from this practice. Here are some of their reasons:
1. Approximately 50-60% of adult horses has a natural immunity to gastrointestinal parasites and don’t require frequent treatment.
2. Frequent deworming is leading to drug resistance in parasites. This means that some deworming drugs are becoming less and less effective at killing the parasites and when they do work, they last a shorter period of time. This is well documented in many daily feed dewormers. This resistance is very serious since drug companies are not currently developing new deworming medicines.
3. Environmental issues: Deworming medicines travel through the horse’s digestive system and is excreted still at full strength! These chemicals end up in the soil and contaminate the environment.
NEW CONCEPT: So what is the present-day thinking on equine deworming? The current recommendation is to use a strategy called “targeted” (or “selective”) deworming. This philosophy is to identify horses with a high parasite burden (high egg shedders) and treat only those horses. Research has shown that the major benefit to this strategy is to slow down the progression of resistance in the worm population. It is also better for the environment and prevents unnecessary medication of healthy horses.
For those 50-60% of adult horses with low egg counts (200 eggs per gram or less), a twice yearly treatment with a broad-spectrum dewormer (such as an ivermectin-praziquantel product) is sufficient. This regiment will also eliminate any tapeworms (which are usually not seen on fecal egg counts).
For the 25% of horses that shed a moderate amount of eggs, 3-4 treatments per year are recommended.
The remaining 20-25% of the horses are the “high egg shedders” and are responsible for about 80% of the total egg contamination on a farm and may need to be treated 4-5 times per year.
TEST, TREAT, MONITOR: To determine the effectiveness of your deworming program, serial Fecal Egg Counts (FEC) should be performed. If your horse has a high fecal egg count, it should be treated with an appropriate deworming product and then a second FEC should be performed 10-14 days later. The FEC should be close to zero at this point. If it remains high, you have evidence that the worms in your horse have developed resistance to that particular drug and a dewormer from a different drug class should be used. Your veterinarian can work with you to create a special deworming program for your horse. Knowing the egg reappearance time for high egg shedders is also important information as it tells you if the drug is effective for a shorter than expected period of time. This can be determined by performing a fecal egg count anywhere from 4 -12 weeks post-treatment (depending on the type of dewormer used).