Deworming: It’s Not One Size Fits All

By Wendy Mollat, DVM, DACVIM, Pilchuck Veterinary Hospital

This article also could be titled “Out With the Old, In With the New.” Why? Most of the current deworming recommendations for our equine friends are more than 40 years old. Needless to say, a lot has changed since that time. Historically, parasite control programs have centered on rotational treatment with anthelmintics (dewormers) at 60- to 90-day intervals. This protocol came about because the large strongyle was the most problematic intestinal parasite in the horse at that time. The lifecycle of a large strongyle from ingestion to maturity (egg laying) was about two months; therefore, with the anthelmintics available at that time, treatment every 60 days was recommended. 

Today, the situation has changed dramatically: The small strongyle is now the primary parasite of concern in adult horses, and roundworms are the primary problem in horses less than 2 years old. So not only is the rotational strategy outdated, but the frequent use of anthelmintics has led to high levels of resistance in both of these parasites to our current selection of anthelmintics. Worse yet, there are no new products being developed by the pharmaceutical companies, so we need to make what we have last!

Current protocols favor fecal egg counts (FECs) to measure an individual horse’s parasite load – reported as eggs per gram (EPG) of parasite eggs. In adult horses, the majority of eggs in the manure will be small strongyle eggs. Research has shown that 20% of the horse population is responsible for shedding 80% of the parasite eggs in the environment. We need to target that problematic 20%. This can be done with annual FECs on any horse over 2 years of age, and use this information to categorize the horse as a “Low” (<200 EPG), “Moderate” (200 to 500 EPG) or “High” (>500 EPG) shedder. 

“Low” shedders only need to be dewormed twice per year (spring and fall) with an ivermectin class dewormer (Equimax, Equalvan, Quest, Ivermectin, etc.). “Moderate” and “High” shedders will need one to two additional treatments per year, or they can be treated with a daily dewormer if recommended by your veterinarian.

Additional key points to keep in mind:

  • Use annual FECs to determine an individual horse’s parasite burden.

  • Do not spread manure on pastures – this only spreads the parasites.

  • Routinely (every one to three days) remove manure from stalls, pens and pastures.

  • Know your horse’s body weight and give the appropriate amount of dewormer each time.

  • Quarantine any new horses from communal pastures until FECs have been performed to ensure they are not going to contaminate your pastures.

Pilchuck Veterinary Hospital offers comprehensive wellness and preventive equine care, including strategic deworming protocols specific to your horse or group of horses. Call 360.568.3111 for more information.


Article added 2/21/2014