As equine veterinarians, we have an obligation to preserve the future effectiveness of current medications so that they can continue to help horses in years to come. In the past, parasite control programs involved aggressive rotational deworming at two month intervals. These programs were geared toward eliminating the large strongyle bloodworm (Strongylus vulgaris) and proved to be very successful as these infections now rarely occur.
Small strongyles (cyathostomins) and tapeworms (Anoplocephala perfoliata) are the primary parasites that current deworming focuses on. However, the previous aggressive deworming programs have led to concern for parasite resistance to the drugs that are currently available. There are no new products in development with pharmaceutical companies, and it would take a minimum of 5 years for a new equine dewormer to hit the market. Therefore, we must change the way we approach deworming in horses to ensure that the drugs we have will continue to function appropriately against our horses’ parasites.
The new approach to deworming in horses is to treat the horses with the higher parasite loads the more frequently, and those with the lower parasite loads less frequently. In order to appropriately address the individual horse, a fecal egg count (FEC) is required to determine how many parasite eggs a horse is shedding. A FEC consists of collecting 2-3 fresh fecal balls and examining them under a microscope. The test can be performed on fresh feces, or samples that have been stored in the refrigerator for up to 3 days.
There are several scenarios when it is appropriate to perform a fecal egg count:
- Twice yearly testing on every horse on the farm. Testing should be done no earlier than two weeks after the egg reappearance period of the last dewormer used:
- For ivermectin, fenbendazole, and pyrantel, the egg reappearance period is about 8 weeks. Thus, the ideal time to test would be at least 10 weeks after deworming.
- For moxidectin, the egg reappearance period is about 12 weeks. Thus, the ideal time to test would be 14 weeks after deworming.
- Testing of any new horses prior to being turned out in a new pasture situation.
- Testing the efficacy of a particular deworming product. This requires testing prior to deworming, and then again 14 days after deworming. The reduction in egg count should be at least 90%. If less than 90% reduction, there is evidence of parasite resistance to that dewormer and that class of dewormer should no longer be used.
As a general rule, horses are categorized into the following classifications based on the number of parasite eggs that they are shedding:
1. “Low Shedder” : has 200 eggs per gram or less and typically only needs to be dewormed twice a year in the spring and fall.
2. “Moderate Shedder”: has between 200-500 eggs per gram and typically needs to be dewormed 4 times a year.
3. “High Shedder”: has greater than 500 eggs per gram and typically needs to be dewormed every 2 months.
Other deworming recommendations:
- Horses should be dewormed base on an accurate weight. This is especially important for very large or very small horses/foals to avoid over or under dosing.
- All horses should be dewormed for tapeworms at least once per year with a product that contains the drug praziquantel. The most ideal time to deworm for tapeworms is in the late fall, after a frost.
- Once per year, all horses should be treated for the encysted stage of small strongyles. This requires either a Panacur Powerpac or a moxidectin product, such as Quest or Quest Plus.
- For adult horses (older than 1 year of age), ivermectin and moxidectin are still highly effective against strongyles on most farms.
- Foals should be dewormed every 60 days for the first year of life, starting when they are 30-60 days old. This is to help control ascarids (round worms). Deworm based on product label.
As every horse and farm is unique, it is best to work with your veterinarian to develop the ideal program for your horse.
