Strategic Deworming

A common question that many horse owners ask is “What should I use to deworm my horse?” In the past, parasite control programs involved aggressive rotational deworming at two month intervals. These programs were geared toward eliminating the large strongyle bloodworm and proved very successful as all three major drug classes are very effective against large strongyles and heavy infections are now rare.

Small strongyles (cyathostomins), roundworms (ascarids), bots and tapeworms are the primary parasites that current deworming protocols focus on. Gone are the days of rotating dewormer brands every two months. Dewormer resistance is quickly becoming one of the biggest issues facing equines of today. What were once effective dewormers are now no longer treating horses as effectively for parasite burdens as they once did. To compound the issue, there are no new dewormer drugs in development for the horse. 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 more frequently, and those with lower parasite loads less frequently. A fecal egg count (FEC) is a simple, useful tool for evaluating a horse’s parasite load. It involves analyzing a horse’s fresh fecal sample to gauge the number of parasite eggs per gram (epg) of manure.

Generally, horses are categorized into the following classifications based on the number of parasite eggs that they are shedding:

·         Low shedder: has 200 eggs per gram or less and typically only needs to be dewormed twice a year in the spring and fall.

·         Moderate shedder: has between 200-500 eggs per gram and typically needs to be dewormed three times a year.

·         High shedder: has greater than 500 eggs per gram and typically needs to be dewormed four times a year.

Even horses who come back with an FEC of zero, does not mean that they are free of parasites. It simply means that they are currently not shedding any eggs. Besides, an FEC will not detect bots or tapeworms, so should still be treated as low shedders during the spring and fall with an appropriate drug class.

Beyond an FEC is a fecal egg count reduction test (FECRT). An FECRT is the only way to accurately assess parasite resistance to a particular drug class. This test involves obtaining a fecal sample, testing it for an epg count, administering a specific drug class, and then coming back in 10-14 days and retesting another fecal sample. The difference between the two samples (pre and post-treatment) are calculated as a percent reduction. If the percent reduction is not at least 85% or greater, then one should suspect resistance to the particular drug class used.

For the Southeast region, spring and fall deworming is all most horses need, unless regular FECs say that a given horse needs to be dewormed more frequently. Fall, after a good freeze (or about 6 months after the spring treatment), is the best time to treat for bots, as the temperatures will all but eliminate them. It is also a convenient time to treat for tapeworms. Since the macrocyclic lactones (ivermectin & moxidectin) are the only things that kill bots, you would want to use Equimax, Quest Plus or Zimectrin Gold with the added praziquantel to also address tapeworms. Then in the spring, it is still a good idea to address any lingering bots and tapeworms that might have overwintered in the horse and treat once the temperatures are consistently above 45-50 degrees. For moderate and high shedders you would want to add one or two additional treatments through the winter months and may include one of the following drug classes: macrocyclic lactones (ivermectin & moxidectin), benzimidazoles (fenbendazole & oxibendazole) and/or pryrimidine salts (pyrantel pamoate). For the latter two drug classes mentioned, these are also ones where an FECRT should be employed to determine the efficacy of the drug class used, as they are often associated with increasing resistance issues.

In summary, changing the thought process about equine parasite control has been an uphill battle. We have tried for 50+ years to clear a horse on pasture entirely of internal parasites and have failed miserably. Thus, the new goal is to focus on those horses that are shedding the most and treat them accordingly. For the rest of the population that are routinely low shedders, we simply need to focus on maintaining a small population of internal parasites that are still managed by the drug classes we currently have.