Q: How often should I deworm my horse?

A:    Industry wide, we are all changing the way in which we deworm horses. What we have been accustomed to doing in the past is no longer the best practice. There are obviously many details and each horse/client/farm will have to be addressed individually.

The main idea is to deworm the horses that need it the most, the most often - and those that need it the least, the least often. The reason that this is important is because we are recognizing parasite resistance to the current dewormers that are on the market.

Currently, there are only three main classes of dewormers for horses. The pharmaceutical companies do
not have any new products in the pipeline and we have been told that it would take a minimum of 5 years for a new equine dewormer to hit the market. They have also indicated that if they bring in new products, they will likely be available by prescription only and would likely cost $40-50 per dose!

As a general rule, horses are divided into three categories based on the number of parasite eggs that they are shedding. A "low shedder" has 200 epg (eggs per gram) or less, a "moderate shedder" has 200-500 epg, and a "high shedder" has >500 epg. As a minimum, we would recommend performing FEC (Fecal Egg Count) twice per year. A FEC consists of collecting 2-3 fresh fecal balls and bringing them in to be examined under a microscope. If it is not possible to bring the sample immediately to the clinic it can be stored in the refrigerator for up to 3 days. Most horses will tend to stay in the same shedding range once
that is determined - but it is important to keep in mind that the more samples that are collected and tested on a given horse, the more accuracy there is with that count.

There are several scenarios when it is appropriate to perform FEC. Initially, you have to start sometime and test the entire farm. In order to test the efficacy of a particular deworming product, it is best to test the horses in the beginning (prior to deworming) and then test them again 14 days after deworming. If there is not a 90% reduction in the numbers of eggs after deworming, then there is resistance to that dewormer. If resistance is documented, then that particular class of dewormer should not be used any further.

Once we think we have a handle on what level of shedding is going on with each horse on a farm, then we can move to testing 2-3 times per year for resident horses and testing all new arrivals prior to turning them out.  Another thought about when to perform the FEC has to do with when the horse was dewormed last. Assuming that we are not testing the efficacy of a dewormer, it is best to perform the FEC no earlier than two weeks after the egg reappearance period (erp). For ivermectin, fenbendazole, and pyrantel the erp is about 8 weeks, so the ideal time to test would be at least 10
weeks after deworming. For moxidectin (Quest), the erp is about 12 weeks, so the ideal time to test would be at least 14 weeks post deworming.

Other deworming thoughts:
1. All horses should be dewormed for tapeworms at least once per year (praziquantel).
2. Low shedders typically can be dewormed twice per year (as long as they continue to be low shedders based on FEC)...spring and fall.
3. Moderate shedders typically can be dewormed four times per year.
4. High shedders typically need to be dewormed every 2 months.
5. For horses less than 1 year of age, fenbendazole – double dose (Panacur) and pyrantel (Strongid) are still effective on most farms for ascarids (roundworms).
6. For adult horses older than 1 year of age, ivermectin and moxidectin are still effective on most farms.
7. Once per year, all horses should be treated for encysted small strongyles. This would require a Panacur Powerpac or Quest / Quest Plus.

(Reprinted with permission from Dr. Kerby Weaver, DVM)

Previous Ask a Vets articles

• February 2009