Effectiveness of dewormers and parasite resistance are current topics of concern in equine medicine. There are reported cases of small strongyles being resistant to many commonly used dewormers and there are no new classes of anti-parasitic drugs on the horizon. This has led us to change our recommendations regarding how we treat parasites in horses. The newer recommendations center around “targeted deworming” and identifying horses who are more susceptible to infection/shedding of parasites and treating them. Even horses on a deworming program or daily dewormer should be checked as they may develop parasites that are resistant to these medications requiring special treatment. Horses on the same farm may have differing levels of infection based on age, health, and immune status so it is important to check all horses and follow up on those that appear to be affected.

***Please note that rotational deworming is no longer recommended and promotes resistance.***

Here are the basics:

  1. You will need to deworm all horses twice yearly (after the first frost and again in the spring) with an ivermectin or moxidectin product to kill large strongyles and bots. Moxidectin also kills encysted strongyles that live in the wall of the gut, so it is a good idea to make sure to use that as one of your minimum twice yearly dewormings.  In addition, at least one of these dewormers should contain praziquantal to kill tapeworms which are not always found on fecal examination.
  2. We are recommending fecal egg counts be done on ALL horses in the spring PRIOR to giving any dewormers, this will help to identify horses who are heavily infected and may require continual deworming. It is estimated that only 20% of horses will be “heavy shedders” and need to continue on a regular deworming program every 8-12 weeks throughout the spring, summer & fall. These horses should also have an additional fecal egg count 2 weeks after deworming to determine if the parasites are being effectively killed by the dewormer(s) selected.
  3. It is estimated that 80% of horses will fall into the not infected or “low shedder” category. In general, these horses do not require deworming beyond twice yearly.  Through the spring, summer, and fall these horses should have fecal egg counts checked every 8-12 weeks to see if additional deworming is needed.
  4. Horses under 2 years of age should be routinely dewormed every 8 weeks due to risk of roundworm infection.  This is very important, as roundworms can cause ill thrift, poor growth, and impactions colics (these typically occur after a deworming is missed, and then a dewormer is given).

Please start by submitting a sample (one manure ball per horse is adequate) in a plastic bag or container labeled with the horse’s name, deworming history, your name & date. The sample can be refrigerated but should be <24 hours old and not frozen. The best time to get this sample would be in the spring prior to giving any dewormers; it must be a minimum of 9 weeks after fenbendazole/oxibendazole/pyrantel pamoate, 12 weeks after giving any ivermectin product, or 16 weeks after giving moxidectin. You can bring these samples to the office or we would be happy to collect them during your horse’s spring visit.

After your horse has had its spring fecal test, check this chart to see what treatment & testing strategy to follow:

This chart (click on link at top of page) only applies to “healthy” horses > 2 years of age. Younger horses should be treated as heavy shedders.

Remember: The goal is to decrease shedding of parasite eggs in the environment, not achieve an FEC of 0 epg.