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Medical Index
First Posted: Jan 12, 2008
Jan 21, 2020

Equine Worming Schedule

by Debora Johnson

"There are only three classes of chemical dewormers (benzimidazoles, tetrahydropyrimidines, and macrocyclic lactones), and parasites have developed either established or developing resistance to all three. Combined with the fact that there are no new chemical dewormers in the pipeline for horses (a process that usually takes a minimum of five years), horses with once-treatable parasite-related health issues are now in the pre-1960s position of having no (or very few at the least) treatment options."
Novel Equine Deworming Principles, Procedures in the Works

AAEP Parasite Control Guidelines Developed by the AAEP Parasite Control Subcommittee of the AAEP Infectious Disease Committee/Revised 2013
Delaying Dewormer Resistance: Advice Offered in Study
Drugs for the Deworming War

The results of a recent study, conducted by Kentucky, researchers indicated that roundworms and small strongyles are developing resistance to most commercially available worming products. Apparently, there are no new drugs in the pharmaceutical pipeline to take up the slack.

"Evaluation of parasiticidal activity of fenbendazole, ivermectin, oxibendazole, and pyrantel pamoate in horse foals with emphasis on ascarids (Parascaris equorum) in field studies on five farms in Central Kentucky in 2007" Published in the July 2008 edition of Parasitology Research.

The following was a recommended worming schedule. Most professionals suggested that horses should be wormed every six to eight weeks alternating among Strongid, Panacur, and Ivermectin, including one dose of Equimax, Zimectrin Gold, or Double Strongid, yearly. The last three mentioned wormers are designed to keep tapeworms under control. I have a fecal done in the spring when the vet does the shots. That way, the horse's parasite load is known. If there is a problem, a panacur pack or similar treatment can be given to reduce the parasite load and get it under control. If one horse is given the pack, all should be given the pack at the same time. Horses should all be wormed at the same time to keep parasite infestation under control.

  • January-Ivermectin
  • March-Strongid
  • May-Ivermectin
  • July-Panacur
  • September-Ivermectin
  • November-Equimax

If your horse tends to be prone to parasites, you may want to worm on the off months with Ivermectin. Neck threadworms (Onchocera, Microfilariae) will usually respond to extra doses of Ivermectin. I have a special compounding done at Wedgewood Labs. If your horse has Neck Threadworms (Onchocera, Microfilariae) contact me and I will give you the compounding formula that I use. It has so far worked really well on one of our horses who came to us with threadworms in his eyes. Neck Treadworms (Onchocera)

Now there have been some changes in worming habits because of parasite resistance to the wormers. Check with your vet to see what they are now suggesting. Below you can read updates that we have learned by attending seminars concerning this issue. We try to keep on top of this issue. The latest research indicates that there is Ivermectin Resistance in Small Strongyles.

Also, I recently attended a seminar which was outstanding. Harold C. McKenzie III DVM. MS, DACVIM, Marion duPont Scott Equine Medical Center, Leesburg, VA was one of the guest speakers. He was beyond excellent. Dr. McKenzie shed lots of light on the problems in worming and gave lots of information. Parasites and Worming

Note: In 2009, it was suggested to us that because of parasite resistence to the wormers we change our worming habits. Now we have a fecal done in the Spring, at least two or three months after the last worming. All three horses at the barn were done at the same time. (It is important that all horses be wormed at the same time so that they do not continue to reinfect each other) The lab results showed negative on all three horses, so therefore, we were told to worm as follows:

We now worm approximately every 4 months varying the wormers to the time of year the horses are being wormed. This way the parasites are less likely to build up resistence to the different wormers. We also do a wormer in the late Fall using Zimectrin Gold to get rid of tapeworms. In the Spring if the fecal shows that there is a heavy load of parasites we use a Panacur Pack. For the horse that has Onchocera, we give him an Ivermectin, in the off months, only if he begins to show any symptoms or increased activity of that parasite. We also increase his compounded pills for two weeks. Instead of giving one pill once a week we give him two pills each week for two weeks. So far this has worked very well for our horses.

Note: This came to me in an e-mail and I thought it was excellent information that should be shared:

Hi Debora,

Thank you for putting this very helpful information on equine parasites on the web, especially with pictures to help identify the different worms. I thought I should ask that you add some information and advice to other horse owners.

Before and after worming, horse owners should do a fecal check for parasites to find out if their horses have a heavy worm load and if their wormer is effective. Before worming a fecal test will give an indication of the parasite load on the horse and about 5-10 days after worming a fecal test will tell you if your wormer is working. There should be a significant reduction in the egg count in the fecal sample after worming. Simply worming every 2 months may cause resistance in worms. Horse owners should also rotate their class of wormer to prevent resistance (ie ivermectin and praziquantal are two different classes of wormer).


Monica Treidlinger
Oxford Mills, Ontario

For More Information:

Equine Parasites
External horse parasites: flies, nats, lice, mites, chiggers, mosquitoes, ticks, bots

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