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Deworming Q & A


I’m looking for the 3-way rotation chart. Can you repost this on your website?


We actually removed the rotation chart from our website and catalog because experts now agree that switching between classes of dewormers based on the calendar is neither the best way to protect our horses from parasites nor prevent resistance from developing in the worms themselves. While rotating between the three chemical classes of dewormers made sense when it was introduced  years ago, it’s no longer the best defense against parasites and, in fact, may be doing more harm than good.

First, each of the three chemical classes of dewormers (benzimidazole/pyrantel/ivermectin) has a different egg suppression period. That is, each class prevents eggs from being laid by adult female worms for a different amount of time. While some dewormers “last” for just four weeks, others are effective for six weeks, eight weeks or even more. So a parasite control program that rotates between these classes every eight weeks isn’t taking into account the differences in how long each dewormer “lasts.”

Second, experts have detected resistance to almost all of the chemical classes of dewormers. Throughout the country, there are pockets of parasite resistance where the benzimidazoles or the pyrantels are no longer effective against small strongyles in adult horses. Scientists have also found that, on some farms, ivermectin is ineffective in controlling ascarid (roundworm) infections in young horses. Therefore, it’s important to use a chemical class on your farm that you know works by conducting fecal egg counts.

Third, parasites have life cycles that depend on external environmental cues like temperature and moisture. When it’s below about 40°F, eggs continue to be laid in manure, but they are no longer able to hatch into infective larvae. So there’s no need to deworm after the first hard frost in the fall in northern climates because horses aren’t picking up any new parasites. However, when temperatures begin to warm up in the spring, eggs that were lying dormant on your pasture all winter now hatch, which is why you should begin deworming again when temperatures remain consistently above freezing (it’s the opposite for southern climates). This is also why it’s a good idea to deworm your herd in the spring before they go out on pasture and deposit even more eggs that can then hatch and infect the horses.

I encourage you to sit down with your veterinarian and develop a parasite control program that is appropriate for your horse and geographic area. Take into account whether your horse is a low, medium or high egg shedder and also incorporate other parasite control measures like manure management.




Lydia F. Gray, DVM, MA SmartPak Staff Veterinarian and Medical Director Dr. Lydia Gray has earned a Bachelor of Science in agriculture, a Doctor of Veterinary Medicine (DVM), and a Master of Arts focusing on interpersonal and organizational communication. After “retiring” from private practice, she put her experience and education to work as the American Association of Equine Practitioner’s first-ever Director of Owner Education. Dr. Gray continues to provide health and nutrition information to horse owners through her position at SmartPak, through publication in more than a dozen general and trade publications, and through presentations around the country. She is the very proud owner of a Trakehner named Newman that she actively competes with in dressage and combined driving. In addition to memberships in the USDF and USEF, Dr. Gray is also a member of the Illinois Dressage and Combined Training Association (IDCTA). She is a USDF “L” Program Graduate and is currently working on her Bronze Medal. Find Dr. Gray on Google+

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7 comments on “Deworming Q & A
  1. Rene says:

    Considering the wormers now are less effective, have there been any studies or hard evidence in using Food Grade DE as a feed through de-wormer? I’m presently using DE & will have a fecal count done soon to see if there is any change in parasite load. I used a de-wormer back in November 2012 before I had heard of DE. I have seen a rescue use it on a horse that otherwise wouldn’t have been able to handle the chemical dewormer & they said it worked great. I’ve also read many other articles & business accounts of this product before trying it myself. I was just wondering since you are a vet if you have heard anything good/bad about using DE instead of chemical de-wormers that may be ineffective? Thanks!

    • SmartPak SmartPak says:

      Hi Rene, thanks for your question. We are not aware of any studies that have shown effectiveness of diatomaceous earth and therefore do not recommend its use as a dewormer. We suggest you discuss your deworming schedule with your horse’s veterinarian to come up with the best plan for your area.

  2. Joan Siler says:

    Although I do understand that there may be resistance issues in the rotation wormers, and that the 8 week practice could prove ineffective, I do not understand how you will be able to convince the average horse owner to spend over $25 per fecal egg count (per horse), when they could just throw a dart, pick a wormer, and give their horse a dose that, in most instances, can be picked up for as little as $2.99. I know very few horseman who can afford to drop $25, eight times a year, per horse, for fecal testing.

    I have been fortunate to live in the same area for the 40+ years I have been a horse owner and have always used the rotation wormers suggested for my area of the country, at 8 week intervals with every horse I have ever owned. I have never had a parasite issue. These last three years I have been at a farm where fecal analysis were done twice a year and my mare has been the only one in the barn to repeatedly come up with zero egg counts. We are lucky to live near a state-run lab that will do the fecal counts for us at a very reasonable cost. Our local vets charge over $25/per horse.

    Unless the testing costs charged by local vets can justify the means, I just worry that horseman will find this practice very costly, and continue to use their worming schedules and rotations as they always have.

  3. Lynn says:

    I totally agree with Joan. I had my vet do a fecal egg count on my 2 horses. One came back with zero and the other high for strongyles. So I asked what to do with this information. I was told to deworm both of them with either ivermectin or moxidectin because there is resistance to most other medications. It seems I could have done that without spending the money on fecal egg counts. Unless they both had zero egg counts, its appears you need to continue to deworm regularly.

    • SmartPak SmartPak says:

      Thanks for keeping the conversation going! I can see where it might seem unnecessary to do fecals on your horse when the recommendation might be to deworm anyway. That’s because there is little to no correlation between the number of parasite eggs seen on fecal exam and the number of parasites within the horse’s body! So fecal egg counts are NOT to find out if your horse has worms and therefore needs dewormed (ALL horses have worms ALL the time), it’s to 1) test for dewormer resistance in a herd and 2) categorize individual horses as low, medium, or high shedders. Armed with this knowledge, you and your veterinarian can then choose the correct dewormer and the correct intervals: once or twice a year for low shedders in a well-managed environment or at the end of every egg reappearance period for the various chemical classes for medium and high shedders or any horse in a high-risk environment. Sounds hard but it’s really not, not when you really sit down and think about it. – Dr. Lydia Gray

  4. Julie says:

    Okay, so I just bought my first horse (Percheron mare, 1500 lbs, if that means anything). I just had a fecal done on her through the Smartpak service and she came back as a high shedder (3000/gm for small strongyles). The barn where she lives has a Paddock Paradise-style track and the manure is removed from the entire track every single day. She is part of a small herd of five. We are in Michigan and winter is settling in. Anything you can recommend for a worming strategy?

    • SmartPak SmartPak says:

      Thanks for all the great details. I’m assuming you’ve shared all this with your veterinarian? He or she, being local, is a great resource for controlling parasites in your horse. Don’t be alarmed at the high epg count, she may not have been dewormed appropriately in the past or have built up much of an immunity. I encourage you to continue to run fecals on her (during parasite transmission season and not right after you’ve dewormed her) because this number may continue to go down. While at this point she is technically a high shedder, since you live in Michigan and are going into winter you just need to deworm her once more this fall and then you’re done until next spring. However, during the spring, summer, and fall, she’ll need dewormed every 1, 2, or 3 months, depending on which dewormer you use. It sounds like your barn does a good job of manure removal, but you may want to consider using daily dewormer on her during parasite transmission season to suppress the number of eggs she’s shedding. – Dr. Lydia Gray

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