October 14, 2017

As we’ve discussed elsewhere on this site and regularly remind our clients, the inappropriate use of equine dewormers — administration of the same wormer repeatedly and deworming too frequently — is a major cause of concern for horse owners.

The evidence shows that overuse of deworming medications can lead to strains of drug resistant parasites. Perhaps this a contributing factor to the increased incidence of acute larval cyathstominosis (small strongyles) in Ontario in recent years.

Reviewing your horse’s parasite management program

To protect the health of your horse, it’s more important than ever to implement a comprehensive parasite management program.

Good environmental management is essential

Throughout the year, the number one method of keeping parasite loads to a minimum is an effective pasture management program. Indeed, it’s been shown that, when it comes to minimizing pasture larval counts, the elimination of feces from a horse’s pasture is just as effective as administering dewormers.

Establish a pasture management routine

We encourage the following as part of your pasture management plan:

  • Daily removal of feces
  • Low stocking density in paddocks
  • Rotate pastures if possible
  • Harrow only in periods of hot/dry weather
  • Regular fecal examinations to assess worm burdens

The main parasites of concern to horses are:

  • Ascarids
  • Strongyles
  • And, to a lesser degree, tapeworms and bots

(Ascarids tend to be a bigger issue in foals. We’ll address this topic separately in a future issue of our newsletter. In the meantime, don’t hesitate to contact us if you have any question about foal or brood-mare deworming.)

In this article, we will talk mainly about the management of strongyles, bots and tapeworms.

Deworming medication

Deworming medication is a necessary part of a comprehensive deworming program to control gastrointestinal parasites in your horse.

When is the best time to administer it?

As the weather gets colder, the strongyle larvae can become encysted in the lining of the gut of the horse, making it more difficult for deworming products to reach them and kill them off. Come spring, the surviving encysted larvae can “wake up,” mature and start reproducing like crazy.

Currently, the recommendations for equine parasite management in our geographic area are as follows:

  • Deworm in the late fall, before the strongyle larvae have a chance to encyst
  • Deworm again in the spring, when these encysted forms start to awaken and become active

In the late fall, your best option is to use a deworming medication containing both moxidectin (for the stronglyes and bots) and praziquantel (for tapeworms). We’ve had good results with, and recommend, a product called Quest Plus.

There is often no need to deworm healthy, adult horses through the cold winter months, unless specific horses are being targeted. When you start your deworming program in the spring again, it is recommended to use the same dewormer throughout the spring and summer months. As opposed to switching between deworming medications in the same year, this will help to reduce the resistance issues that we are seeing.

When multiple horses are living together on a farm and all are receiving deworming medication, it’s helpful to place them on the same treatment schedule.


Parasite management is an area of growing concern for horse owners. Remember, when developing a deworming strategy your horse, you must consider not only the animal’s identified parasite burden, but also the physical environment.

If you have questions about parasite management, don’t hesitate to ask. We’re here to help!


Allossery Equine Veterinary Services
11499 York Durham Line
Mount Albert, Ontario
L0G 1M0

(289) 338-2068