Dealing With Equine Resistance to Dewormers.


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Dealing With Equine Resistance to Dewormers.

Morgan Murphy

Dealing With Equine Resistance to Dewormers.

Most horse owners knows the risks of parasitic infection in their horse. In some of my previous blog posts we have discussed the species of worms that commonly attack horses, such as tape and redworms. For years, horse owners have taken preventative measures to maintain their horse's overall health through regular worming.

Over the past decade, equine research facilities have reported a worrisome trend: some worms, particularly cyathostomes, are growing increasingly resistant to traditional dewormers. This is a growing concern to veterinarians, equine professionals and owners as there are currently no new drugs under development to replace "anthelmintics," the class of drugs used to fight parasite worms. Many professionals are concerned that equine worm populations are quickly becoming totally resistant.

All Grazing horses at any age should be subject to a deworming program *



How Do You Combat Wormer Resistance?

Currently, "Interval Dosing" is commonly used and promoted by drug companies. This is a year round program where wormer administration is based on the egg reappearance periods after treatment with wormer: e.g. dosing with Ivermectin every 8 weeks as this is how long it takes for eggs to reappear in the manure after treating with Ivermectin. If you are using this method, it is important to understand the various efficacy periods for each active ingredient.

Although worming is a sound and respected strategy, many horses are actually "over-wormed." Only 20% of all horses have a severe parasite burden. “Overworming” may actually play a major role in the growing wormer resistance problem, we now face.

"Strategic Dosing," is a program where horses are only wormed at times when parasite burdens are thought to be highest: deworming for encysted cyathostomins in winter is a good example. "Targeted Strategic Treatment," is a Fecal Egg Count lead program (FEC) which takes into account both the life cycle of parasites and environmental factors (e.g. pasture management) and the individual levels of worm burden in individual horses. Regular fecal egg counts are taken from horses to identify their individual worm burdens and their worming needs. Horses with high or medium egg counts are de-wormed while horses with acceptable eggs counts are left untreated. This reduces overuse and helps to curtail resistance. Low egg counts, though they clearly indicate some worm presence, should be left untreated to help the horse build more natural immunity. However, bear in mind that FEC testing only detects adult redworms. You should still use a dewormer effective against larvae, encysted larvae and tapeworm for all grazing horses during the winter months, regardless of FEC results.

In addition to deworming schedules, pasture care and maintenance can assist owners in productive worm control, helping to further minimize dependence on wormer drugs.

1. Do not allow horses to over graze

2. Remove feces regularly from pasture (twice weekly)

3. Ensure the dose of wormer is carefully calculated for each horse: do not over or under dose.

4. Rotate grazing wherever possible

For more hints and tips on horse care, visit Morgan’s blog again soon.

*Image courtesy of Bigstock Images

  • This is one of those really worrying topics that I am really glad you have addressed, I have been rotating wormers for the past few years and I am very conscious of not over using any particular groups but I also feel as horse owners we lean towards the chemicals with longer efficacy ( 13 weeks plus) as they also appear on teh outset to offer greater protection