Mythbuster Monday: Dewormers Lose Their Effectiveness if Given Too Often

On Mythbuster Monday, we tackle a variety of equestrian myths to either bust or confirm. Today’s discussion: Do dewormers lose their efficiency when given to horses too often?

It’s Mythbuster Monday, where Horse Nation dives into different equestrian myths and provides research-based evidence to either bust or confirm those myths. Today’s topic: Do dewormers lose their efficiency when given to horses too often? Does this happen when horses are given the wrong paste for the type of worms the horse has acquired? How long does it take for horses to create resistance to the deworming pastes? Read further to find out!

Myth: Dewormers lose their effectiveness if given too often

Myth or Fact: Fact

Dewormers are the treatment that fights against internal parasites in horses. There are many different products for the different kinds of parasites that can be contracted by equines. The mechanism of a dewormer is to destroy and inhibit the development of internal parasites. Dewormers help to manage the load of parasites so that horses can avoid the health problems that come with an over abundance of these pesky bugs.

But, does giving the dewormers routinely without knowing egg counts and type of parasite create resistance to the medication later?

According to an article from Penn State Extension, Anthelmintic resistance, the ability for parasites to survive dewormer treatment, is a growing concern. Some parasites have become highly resistant, making treatment less effective. One of the reasons for this built up resistance is due to the frequency of exposure to the deworming products. The parasites that are surviving the deworming medications are the parasites reproducing, due to survival, and therefore passing these genes to the next generations.

To stop the parasite population’s resistance to deworming medications, the treatment needs to be individualized to each horse. The American Association of Equine Practice recommends that each horse be dewormed at least one to two times per year. Even if a horse’s fecal count is zero, it should still receive one dose annually because many times fecal counts cannot account for encysted small strongyles that are within the horse’s intestinal system or tapeworms.

Strongyle Egg Shedding Category FEC Results (EPG) Number of recommended deworming
treatments per year
Low shedders 0 to 199 1-2
Moderate shedders 200 to 500 2-3
High shedders >500 3-4

Note: Information in table gathered from the AAEP Internal Parasite Guidelines.

Kentucky Equine Research also published an article discussing the resistance of parasites to deworming strategies. They state that despite extensive research on proper usage of dewormers, many people continue to deworm with a constant rotational schedule. Deworming schedules are no longer proven to be appropriate or economical. This article affirms the importance of only deworming when parasites are proven by a fecal count. The overuse of dewormers when not needed creates resistance to the therapy and no new products have been created for decades.

The Horse writes in their article that historically the recommendation was to deworm horses routinely every two months because research showed that eggs would return approximately two months after initial treatment. However, this strategy quickly created resistance to the medications. Veterinarians and researchers are now recommending that only horses with confirmed parasites should be treated. Fecal egg counts should be performed and only horses with parasite loads should be treated. This way, the correct treatment can be administered the first time and horses who do not need treated at all are not exposed to the medication.

After diving into the research, it’s important to note that horses who are on regular worming schedules may become resistant to deworming pastes. Using these mechanisms on horses who do not need treated decreases the efficacy of the drugs. To minimize resistance, fecal counts should be performed and horses who need treated should be done so with the correct medication.

Do you have an equine myth you’d like us to tackle? If so, send it our way! Email your suggestions to [email protected]. Put Mythbuster Monday in your subject line.