Horse Side Vet Guide ®

Equine Health Resource

Deworming, Ivermectin



Ivermectin has been the most important equine anthelmintic (anti-worm) drug since its development in the early 1980’s. For equine use, it has been the main representative of a revolutionary drug class called the macrocyclic lactones. Macrocyclic lactones work by blocking the parasite’s nerve transmission, causing paralysis and death. The drug is effective against most species of internal parasites and some external parasites. It is especially effective against bots.

Ivermectin is a potent worm killer, even at doses much lower than other classes of dewormer. Both moxidectin and ivermectin also kill external parasites, such as lice, mites, ticks and the skin-dwelling larvae of parasites such as Onchocerca and Habronema.

Ivermectin is relatively safe in horses, and has been shown to be safe in foals, breeding stallions and pregnant and lactating mares.

Ivermectin is mostly used as 1.87% paste formulations but can also be purchased as a 1.87% liquid, which may be more cost-effective for large numbers of horses. Injection of ivermectin is not recommended in horses.

The problem with ivermectin is common to all the anti-parasitic drugs at this time. Inappropriate use is resulting in the emergence of drug resistance in the parasite population. For this reason, it is critical that this drug be used as part of a targeted worming program based on veterinary fecal testing.


The usefulness of any anthelmintic medication depends not only on the effectiveness of the drug against a particular parasite species, but also on the drug’s pharmacology (how much of the chemical is exposed to the parasite in question and for how long), characteristics of the host animal (general health and immunity) and characteristics of the parasite (not only its susceptibility to the anthelmintic but also its location in the body and susceptibility at various life stages to the drug).

Your vet suggests ivermectin as the best choice deworming compound based on results of fecal diagnostic testing and identification of specific parasites affecting your horses.


Horse owners play a vital role in the use of anthelmintic drugs. All anthelmintic drugs should be administered as part of a targeted deworming program along with management techniques to reduce intake of infective parasites.

Random rotational use of dewormers without fecal testing is leading to parasite resistance and the rapid loss of effectiveness of this and other deworming compounds.

Consider Potential Side Effects & Complications

Ivermectin is a relatively safe drug in horses. Very rarely, ivermectin causes neurologic signs in horses. These signs include wobbliness, weakness, depression and even seizures.

In horses with brain disease or injury (loss of the blood-brain barrier), the use of ivermectin could be more dangerous.

Ivermectin can be toxic to Collie breed dogs. Ivermectin paste intended for a horse but inadvertently ingested by a Collie breed dog could be toxic.

Consider Reasons Not To Use This Treatment

Ivermectin should not be administered to horses that have brain disease or injury.

Ivermectin should no longer be used on a rotational basis without fecal testing.

Ivermectin is not effective against tapeworms (Cestodes), Flukes (Trematodes), or encysted small Strongyles.

Ivermectin has poor activity against ascarids. For this reason it is no longer recommended in horses under 1 year of age.

Is It working? Timeframe for effect

In most cases, you will see nothing after administration of an anthelmintic drug. However, in horses that have large visible worms (ascarids), you may see dead worms in the manure for a day or two after treatment.

In horses for which chronic parasitism is suspected, you can expect to see improvements in weight and haircoat appearance within weeks following treatment.

The only reliable way to determine the effectiveness of a particular deworming compound is to perform a fecal egg count reduction test (FECRT). A fecal egg count is performed on the horse. The dewormer is administered. Two weeks later, a second fecal egg count is performed. If the dewormer is effective, then the egg count of the second sample should be zero. Talk to your vet about creating a proper deworming protocol for your horses.

Questions To Ask My Vet

  • Do ivermectin products have any place in my parasite control strategy?
  • How often should you do fecal exams?
  • What management changes should I make to reduce the worm burden in my horses?
  • When should fecal testing be performed to determine the effectiveness of my deworming program?
Bimedia Inc. Equimax® Paste (ivermectin 1.87% praziquantel 14.03%), Deworming
Bimedia Inc. Equell® Paste (ivermectin 1.87%), Deworming
Bimedia Inc. Bimectine® Paste (ivermectin 1.87%), Deworming
Merial Zimecterin® Paste 1.87%, Deworming
Merial Eqvalan® Paste 1.87%, Deworming
Merial Zimecterin® Gold Paste, Deworming
Horse Health Horse Health Equine Ivermectin Paste 1.87%, Deworming
Farnam IverCare® Paste (ivermectin 1.87%), Deworming
Zoetis Animal Health Equimax, Deworming
Med-Pharmex Animal Health Iversol, Deworming
AgriLabs Agri-Mectin® Paste 1.87%, Deworming
Durvet Animal Health Inc. DuraMectin™ Paste (ivermectin 1.87%), Deworming
Med-Pharmex Animal Health Ivermectin Paste 1.87%, Deworming
First Companion Iverchoice® Equine Paste 1.87%, Deworming
First Companion Ivermax® Equine Paste Dewormer, Deworming
Henry Schein® Animal Health Parid EQ™ Liquid for Horses, Deworming
Henry Schein® Animal Health Parid EQ Paste™ 1.87%, Deworming
RXV Ivermax™ Equine Oral Fluid, Deworming
Phoenix Pharm Ivercide™ Equine Paste 1.87%, Deworming
VedCo ProMectin E™ Paste, Deworming
VedCo Equi-Phar ProTal™ Suspension, Deworming
Sparhawk Laboratories Inc. SparMectin-E, Deworming
VetOne Vetrimec™ Equine Oral Liquid for Horses, Deworming
VetOne Vetrimec™ Paste 1.87%, Deworming
Vetoquinol USA Equimax (ivermectin/praziquantel), Deworming
Vetoquinol USA Bimectin (ivermectin), Deworming
Durvet Animal Health Inc. Ivermectin Paste 1.87%, Deworming
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Author: Doug Thal DVM Dipl. ABVP


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