Horse Side Vet Guide ®

Equine Health Resource

Deworming, Moxidectin



Macrocyclic lactones (ivermectin and especially moxidectin) are potent worm killers, 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.

Moxidectin is the newest compound of this class. It is even more potent than ivermectin and is considered the most broad spectrum worming compound available. The most important difference between it and ivermectin is its ability to penetrate the intestinal wall and kill encysted (hypobiotic) larval stages of small strongyles.


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 moxidectin 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.

Do not use moxidectin in young and small equines.

Consider Potential Side Effects & Complications

A neurologic syndrome can occur following overdose of this medication in any horse. This is most common in small horses and foals. Signs associated with this syndrome are 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.

Consider Reasons Not To Use This Treatment

Generally not used in horses under two years of age or smaller equines.

Is It working? Timeframe for effect

These dewormers kill parasites quite rapidly. In cases in which there are large numbers of adult worms or larval bots in the horse, dead worms will appear in the manure within hours to days.

Questions To Ask My Vet

  • Is this medication safe for my horses?
  • How often should moxidectin be given to my horses?
  • What management changes should I make to reduce the worm burden in my horses?
Zoetis Animal Health Quest®, Deworming
Zoetis Animal Health Quest® Plus Equine Oral Gel, Deworming
Zoetis Animal Health Quest® 2% Equine Oral Gel, Deworming
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Author: Doug Thal DVM Dipl. ABVP


Bowman DD. Georgis' Parasitology for Veterinarians 9th Ed. Elsevier 2009.


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