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.

YOUR VET'S ROLE

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.

YOUR ROLE

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.