There are 4 main classes of deworming compounds (anthelmintics) used to control internal parasites in horses: benzimidazoles, pyrantels, macrocyclic lactones (ivermectin, moxidectin), and praziquantel. Different classes have different effectiveness against different types of parasites.

Unfortunately, indiscriminate use of worming compounds has caused internal parasites to develop varying levels of resistance against these compounds. This means that many of the compounds are less effective than they once were.

Worming on a rotational basis (as has been recommended over the past 30 years) is likely contributing to the resistance problem. Rotation is no longer recommended. Deworming should now be performed based on need for a particular population of horses, and on fecal examinations performed by your vet.

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).

Management techniques are the true cornerstone of effective parasite control – even more important than the use of deworming drugs. The most important aspect of management is prompt removal of manure from the environment, before worm larvae can be ingested by horses.

Common deworming drug types used in horses include:

A class of compounds called “Benzimidazoles” have been a mainstay of equine parasite control for over 40 years. These chemicals interfere with a worm’s energy metabolism on a cellular level, causing a slower kill of the parasites than the so-called “paralytic compounds”. Familiar examples of benzimidazoles are fenbendazole (Panacur®) and oxibendazole (Anthelcide® EQ). These continue to have good activity against Ascarids, but small strongyles are now mostly resistant to this class. For this reason, these drugs should probably only be used in young horses. They should not be used as part of a rotation unless recommended by your veterinarian based on fecal results. Benzimidazoles are relatively safe and unlikely to cause side effects.

Another class of de-wormer includes pyrantel pamoate and pyrantel tartrate (the familiar trade name Strongid®). These drugs act at the junction between nerve cells and muscle cells, causing paralysis, and a more rapid kill of worms than the benzimidazoles. Pyrantel does not penetrate the intestinal wall and so will not kill encysted strongyles. There is now significant resistance to pyrantel among strongyles. Because of this fact, pyrantel should only be used in selected circumstances as guided by your veterinarian.

Pyrantel comes in several forms, a paste, suspension for tube worming, and at low levels in a pellet (continuous wormers like Strongid-C®). Pyrantel at very high dose may have activity against tapeworms but praziquantel is likely more effective. Pyrantel is generally very safe for all age classes of horse.

While pyrantel-containing continuous wormers have been implicated in resistance, they still may have a niche role for select horses on a farm, for a selected time period, but never for life.

Macrocyclic lactones (Ivermectin & Moxidectin) – Ivermectin has been around for about 30 years and has been our most relied upon wormer during that time period, but there is evidence now that certain parasites are developing resistance to it.

Ivermectin and Moxidectin are potent at even low levels. They work by blocking nerve transmission and paralyzing worms. Unlike the other drug classes, macrocyclic lactones also kill external parasites like lice, mites, and larval skin forms involved summer sores. They kill bots very effectively.

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. It is also marketed in pastes combined with praziquantel to cover Tapeworms. Ivermectin is quite safe at doses up to 5 times the recommended dose.

Moxidectin is a more recently developed drug than ivermectin, and has a superior ability to penetrate into the intestinal wall and kill encysted strongyles. It probably is the most effective compound for this purpose. This drug is not recommended in horses less than 2 years of age, or for smaller equines.

One to two treatments of macrocyclic lactone (either ivermectin or moxidectin) per year will probably control both large and small strongyles, bots and other important parasites in most horses. Unlike other classes of dewormer, both ivermectin and moxidectin are effective against bots and many external parasites.

Praziquantel only kills tapeworms. It is currently marketed for horses only in combination pastes with either Ivermectin or Moxidectin. Praziquantel is probably also being overused, especially in regions that have very few tapeworms. Talk to your vet about whether this medication is needed in your region.

To date, natural or organic remedies (including diatomaceous earth) have not been shown to effectively kill parasites. If you rely on these products, you may be putting your horses at risk.