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.

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).

Keep in mind that no dewormer is effective if not used in conjunction with good management practices to reduce ingestion of infective parasite eggs or larvae.

Most anthelmintics are administered as oral pastes. In the past, injectable formulations (ivermectin) have been given to horses, but this has proven to be unsafe. Some anthelmintic formulations are available as granules to be top-dressed on feed. Continuous wormers are mostly pyrantel tartrate.

Oral paste wormers are dosed to horses by weight. In most cases, there is a weight measure on the packaging that allows accurate dosing.

YOUR VET'S ROLE

Determine the most appropriate deworming compound for your horses and the best timing of administration. This is done with fecal testing (fecal egg count reduction test).

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.

You also need to properly provide fecal samples to your veterinarian to test the effectiveness of your parasite control program.