How Dewormers Work

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With all the discussions and worries about internal parasites becoming resistant to dewormers, it would be good for us as horse owners and managers to actually understand how anthelmintics work. In this article from Kentucky Equine Research's Bryan Waldridge, DVM, DACVIM, we can get some basic information about parasites and dewormers.

Resistance to dewormers (anthelmintics) has been a huge problem in goats and sheep for many years. In horses, anthelmintic resistance is becoming more frequent worldwide.

The most common reasons for anthelmintic resistance are underdosing the animal, overuse of dewormers and not tailoring deworming programs to fecal egg per gram counts or the times of year when anthelmintics are most likely to be effective. When parasites develop resistance, they are likely to be resistant to all the anthelmintics in that same drug class. Parasites can also be resistant to different anthelmintic drug classes.

When resistance to an anthelmintic occurs, a different class of drug usually must be used to effectively deworm the animal (if there is no other resistance). This article will discuss different anthelmintics currently used and how they work against parasites. The active drug contained in a dewormer can be found on the box or in the product insert. Your veterinarian is the best source of advice for anthelmintic choice and strategic deworming protocol.

Fenbendazole interferes with parasite growth and energy pathways.

Ivermectin and moxidectin enhance the release of inhibitory neurotransmitters, which cause paralysis of worms. Ivermectin and moxidectin are used against bot larvae and several other parasites.

Praziquantel is used against tapeworms. Its exact anthelmintic actions are not known, but it is believed that praziquantel damages parasite cell membranes that interfere with electrolyte concentrations. Praziquantel may also inhibit the ability of tapeworms to move and may directly kill tapeworm segments.

Pyrantel also paralyzes worms, but acts similarly to neurotransmitters that cause muscle to contract. Double the normal label dose of pyrantel is effective against tapeworms, as well as some other parasites.