Dewormers rank near the top of horse care advancements of the last 50 years, and horses are living longer, healthier lives because of them. Yet, paradoxically, because serious colics and other life-threatening conditions resulting from internal parasites are increasingly rare, it's easy to become complacent about the very routines that made those issues largely a thing of the past.
That's why it's worthwhile to periodically review the basics of parasite control as well as your management routine. You may find that, with a few minor adjustments, you'll be able to improve your horses' health that much more.
A Well-Stocked Arsenal
A wide variety of anthelmintic products are available, but all fall into one of three chemical classes, which are most effective against particular types of worms.
Macrocyclic lactones (avermectins and milbemycins) have the broadest range of activity. Prominent in this class, ivermectin is effective against adults of all the common equine parasites except tapeworms. It is also effective against some larvae and is credited with greatly reducing colic associated with the migrating larvae of Strongylus spp., but it does not kill encysted small strongyle larvae. Ivermectin and related compounds are found in products including Agri-Mectin, Bimectin, Durvet Ivermectin, Equell, Equimax, Equimectrin, Eqvalan, Horse Health Ivermectin, IverCare, Ivercide, IverEase, Phoenectin, Rotectin, Zimecterin and Zimecterin Gold. This class also includes moxidectin, which is similar in action to ivermectin but is also effective against encysted small strongyle larvae. It is the active agent in ComboCare, Quest Gel and Quest Plus.
Benzimidazoles (including compounds that end in "-endazole") are effective against a number of adult parasites. In this class, fenbendazole kills large strongyles, pinworms, lungworms and ascarids; oxibendazole is effective against large strongyles, pinworms, roundworms and threadworms. These are the active agents of products such as Anthelcide EQ, Panacur and Safe-Guard Equi-Bits and Safe-Guard Paste.
Tetrahydropyrimidines (pyrantel salts) have several applications. In this class, pyrantel pamoate controls large strongyles, pinworms and roundworms and, at double-dosing rates, kills tapeworms. Pyrantel tartrate is the basis for daily feed-through dewormers. It controls large strongyles, pinworms and ascarids. These are found in products such as Equi-Cide, Exodus, Kaeco Equine Wormer Pellets, Liqui-Care P, Pellet-Care P, Primex Equine Liquid, Pyrantel Pamoate Paste, Rotectin P, Strongid, Strongid T, StrongyleCare, TapeCare Plus and daily feed-through dewormers such as Continuex, Equi Aid, Nu-Image Guardian and Strongid C.
Another anthelmintic agent, praziquantel, is used to control tapeworms but on its own it has no effect on strongyles or other nematode parasites. Praziquantel is often combined with other deworming compounds in such brand names as ComboCare, Equimax, Quest Plus and Zimecterin Gold.
For optimal impact, you need to use the right dewormers, in effective dosages, at the right times. Which products and schedule are best for your situation depend on many factors, including where you live, the number of horses you keep and your pasture-management practices. Schedule an appointment with your veterinarian to review and possibly revise your schedule, especially if your routines have been in place for some time. Topics to cover include:
Rotation. To achieve the greatest impact, you need to rotate among different chemical classes. Some parasites will survive treatment with a particular dewormer, and if you use the same type of chemical in successive treatments, the survivors will be able to reproduce, with new generations resistant to that anthelmintic. However, if you use a different class of chemical in successive deworming treatments, parasites which survived the previous treatment will be eliminated. But to rotate properly you need to do more than switch brand names-different products can contain identical active ingredients. Check the labels of your dewormers and make sure you are rotating among the three major chemical classes.
Dosing. The dewormer dosage is based on a horse's body weight, and if you underestimate how much your horse weighs, you might not administer enough of the product. Use a measuring tape to calculate your horse's weight based on his girth and length. If you're at all unsure exactly how much your horse weighs, err on the side of giving him too much dewormer. All of the products on the market have been proven to be safe at doses at least five times higher than the labeled use.
Technique. Getting all of the dewormer into the mouth and swallowed can be tricky. Practice your technique to make sure you deliver the full dose. If you're still not confident, consider switching to pelleted products. Some horses may even be induced to voluntarily eat a dose of paste dewormer if it is mixed with their feed or hidden in a treat.