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.

Targeted Treatment
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.

Additional Efforts
Dewormers are a central part of your parasite-control program, but some management measures can contribute to their effectiveness:

Minimize exposure to manure for pastured horses. Given enough space–about two acres per animal–horses will naturally divide a pasture into eating areas, called lawns, and toileting spaces, called roughs. Careful management can prevent roughs from taking over too much space and exposing horses to contaminated grass.

Rotational grazing, moving horses periodically to other enclosures or alternating pastures with other livestock, allows the larval stages to emerge and then die off without finding hosts.

Mowing and harrowing pastures. Open up manure balls and expose eggs and larvae to the drying effects of air and sunlight. To be most effective, these tactics need to be implemented during the strategic points in the parasites’ life cycles-usually in the hottest, driest weather of the summer. Ask your veterinarian for more detailed advice about your locale.

Remove manure at least weekly from crowded pastures as well as paddocks and pens. Without routine manure removal, smaller or crowded turnout areas would become nothing but roughs. Manure buildup isn’t as critical a health issue in freezing or extremely dry weather, but sticking to a habitual year-round schedule will help keep the chore manageable.

Compost manure before spreading. Avoid spreading fresh manure on active grazing areas. If you want to improve your soil, composting is a good option: the heat generated during the process kills parasite eggs in manure, making it safe to return it to pastures.

Feed horses in manure-free conditions. Ground-level feeding is healthiest for horses in several ways, but the practice does increase the opportunity for fecal contamination. Place hay and feeds well away from established roughs. Remember that horses will also scavenge for dropped hay and feed around raised feeders; be vigilant about removing manure from these areas.

Deworm new horses before turnout with residents. Also keep transient visitors separate from permanent farm residents, and clear their manure promptly and thoroughly from stalls and confinement areas.

Progress Assessment
Fecal egg counts can help you plan your overall parasite-control program and monitor how well it is working. As the name implies, these tests determine the concentration of parasite eggs in manure, primarily the eggs of small strongyles, the most common parasite of mature horses.

Do-it-yourself kits are available, and with the purchase of a few pieces of readily available laboratory equipment, most people could learn to perform the tests and interpret the results. You can also have the tests professionally done by your veterinarian or an independent laboratory.

The standard protocol calls for performing a fecal egg count on each horse in a herd just prior to deworming and then again 10 to 14 days later. Ideally, the count should decrease by 90 to 100 percent in that time. If it doesn’t, then you have an indication that your efforts were not effective at killing egg-laying adult parasites. If that is the case, then you need to reassess whether you administered the correct dosage or whether you rotated active ingredients properly. Most modern anthelmintics are still highly effective against parasites in horses, but if you think you did everything right, ask your veterinarian about managing resistance issues.

Fecal egg counts can help you plan your deworming schedule. After an anthelmintic is administered, egg counts will remain low for a predictable interval called the egg reappearance period (ERP). The normal ERP is four weeks for benzimidazole- and pyrantel-based products, six to eight weeks for ivermectin, and nearly 12 weeks for moxidectin. So, for example, after administering moxidectin, you could wait 12 weeks before scheduling the next treatment. Fecal egg counts can help you confirm when the mature worms resume laying their eggs, and you can optimize your use of dewormers accordingly.

Once you and your veterinarian have established a program tailored to your herd’s needs, yearly recounts will help make sure your efforts stay on track.

This article originally appeared in the June 2008 issue of EQUUS magazine