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Horse Dewormer Drug Resistance
 By Winniefield Park   •   2nd Jun 2016   •   1,501 views   •   0 comments
Horse Dewormer Drug Resistance

A regular parasite control program is an important part of good horse care. The most common way horses get internal parasites is picking up parasite eggs as they eat off of the ground, which then hatch inside their digestive system or lungs. Bots eggs are ingested when the horse licks its coat or pecks at the irritation caused by the bot fly as it lays its eggs. Some live their complete lifecycle within the horse. All can steal nutrition and cause damage to internal organs. If a horse is in poor condition, with a potty belly and rough coat itís a good chance parasites are the cause. Parasites can also cause colic symptoms, decreased energy, coughs, diarrhea and in young horses, can slow their growth. Thatís why horse owners must make de-worming part of their overall horse health care plan.

Most tube dewormers are known as anthelmintics. There are several different drugs that are anthelmintics and the most common of these is called ivermectin. Ivermectin works by paralyzing the worms. Another, called moxidectin does the same. Fenbendazole impedes the parasiteís growth. Pyrantel causes paralysis, but in a slightly different way than ivermectin. Praziquantel, like pyrantel is used to target tapeworms, which other medications are not effective against. This medication may damage the tapeworm's cell membranes, but what it does exactly isnít fully understood. Each of these,along with a few others, is one of four different classes or chemical families of anthelmintics used for horses: avermectins, benzimidazoles, isoquinolines, and pyrantels. Each chemical is used on its own, or in combination with another.

Related: 10 Diseases You Can Catch From Your Horse
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Deworming used to be as simple as going to the feed store, picking up a syringe of the appropriate medication and giving to the horse every six to eight weeks. Some used medications that can be put into feed, and itís still possible to have a vet administer medication with a naso-gastric tube. Weíve relied on these medications for decades. But, the practice of regular and frequent deworming has lead to a new problem - parasites that have become resistant to the medications meant to control them. With repeated exposure to the same chemical, worms have genetically changed to resist the effects of that chemical and stay alive and well after the drug has been administered.

Sometimes, an owner will faithfully administer a deworming medication to their horse, only to find the horse still has a substantial, and sometimes damaging parasite load. This happens because the non-drug resistant worms die off, leaving behind the resistant worms, that now are left with greater resources to reproduce. Slowly, the parasite population overall becomes more and more drug resistant as an increasing number of genetically resistant parasites reproduce.

So, how do we prevent resistance? Regular dewormings every six to eight weeks with the same drug are a thing of the past. Even rotating wormers may not be effective as it once was. Veterinarians are now recommending starting with fecal egg counts to determine what type of parasite should be targeted, or even if the horse needs to be treated at all. Generally, a high fecal egg count indicates a high parasite load, but it also may indicate where the majority of parasites are in their life cycle. So, regular counts need to be taken. If a horse generally has a low fecal egg count, then it probably doesnít need deworming at all. If the fecal egg count peaks, it can be assumed the horse needs parasite control to bring the numbers down. Few horses will ever be completely free of parasites. Young horses may need a regular program every six weeks, but mature horses may need only twice a year deworming. Your veterinarian is the best resource for this information.

In addition to chemical warfare, parasites can be combated by cleaning paddocks and pastures regularly, daily if possible. Scattering manure may only scatter parasites eggs horses might otherwise be able to avoid. If scattering is part of your plan, do it in hot, dry weather, keeping in mind that some parasites are extremely hardy and not affected by weather. Rotate your pastures with other livestock that arenít affected by horse parasites like goats or cattle. Compost manure that comes out of your stable, so the heat and sunlight can kill parasite eggs. Keep new horses separate until a fecal egg count can be done. Research natural parasite controls such as herbs, but keep in mind that even Ďnaturalí substances can be harmful, and were replaced by chemical medications because they were ineffective. Parasite control may have become more complicated, but it isnít impossible.

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www.merckvetmanual.com
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