Horse Dewormer (Wormer)
What Kind and How and When to Use Them

    A horse that is not dewormed regularly and effectively often has a dull coat, dull attitude, pot belly, persistent winter coat, more frequent episodes of colic, and is repeatedly depressed or "off" in performance.  When a parasite-infested horse is fed, the resident parasite population grabs its share first.  The wormy horse soon becomes debilitated.

How Often?

    To most effectively break the reproductive and developmental cycle of common internal parasites, most horses should be dewormed every eight weeks, year round.  There are four exceptions to this rule:

    1.  Horses in crowded turn-out areas with continually changing population.  This environment, prevailing at many boarding stables, usually is host to a high parasite concentration.  In this case, deworming every six weeks or possibly daily (Strongid C) may be beneficial. This decision should be made in conjunction with post-treatment fecal exams and veterinary consultation.

    2.  Foals under one year of age and very old horses. Foals under one year of age are susceptible to heavy parasite infestation. We recommend deworming at one month of age, and then every month until weaning. After weaning, deworm every six weeks until 12 months of age, then every eight weeks thereafter.

Threadworms can also be a problem for foals under six months of age. They are not common in the older horse. Ivermectin and oxibendazole (a benzimidazole) are effective on threadworms.

    3.  Older horses, in their late teens, often have a weaker immune state than adult horses in their prime and are more susceptible to parasites. Older horses may require deworming every six weeks, especially if they experience any of the digestive tract problems that often accompany old age.

    4.  Horses that share a large pasture with a relatively unchanging population. If a small, unchanging herd is on a large pasture and has been on a six times a year deworming program for a year or two and fecal egg counts are low, it may be practical to decrease the deworming frequency to four times per year.

Forms of Dewormer

    Dewormers are available as paste, pellets, powder and liquid suspension.

    • Paste in a syringe-type tube is the most common form available to horse owners. Some dewormers (ivermectin and febantel) are low volume so are more convenient to administer.

    Larger volume dewormers can be messy in paste form and significant amounts can end up on the ground if the horse is uncooperative or has food in his mouth when the paste is given. For best results, rinse horse's mouth with several large syringes of water.  Wait until all water has dripped out and then administer paste.

    • Pellets are designed to be fed mixed with grain. Some horses will not eat grain with dewormer pellets in it, however.  This is a major drawback to purge dewormer pellets, those that are fed once every eight weeks to rid a horse of parasites.  You can't count on all of your horses eating all of the dewormer at once.

    There is an important distinction between the two types of pelleted dewormers currently on the market.  One type is designed to be fed once every 8 weeks as a regular purge dewormer.  The other type of pelleted dewormer is Strongid C which is designed to be fed at a low level (4.8 oz per 1200# horse) every day to prevent parasite infestation.  Strongid C is available over the counter (OTC).  Because this product if fed every day, a horse becomes accustomed to it.  Strongid C2X, a new only to veterinarians (OTV) product from Pfizer is twice the strength of Strongid C so you only have to feed 1/2 the amount (2.4 oz. per 1200# horse) each day.

    • Powder is usually available only to veterinarians and is designed to be reconstituted and used as a liquid.

    • Liquid suspensions can be given as an oral drench or via stomach tube.  An oral drench is simply a large syringe full of the liquid to be squirted in the horse's mouth much of which can end up on your sleeve if you are not careful.

Liquid is traditionally administered by the veterinarian using a stomach tube, a long flexible, plastic tube passed through the horse's nose and into the stomach for a "direct hit."

    Tubing gained popularity when early products were caustic to the lining of the horse's mouth and esophagus and had to be placed directly into the stomach. It was also often the only way to get the dewormer into finicky eaters.

    But tubing can be risky: medication may get into the lungs, the horse's nostrils may bleed, and/or horse or handlers may be injured during the process. Recent  research shows tubing is no more efficient than paste deworming, and I don't feel it is worth the risk.

Chemical Classes   

     The decision on which chemical class to use involves knowing what parasites you want to destroy, what chemical is effective against those parasites, and what brand name product is best.

     You should be concerned with strongyles, roundworms, and pinworms with each deworming. At least once a year, you should use a dewormer that is effective against tapeworms.

     In the early spring and late fall, you should target bots. The drugs that are effective against bots include ivermectin and dichlorvos.

      However, dewormers from the organophosphate category, which includes dichlorvos, have safety and/or palatability considerations so ivermectin is the current drug of choice for bots.

      The lower efficacy of piperazine on pinworms and large strongyles makes it less useful "across the board" than ivermectin, the benzimidazoles, and the pyrimidines. But if you have a persistent problem with roundworms or small strongyles, you might consider a piperazine product.

     The benzimidazoles have many sub chemical classifications that can be confusing and arguable. Fenbendazole is a more effective, broad spectrum drug with a high safety factor. Oxibendazole is a good choice for foals as it is effective against threadworms at a 1.5 dosage.

      Note that benzimidazoles are on the American Horse Shows Association list of forbidden masking substances. This means benzimidazole has been shown to cover the evidence of other drug use in blood samples and is forbidden in AHSA competition.

      You must administer the drug at least 24 hours before your competition. The AHSA does not want your deworming schedule disrupted because of competing. However, you must go to the show office and file the proper form with the show secretary if you have used a benzimidazole anthelmintic within seven days of your show.

     Double-dose Strongid and praziquantel will kill tapeworms. Horsemen rarely had to worry about tapeworms before ivermectin came into the picture. Tapeworms were usually only found in very young horses and were more prevalent in the wetter areas of the country. But tapes are on the rise because while ivermectin effectively eliminates other worms, it leaves the door wide open for tapes to infest the horse. Now there are products that combine ivermectin and praziquantel which are designed to kill all major equine parasites in one dose.

      If you choose to use Strongid to kill tapeworms, you will need to "double-dose" your horse. It is safe. However, read the label for weight and dosage. Most pastes, for instance, are for an 1,200-pound horse.

     So "double-dose" means two tubes of paste for the mature adult horse. However, a 500-pound pony would only need one full tube. If you suspect tapes, consult your veterinarian before administering treatment. He or she may recommend praziquantel.

      Although tapeworms are not as damaging as other worms, especially strongyles, an annual double-dose of strongid or a normal dose of praziquantel should be administered as a precaution. If you have a known tape infestation, you might want to deworm for tapes twice a year. The larvae are very hardy and will live outside the horse for up to seven months. (If you are feeding the daily dewormer Strongid C, ask your veterinarian if you should stop feeding before you dose your horse for tapeworms.)

     Encysted small strongyles and bots are not killed by normal deworming products. Moxidectin kills encysted small strongyles and bots and provides 84-day suppression of strongyle eggs.

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