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Design a Top-Notch Deworming Program

Here's a simple, two-step plan to help you design a top-notch deworming program that is best suited for your horse.

When it comes to deworming your horse, are you flying by the seat of your pants? Time to get grounded. A haphazard deworming program puts him at risk for increased exposure to parasite eggs and larvae.


To learn more about deworming your horse, download a FREE guide—Deworming Your Horse: How to find the best deworming schedule for you and your horse.




My simple, two-step guide will help you design a top-notch deworming program that's best-suited for your horse. The first step helps you decide whether to put him on a periodic ("purge") program, or give him a daily dewormer with his feed. The second step covers three parasitic "troublemakers" that aren't always killed by a general dewormer. Then I give you tips on how to stick with the program you select. Finally, I detail two day-by-day sample deworming programs, and give you the rundown on six effective dewormers.

Step 1: Choose Your Schedule
First, decide whether to put your horse on a periodic ("purge") program with a paste dewormer or give him daily dewormer in his feed. To help you determine which program is right for your horse, I'll briefly explain a worm's life cycle, then how each deworming program works to interrupt that cycle. Next, I'll give you a short quiz to help you determine your horse's specific needs.

A worm's life cycle: Your horse ingests most worms that plague him as larvae. Some of these larvae (such as large and small strongyles) migrate through his body tissues and end up in his gut, where they mature and lay eggs. He then passes these eggs in his manure, where they hatch into larvae and spread into the environment. He ingests the larvae, and the cycle is repeated.

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How a purge program works: It breaks the cycle primarily by killing adult worms before they lay eggs in your horse's intestines. By reducing the number of eggs, it'll minimize his exposure to parasites-unless he shares grazing space with other horses on less-than-effective deworming programs. If you choose this option, rotate different classes of deworming medication. Cost: About $80 per year.

How a daily program works: It kills the early-stage larvae your horse picks up in his mouth before they penetrate his body tissues. Once larvae start migrating through his body, they can cause damage. So, by killing them before they begin migrating, daily dewormers minimize his internal parasite levels even if his environment is heavily infested and/or out of your control. Cost: About $144 per year.

QUIZ: Now see how you score on the following eight questions to determine which program is right for your horse.

1. Does your horse graze on pasture all year, increasing his chances of exposure to parasite larvae? (Yes=2. No=0.)

2. Do other horses, on different or unknown deworming programs, graze on the same pasture, increasing your horse's chances of ingesting parasite larvae? (Yes=3. No=0.)

3. Does your horse nibble grass at other stables or public horse facilities-such as show grounds, fairgrounds, campgrounds, and/or highway rest stops-increasing his chances of ingesting parasite larvae? (Yes=5. No=0.)

4. Has your horse ever shown signs of heavy worm infestation? (Symptoms include a poor haircoat, weight loss, recurrent colic, or sloppy manure; or a fecal egg count of more than 100 eggs per gram.) (Yes=4. No=0.)

5. Is the collected manure at your horse's facility spread on the pasture as fertilizer, increasing the chance of parasite larvae in his grazing pastures? (Yes=3. No=0.)

6. Is "dropped" manure in your horse's grazing areas spread out with a harrow at least once a year? (Yes=3. No=0.)

7. Do you have a hard time keeping track of which dewormers can be used in a rotation program-possibly disrupting a purge program?(Yes=2. No=0.)

8. Do you delay scheduling your horse's regular-care appointments, such as farriery, dentistry, vaccinations, and deworming? (Yes=3. No=0.)

Here's what your total score means.

0-8: Your horse's management and general condition are good enough that a well-timed purge program probably is adequate. It'll minimize parasite eggs in his manure, and his risk of internal damage from worm larvae picked up in the environment is probably minimal. (Exception: If you answered "yes" to questions 3, 4, and/or 5, risk of damage increases; consider a daily dewormer.)

8-15: You're in a gray area. Although a well-timed purge program will minimize worm eggs in your horse's manure, other factors- such as a high concentration of parasite larvae in his environment- may expose him to internal damage.

15-25: Your horse is exposed to high levels of parasite eggs and larvae in his environment. Use a daily dewormer to protect him from internal damage caused by larvae migration.

Step 2: Target Troublemakers
Whether you choose purge or daily deworming, you won't kill some dangerous parasites unless you take additional steps. These troublemakers are bots, tapeworms, and encysted cyathostomes (one of the most destructive immature forms of small strongyles). Here's a general program to fight these parasites, but check with your vet to develop a program right for your horse and your particular area.

Bots. Ivermectin and moxidectin are the only available products effective against bots. In a purge deworming program, you can kill two birds with one stone by using one of these products on your regular late-fall and spring treatment dates. Time of year is critical, because fall's' first frost kills bot flies, giving you a leg up on reducing their population-especially if you follow up in the spring. Here's what to do: After first frost, remove/kill any remaining bot eggs or larvae on your horse's legs with a bot block or knife. Then use a purge dewormer to get rid of adult bots in his system. In spring, remove/kill any external eggs or larvae you may've missed in the fall, and deworm him again to zap any adult bots in his stomach before they lay eggs. Then you'll start bot season (spring through early fall) with a clean slate.

If your horse is on a daily program, give him a dose of ivermectin or moxidectin in early spring and again in late fall, in addition to the daily dewormer.

Tapeworms. Some investigators believe daily deworming effectively controls tapeworms, but the evidence is conflicting. As an extra measure, you have three options: 1) give pyrantel pamoate (Strongid P or T), at twice the usual dose, 2 days in a row; 2) give pyrantel tartrate (daily dewormer), at 10 times the usual daily dose, 2 days in a row; or 3) use of the canine tapeworm medication prazi-quantel (Droncit), which your vet can prescribe for oral use in your horse (about $45 a dose). You can use options one or two to replace your horse's regular deworming treatments in spring and fall. Give Droncit in addition to the regular deworming treatment, but on a different day, to avoid possible drug interactions.

Encysted cyathostomes. Prevent encysted cyathostomes by putting your horse on a daily deworming program, or kill them by: 1) using moxidectin as a spring and/or fall treatment in your purge deworming program; or 2) replacing a regular spring and/or fall purge treatment with fenbendazole at twice the usual dose, for five days in a row.

Stick-to-it Tips

  • For daily programs, it's critical that your horse gets his daily dose daily, as missed doses will decrease the levels of dewormer in his system, rendering it less effective.
  • For purge programs, timing is key. If you treat too early, targeted worms will be too immature to be affected by the dewormer. If you treat too late, adult worms will have the opportunity to produce eggs, infesting your horse's environment and raising his (and other horses') risk of exposure.Here are my four stick-to-it tips.1. Buy the whole year's worth of dewormer at once. If you don't have it on hand, you may miss a critical deworming date.2. Label each tube with each horse's name and the date to be given.

    3. Keep your dewormers handy. But store them out of reach of children and pets.

    4. Post a calendar prominently in your barn. On it, clearly mark the day before each deworming day to give you time to prepare. On the scheduled day, set out your dewormer where you can't miss it.

Karen Hayes is an Idaho-based equine practitioner.

This article first appeared in the June 1999 issue of Horse & Rider magazine.

For more information on deworming, check out Deworming Your Horse: How to Find the Best Deworming Schedule for You and Your Horse, a free guide from MyHorse Daily.

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