Deworming Strategies

Q: My 7-year-old Quarter Horse mare is too thin. My neighbor suggested I have my vet do a fecal egg count to check for parasites. I try to deworm her every month, but she hates tube dewormer, so I add it to her grain—half in the morning, and half at night. I feed her three flakes of hay twice a day, along with a grain ration recommended for her weight. She eats everything I give her, but her ribs are still showing. Could this be a parasite problem?

Katlyn Kelley, Kansas

A: Intestinal parasites are a possible explanation for your mare’s weight loss, especially if she’s recently lost weight while being maintained on the same feed rations. Other signs of parasitism include a rough hair coat and intermittent bouts of diarrhea.

Difficulty administering a proper dose of deworming medication could put your mare at risk for parasite problems. If she’s chronically under-dosed, the healthiest, strongest parasites are likely to survive—and her problem will probably get worse over time. 

To develop an effective deworming strategy, get an accurate assessment of your horse’s weight. Scales for horses are generally hard to come by, but weight tapes, available from any feed store, can provide a reasonable estimate. 

Many owners think their horses weigh less than they do, so you might be surprised to learn your mare needs more than a single tube to be adequately dosed. I suggest you administer approximately 10 percent more dewormer than recommended for your mare’s weight estimate. This amount is well within safety margins for all available dewormers, and will ensure she’s getting an adequate dose.

Administer the dewormer in a single treatment, directly into your mare’s mouth. If you’re having behavioral issues with her during deworming, work with her on accepting oral medications by giving her a syringe full of sugar water every day. When deworming day comes, ask for an experienced horseman’s assistance to make sure your mare receives all the medication. 

Your neighbor’s suggestion for a fecal egg count is sound advice. A large number of parasite eggs in the sample may indicate your current deworming program isn’t effective, and parasites could be causing your mare’s weight loss. 

However, a negative fecal sample can be misleading. Small strongyles, now thought to be the most threatening parasite to your horse, spend part of their life cycles as larvae, encysted (burrowed) in your horse’s intestinal wall—meaning they don’t shed eggs. Your mare could have a large number of these parasites, while having a negative fecal egg count. 

A high count indicates that intestinal parasites are a problem for your mare, and you should carefully examine your deworming program. Your vet can help you devise and monitor an effective deworming regimen for your mare’s situation. A good program should include several different classes of dewormers to ensure all classes of parasites are controlled, and it should be monitored with regular fecal egg examinations to verify it’s working (see “Rethinking Deworming,” April ’08). 

Even if your mare’s egg count is low, however, I recommend you administer a five-day larvicidal deworming treatment consisting of the medication fenbendazole, given at twice the recommended dose for five consecutive days. This deworming protocol will thoroughly clean out your mare’s system, and it’s effective against larvae burrowed in her intestinal walls that aren’t killed by other dewormers. If your current deworming program is lacking, this method is the best way to get on track. This treatment can also be included in your regular program on an annual basis. 

If you administer a five-day larvicidal treatment, consult with your veterinarian about an effective long-term parasite control strategy, and work with your mare to ensure proper dosing, I’m confident you can eliminate any parasites affecting your mare’s health. If your mare’s weight loss continues, however, there are other factors to consider, such as dental problems that could be making it difficult for her to chew. If you suspect this, schedule an appointment with your vet for a general physical exam to thoroughly evaluate her health. 

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