Parasites and Your Horse

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Internal parasites, or worms, are silent thieves and killers. They can cause extensive internal damage without you even realizing your animals are heavily infected. The effects of internal parasites on a horse range from a dull haircoat and unthriftiness to colic and death. Internal parasites lower the horse’s resistance to infection, rob the horse of valuable nutrients , and in some cases, cause permanent damage to the internal organs. The lifecycle of most internal parasites involves eggs, larvae (immature worms), and adults (mature worms). Eggs or larvae are deposited onto the ground in the manure of an infected horse. They are swallowed while the horse is grazing, and the larvae mature into adults within the horse’s digestive tract (stomach or intestines). With some species of parasite, the larvae migrate out of the intestine, into other tissues or organs, before returning to the intestine and maturing into egg-laying adults.

FECAL EGG COUNTS

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One of the most useful tools in a parasite control program is the fecal egg count—microscopic examination of fresh manure for parasite eggs. This test allows the veterinarian to determine which parasites are present and whether the infection is light, moderate, or heavy. This information is important in developing a deworming program for your horse or farm, and in monitoring the effectiveness of the program. It is important to note that a negative fecal examination does not mean the horse is free of internal parasites. Some types of parasites produce eggs only intermittently. Larvae do not produce eggs at all, and may be present in large numbers in a horse with a fecal egg count of zero. And tapeworm eggs may be missed with routine fecal egg count techniques. The results are most useful when several horses on a farm are tested on the same day. This information gives the veterinarian and farm manager a good idea of the level of parasitism on the property.

DESIGNING A DEWORMING PROGRAM

There is no single deworming program that suits all horses and all situations. The ideal program for your horse(s) depends on the type, number and ages of the horses on your farm, pasture management and your geographic location. It is best to have your regular veterinarian help you devise an appropriate deworming program for your horse or farm.

A COMPLETE MANAGEMENT PROGRAM

Chemical control using dewormers is just one part of a complete parasite control plan. As parasites are primarily transferred through manure, good management is essential:

  • Keep the number of horses per acre to a minimum to prevent overgrazing and reduce pasture contamination with parasite eggs and larvae
  • Pick up and dispose of manure regularly (at least twice a week, even in dirt or sand yards)

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  • Do not spread manure on fields to be grazed by horses; instead, compost it in a pile away from the pasture
  • Mow and harrow pastures periodically to break up manure piles and expose parasite larvae to the elements (larvae can survive freezing, but they cannot tolerate extreme heat and drying for very long)
  • Consider rotating pastures by allowing sheep or cattle to graze them, thereby interrupting the life cycles of equine parasites
  • Keep foals and weanlings separate from yearlings and older horses to minimize the foals’ exposure to ascarids and other parasites
  • Use a feeder for hay and grain rather than feeding on the ground
  • Remove bot eggs regularly from the horse’s haircoat (flea combs work well in some instances)
  • Consult your veterinarian to set up an effective deworming program for your horse(s) and monitor its effectiveness.