Most horse owners invest large amounts of money and time in controlling, rather than treating, worms in horses. The reasons for this are simple, but frustrating:

  • Horses often have more than one type of worm. This means that more than one type of de-worming medication is needed, because not all worms respond to the same types of treatment.
  • A horse may show no signs of worm infestation until a severe episode of colic, diarrhea or weight loss occurs. By that point, the damage may be so severe that the horse will die.
  • Adult worms can be killed more easily than worm larvae, especially if the larvae have burrowed into the intestinal walls and become “encysted.” (Think of how caterpillars wrap themselves into cocoons to develop into butterflies). So it’s necessary to set up a regular “de-worming” schedule to catch successive generations of parasites.

As a result of these and other mitigating factors, treating worms in horses demands constant attention to three controls: cleaning pastures, scheduled treatment with de-worming drugs and diagnostic tests to determine heavily infected horses that can be dosed with stronger concentrations of worming drugs.

Pasture cleanliness

Keeping down the parasite populations in horse pastures is a real chore. That’s because when infected horses drop their feces, worm eggs are deposited into the soil and onto vegetation that other horses consume, becoming infected themselves. Many species of parasites can lay dormant in their eggs in pastures for extended periods, developing into larvae when environmental conditions are good for their survival.

Research has found that regularly removing horse droppings from grazing pastures significantly reduces the risks of transmitting worms between horses. In addition, studies show that techniques such as pasture rotation, and grazing sheep or cattle on the same pasture, also reduce horse parasite populations. Sheep and cattle can eat horse worm eggs without becoming infected, so they act as natural controls on the parasites.

Scheduled de-worming drug treatments

Horse owners once dreaded de-worming their horses, because it mean inserting a nasogastric tube through the horse’s nostril, down its throat and into its stomach in order to administer de-worming medication. Now, however, new de-worming drugs come in oral pastes and feed additives, making it possible to treat horses for worms more easily. Tubing may still work best when a horse is diagnosed as heavily infected and needs a high dose of medication to kill off the worms.

What’s more, regular de-worming has been shown to kill off some 90 percent of adult worms. However, a drawback to this method is that it means treating horses that may only be minimally infested with worms. It also means that the horse owner must plan for a regular purchase of expensive de-worming medicines. In some cases, however, this may be the best way to control the worms where pastures have been found to been highly infested or horses seem susceptible to re-infections.

Diagnostic tests

Given the prevalence of horse worms,  horse owners and breeders for years have simply assumed that all their animals were infected and dosed them all accordingly. Today, however, veterinarians have been much more skilled at performing and interpreting diagnostic tests to determine the parasite burden (amount of worm infestation).

For instance, all it takes to identify the severity of adult redworm infestation is to do a worm egg count on a sample of a horse’s droppings. This method also diagnoses roundworm infestation. Tapeworms have been much harder to detect, but a new blood test for tapeworm infection has provide promising.

The use of diagnostic tests to target de-worming treatment is helping horse owners to spend less on medications, to delay or avoid the development of drug resistance in certain worm species, and to reduce the environmental impact that certain drugs have on pastureland.

Consequently, the best treatment for worms in horses is to control or prevent worm infestations as much as possible.

Horse Worm Treatment and Control
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