Written by: Dr. Steve Manning

Targeted deworming practices to prevent drug resistance.

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When new anthelmintic (deworming) medications were first discovered and introduced in the 1960s and 1970s, horse owners were quick to accept the drugs along with the practice of regularly deworming their animals throughout the year.

But in the past five decades, overuse of deworming products has led to resistance, meaning worms can develop genes that allow them to become resistant to the drugs. Resistance limits our ability to treat horses with worm problems. This is a growing issue with serious consequences, since heavy parasite burdens in horses can lead to grave health conditions such as severe colic.

It’s no longer considered good practice to give your horse deworming products indiscriminately. Veterinarians and horse owners need to be more strategic about how to combat and control parasites. Strategic deworming considers all aspects — the parasite life cycle, the environment and the horse.

Before coming up with the best deworming strategy, we need to consider each horse’s overall health and environment. Through research, we also know different horses in the same herd will have different parasite burdens.
While most horses shed very few worm eggs, a small number of horses shed high numbers of eggs and are more responsible for infecting the rest of the herd. That’s why it’s important to customize your deworming practices for each horse in your herd. Treat horses as individuals – or as select groups based on age, immune response and overall health – and appropriately target your high-shedding horses so you don’t overuse deworming medication.

Fecal Egg Counts for Horses

Most horses only need to be dewormed once or twice a year. Before deworming in the spring, have a fecal egg count (FEC) done on each of your horses. This procedure, which allows your veterinarian to measure the number of worm eggs shed in its feces, is indicative of the horse’s parasite burden. Based on the results, your vet can recommend whether you need to deworm your horse or not.

It is recommended that horses be dewormed in the late fall, after a hard frost, with an appropriate deworming product. It is a good idea to conduct a second fecal egg count in the fall, but many owners elect not to do this second procedure. Your local veterinarian can advise you about what deworming product to use in the fall based on the common parasites in your area.

Horses that are Vulnerable to Parasites

Pregnant mares should be dewormed in the spring before they foal with a dewormer chosen based on the results of a fecal egg count. Mothers should be dewormed with an ivermectin product 24 hours after foaling.

Foals need much more frequent deworming than other horses. They should first be dewormed around two months of age with fenbendazole, and then retreated with this product every two months until they are yearlings. In the prairie provinces and other areas where temperatures reach below freezing, foals do not need to be dewormed through the winter. Foals should be dewormed with an ivermectin-type product around 12 months of age and then every three to four months with a product chosen based on their fecal egg counts.

Immune-compromised horses are also more susceptible to parasite development. Help your horses develop a strong immune system by providing them with good quality hay and feed along with 24-hour access to clean water. Ensure that their vaccines are up to date and schedule regular dental examinations with your veterinarian.

Controlling Parasites in Your Horse’s Environment

Parasites can also be found in pastures where they develop from eggs in the feces to larvae that can infect your horse. Since parasites require specific temperatures, humidity and other circumstances to develop into infective larvae, one effective way of controlling them is to break the life cycle of parasites before they reach your horse. To control parasites, use the following practices:

  • Pick up manure and remove it from your horse’s environment
  • Prevent overgrazing and reduce pasture contamination by limiting the number of horses per acre
  • Avoid spreading non-composted manure in any pasture currently being used for grazing, as this practice spreads parasite eggs
  • Rotate pastures with other species to break parasites’ life cycles
  • House young animals separately from adult animals as they carry different parasites

By targeting high-shedding horses in your herd, maintaining your horses’ health and managing their environment, you can minimize your use of deworming medication and reduce your contribution to the development of parasite resistance while still keeping your horse healthy.