Horses’ parasite burdens
- Every horse is an individual and different horses in the same herd will have different worm (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.
- It’s important to deworm horses strategically so owners appropriately target their high shedding horses and not overuse dewormers.
- Overuse of deworming products leads to resistance. Worms can develop genes that allow them to become resistant to the dewormers — making the medication ineffective.
- Resistance limits our ability to treat horses with worm problems. These issues can lead to severe colic and death in affected horses.
- Most horses only need to be dewormed once or twice a year. Before deworming in the spring, we recommend having a fecal egg count (FEC) done. This procedure allows us to measure the number of worm eggs a horse is shedding in its feces. Based on the results, we will recommend whether you need to deworm your horse.
- We recommend that horses be dewormed in the late fall, after a hard frost, with an appropriate deworming product. Your local veterinarian can advise you about what deworming product to use in the fall based on the common parasites in your area.
- We strongly recommend conducting a second fecal egg count in the fall, but many owners elect not to do this second procedure. A fecal egg count is included in the spring herd health packages offered at many local boarding facilities.
- 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, foals do not need to be dewormed through the winter when temperatures are below freezing.
- Foals should be dewormed with an ivermectintype product around 12 months of age and then every three to four months with a product chosen based on their fecal egg counts.
Roundworms (Parascaris equorum)
- Most common in horses less than one year old and in very old animals due to decreased immunity
- Migrate through the trachea and live in the small intestine
- In young horses, roundworms cause poor growth and development as well as respiratory signs. Young animals with very heavy burdens can have obstructed intestines — leading to colic and possibly death.
- Older affected horses may have a poor hair coat, weight loss or decreased performance
Bloodworms (Strongylus vulgaris)
- Can migrate through the arteries around the large intestines and cause colic
- Bloodworms used to be a major problem in horses, but their occurrence and significance has decreased with appropriate deworming
- Live in the large intestine and migrate into the wall of the intestine
- Cyathostomins cause severe gastrointestinal signs such as decreased appetite and diarrhea
- Resistance to dewormers is increasing in these worms, so targeted deworming is especially important
Pinworms (Oxyuris equi)
- Most common in horses less than two years old or in horses with poor management
- Worms live in the large intestine, but they deposit their eggs around the horse’s anus — causing the horse to have an itchy hind end.
- It’s difficult to treat and control pinworms so good horse and pasture management is important
- Tapeworms are uncommon on the prairies, but the most common tapeworm in Canada is called Anoplocephala perfoliata. Since these worms spend part of their life living on mites, they are difficult to control.
- Tapeworms don’t seem to cause much disease in horses. While colic is considered a risk with this type of worm, the incidence is quite low.
Bots flies (Gasterophilus)
- Bot flies lay their eggs on the horse’s coat in late summer and early fall. The eggs show up as small yellow specks, mainly on the horses’ legs.
- Horses ingest bot eggs while licking or scratching their legs, then the eggs develop into larvae in the equine stomach and small intestine. The larvae are passed out through feces and hatch in the spring.
- Bots rarely cause clinical signs in horses, but they can cause gastric ulcers.
Ask your veterinarian for advice on what specific products to use.
Fenbendazole: Targets roundworms, bloodworms and pinworms
Ivermectin: Targets all parasites except tapeworms. However, resistance to ivermectin is high in roundworms.
Ivermectin and praziquantel: Targets all parasites. However, resistance to ivermectin is high in roundworms.
Moxidectin: Targets all parasites except tapeworms. However, resistance to moxidectin is high in roundworms.
Moxidectin and praziquantel: Targets all parasites. However, resistance to moxidectin is high in roundworms.
Pyrantel: Targets bloodworms, pinworms and roundworms.