During a horse's lifetime, it often becomes infected with one or more species of parasitic worms. All horse owners should undertake a regular deworming schedule to maintain their animal's health. Parasitic worms infect the digestive tract. The worm's immature larva travels through the horse's tissues. Uncontrolled, the worms can cause serious injury or death to the horse.
Stables, paddocks and pastures can remain infected by the eggs of the ascarid for years. The parasite's eggs have a hard exterior that resist even the most adverse weather conditions. The eggs lay dormant until a horse accidentally ingests the egg while grazing. Once in the horse's system, the egg hatches. The larva penetrates the tissue of the horse's gastrointestinal tract and migrates through the blood stream. It takes up residence in the horse's lungs. The larva matures further in the horse's lungs until the animal coughs up the worm and re-ingests it. The worm reaches full maturity in the horse's small intestine and begins to multiply. The horse passes the eggs in its manure. The whole process takes approximately 10 weeks to complete. The large worms measure up to 12 inches (31cm) in length. A heavy infestation can rupture the horse's intestine and lead to its ultimate death if not controlled.
All broodmares should undergo worming within 24 hours of foaling to prevent the foal from suffering from a threadworm infestation. The mare can pass the parasite to her foal while nursing if not properly wormed prior to birth. The foal can also become infected when the larva enters the young horse's skin in the pasture. The larva travels through the foal's bloodstream and into its lungs where the horse coughs up the larva and swallows it. Once inside the intestinal tract, the larva grows. A 4-day-old foal may suffer from an infestation of the worms. The horse may exhibit diarrhoea, weight loss and failure to grow. Adult horses rarely suffer from threadworms.
The eggs of the bloodworm hatch from the manure within one week of defecation. The worm's larvae can survive for months in the soil of paddocks, stables and pastures. The horse can accidentally swallow the larvae while grazing or drinking from contaminated water. The larva often climbs to the tips of the grass, so a horse will more easily swallow it. The larvae use the horse's bloodstream to drift around the horse's gastrointestinal tract. The larvae can cause aneurysms to form in the horse's blood vessels. Blood clots can form and cause the horse to experience life-threatening colic.
Although not life-threatening, the pinworm can cause severe anal itching and discomfort. The horse will rub its behind against various surfaces to relieve the itching. The animal's tail hair often becomes rubbed off. The horse can even cause tears and lacerations to occur to the area as it seeks to relieve the itching. The parasite resides in the horse's large intestine. The female worm leaves the horse's rectum to lay her eggs around the anus. The eggs form a crusty residue. The egg-filled residue flakes away from the horse's anus, so other horses inadvertently swallow the eggs.
- University of Missouri Extension; Controlling Internal Parasites of Horses; David K. Hardin; May 1997
- The Equestrian and Horse: Intestinal Threadworms
- Medic Animal: A Complete Guide To The Types Of Worms Affecting Horses And The Drugs Effective Against Them
- Horse and Horse Information: In-Depth Information about Worms and Other Common Horse Parasites
- North Dakota State University; Internal Parasites of Horses; Charles L. Stoltenow, et al; Jan 2003
- Practical Horsekeeping: Equine Pinworms: Ick.