605 West 5th Street

Morris, MN 56267 US




Internal Parasites

Internal parasites are a primary cause of colic in horses and can cause or contribute to many respiratory, digestive, and performance problems. Although parasite load is a continuous problem among horses, it can be managed very well using a deworming protocol. A horse with substantial parasite load can be perfectly healthy on the outside. Some horses will show signs of parasite infestation, such as a dull coat, lethargy, decreased appetite, diarrhea or loss of body condition.

Equine Deworming Protocols
Deworming protocols should be tailored to best suit your horse's living environment and age. For example, isolated horses may need fewer dewormings than those horses on larger farms. Also, to ensure your protocol is successful and cost effective, the appropriate dewormer must be used at the proper intervals, and rotating dewormers is recommended. Fecal exams are used to determine which dewormer would be the best treatment for your horse.

Common Deworming Protocol

  • Foals should be dewormed every 2 months starting at 6-8 weeks of age until they reach 1 year of age
  • Adult horse should be dewormed a minimum of twice yearly and as often as every 2 months if the parasite load is high.
  • Dewormers should be rotated to minimize resistance among parasite populations
  • Use an ivermectin-based dewormer after the first frost in the fall to control bot flies

Strongyles (Small and Large)
Strongyles are also known as bloodworms. Strongyle eggs are passed in the manure and distributed in the horse's environment. Unlike ascarid eggs, which remain in egg form until they are ingested by the horse, strongyle eggs hatch into larvae in the manure. The larvae then go through a series of three molts to advance through the larvae stages. The third stage can infect the horse when the horse ingests the larvae shortly after the third molt. Strongyle larvae are ingested by the horse while they graze their pasture. This is why a more concentrated horse population on a smaller pasture will likely have a larger load of parasites. Strongyles lay many eggs and one horse can pass up to 100 million eggs per day!

Small Strongyles (Cyanthastomes)
Small strongyles are similar to large strongyles, but they have the ability to encyst or burrow themselves into the intestinal wall and remain there. Inflammation of the bowel is the end result of this encysting. These small strongyles can remain encysted for 2-3 years. The encysted small strongyles will come out of the gut wall when the number of adults goes down. This means that horses only have to pick up this parasite once every 2-3 years for the worm's life cycle to occur.

Large Stongyles (Strongylus vulgaris)
This parasite is a large strongyle and has a peculiar migration pattern. Eggs are passed in manure and develop after they are ingested. When the egg reaches the large intestine, it penetrates the intestinal wall and into small arteries. Migration continues to the artery that supplies blood to the intestinal tract. Larvae then return to the gut to reproduce. This migration can cause serious inflammation and damage to the tissues. This parasite can cause chronic diarrhea and other serious disease processes.

Tapeworms (Anoplocephala perfoliata)
Most tapeworm infections do not cause visible symptoms that we can see, but do cause damage that we cannot see. Horses that develop signs usually have a serious tapeworm infestation and may be unthrifty (poor coat, hard to keep on weight, dull mentation etc.), or anemic (low red blood count). Tapeworm infestations can cause severe damage to the inside of the horse's intestinal tract and are a risk factor for decreased gut motility and multiple forms of colic in horses.

The size of a mature tapeworm can vary from 3 to 8 cm. The head of the parasite has 4 suckers which the parasite uses to attach to the bowel wall. Tapeworms have an indirect life cycle and require an intermediate host to develop. The intermediate host of A. perfoliata, for example, is a mite that is commonly found in pastures and hay. Part of the tapeworm egg's development occurs in the mite and takes about 12-15 weeks. When these mites are ingested by a horse, the horse develops a tapeworm infection.

Unfortunately, tapeworm eggs are also difficult to detect in our traditional fecal floration methods. In general, we recommend deworming your horse for tapeworms every 6 months (in the spring and fall each year).

Roundworms (Parascaris equorum)
Roundworms, otherwise known as ascarids, can lay as many as 200,000 eggs each day. The eggs are passed in the manure and the eggs develop into infective larvae within a tough outer layer. These infective larvae can remain viable for up to 10 years in your pasture! When they are ingested, they hatch. The larvae then penetrate the intestinal wall and a 30-day migration begins. The larvae can travel through the liver and lungs and cause permanent damage. Damage to the lung can cause pneumonia in foals and can cause respiratory difficulties later in life. A large burden of roundworms can cause colic, sometimes even after deworming.

Bots (Gasterophilus intestinalis and Gasterophilus nasalis)
Adult bot flies are commonly seen around horses. The females lay eggs on the horses' hair. Moisture from the skin or from the horse's licking causes the eggs to hatch into larvae. After a 3 week development period in the mouth, the larvae migrate to the stomach and attach themselves to the wall. They remain there for the winter and in 10 months they detach and pass out through the feces. The larvae burrow into the ground and mature. Adults emerge in 3-10 weeks and the life cycle starts again. Horses should be treated for bots in the fall, after a frost that kills the adult flies, and again in the spring.

Pinworms (Oxyuris equi)
Infective pinworm eggs are ingested orally and travel to the colon. While in the colon, the larvae go through various stages and become sexually mature adults in five months. Female pinworms crawl out of the horse's rectum, lay eggs in the sticky substance around the horse's anus and then crawl back into the rectum. Pinworms create very little physical damage to the horse but cause moderate itching to the tail head. Some horses will rub their tails resulting in fraying of the hair at the tail head.

External Parasites

Flies, ticks, mites, etc. are controlled with a variety of medicated topical applications
depending on individual needs. Sprays and pour-ons are available.

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