Equine Worms - what they are & how to combat them | All Creatures Healing

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Equine Worms - what they are & how to combat them


Equine worms are a constant problem for horse owners everywhere. Heavy worm burdens kill foals and elderly horses, reduce the performance of show horses and severely reduce any horse's vitality.Worming is a vital part of equine health care and horses that have a high worm burden will be prone to losing condition or weight, having a poor appetite, and diarrhea. In severe cases, worms can also lead to digestive problems (such as colic) because of the damage they can do to the bowel. (Couzens, 2006)

Most conventional wormers are divided into four different chemical groups:

  1. Macrocyctic lactones (Ivermectins and milbemycins), EqvalanEquest, Eraquell, Bimectin, Vectin, and Equimax
  2. Benzimidazoles, Panacur (fenbendazole) Telmin (mebendazole)
  3. Pyrantel Embonates, Strongid P, and Embotape
  4. Praziquantel, Equimax, and Equitape

Macrocyctic lactones treat bots, small redworms, large redworms and lungworms.

Benzimidazoles treat roundworms, large redworms and small redworms.

Pyrantel Embonates treat large roundworms, large redworms, small redworms, seatworms/pinworms and tapeworms (all species).

Praziquantels treat all three species of tapeworm. (Worming wordpress, 2008)

Most conventional worming is done every 6-8 weeks as a preventative measure, and typically there are general worming schedules that are suggested for horse owners to follow. Most programs suggest performing fecal egg counts (FEC) and blood tests to make sure a worming program is successful. Generally a worming schedule would go as follows:


Worming Performed

January to March

Routine Worming



May to September

Routine Worming




Encysted Redworm



 Table 1: Recommended worming schedule for horses (NSW AG Dept, 2008)

More and more people are choosing to use natural methods of worming for their horses, especially since worms have started to build a ressistance to chemical methods.

Some natural ways of worming:

  • Homoeopathic remedies
  • Herbal remedies
  • Raw Fresh garlic in the feed
  • Copper sulfate in the diet

There are 6 types of worms most commonly found in horses: Small strongyles, large strongyles (bloodworms), Pinworms (Oxyuris equi), Tapeworms, Ascrid worms (large round worms) and Bots (Gasterophilus spp).

Small Strongyles

There are up to 40 species of small strongyles in several genera have been found in the cecum and colon of domestic equids, each with its own site of preference. They belong to the subfamily Cyathostominae of the family Strongylidae and approximately 10 species are particularly prevalent. Most are appreciably smaller than the large strongyles. (Merck manual, 2008)

Small strongyles do not migrate extra intestinally, as early development is confined to the wall of the intestine. When these worms emerge from the gut wall, they feed superficially on the mucosa and may rupture capillaries but are less pathogenic than the large strongyles, as their buccal cavities are much smaller. Encysted small strongyles can cause severe clinical signs and even death when thousands to millions of the larvae emerge simultaneously from the intestinal wall. The resulting damage to the intestinal mucosa shows up in a horse as weakness, recurring colic, diarrhea, weight loss, peripheral swelling and in severe cases, death. (Merck Manual, 2008) In less severe cases, horses may have decreased performance, poor food utilization and dull hair coat. Encysted small strongyles can remain in this state for up to three years. Unfortunately, there's no way of knowing how heavy of an encysted small strongyle load a horse is carrying, since fecal analysis cannot measure worms in the encysted state.

Large Strongyles

Large Strongyles are also known as the artery Bloodworm (Strongylus vulgaris), palisade worms, sclerostomes, or red worms. The 3 major species are Strongylus vulgaris (up to 25 mm), S. edentatus (up to 40 mm), and S. equinus (up to 50 mm) and large strongyles has historically been considered to be the most damaging of all internal worms. (Merck Manual, 2008)

Their migrating larvae damage vessels that supply the gut and internal organs, and the adults suck on plugs of the bowel wall causing poor condition, weakness, ill-thrift, anemia, diarrhea, colic and death. The larvae develop to the infective stage within 1-2 weeks after the eggs are passed.  Horses can get infected by ingestion of infective larvae, which exsheath in the intestine and migrate extensively before developing to maturity in the large intestine. This period from 6-11 months. The larvae mature in the intestinal tract and burrow out into blood vessels where they migrate throughout various organs and eventually back to the intestine. The larvae can cause extensive damage to the lining of blood vessels. The larvae of S. vulgaris migrate extensively in the cranial mesenteric artery and its branches, where they may cause parasitic thrombosis and arteritis.

Adult large strongyles have large buccal capsules and are active blood feeders; they ingest mucosal plugs as they move about in the intestine. The associated blood loss may lead to anemia in severe cases. Weakness, emaciation, and diarrhea are also common.

Symptoms of large strongyle infestations may display weight loss, anemia, or colic. In extreme cases, the blood supply to the intestine may become completely blocked by the strongyles resulting in severe (and even fatal) colic. In heavily infested horses, blood vessels may become distended and may even rupture, leading to sudden death.


Ascarids (large roundworms) affect young horses more often than mature horses. The 6 to 12 inch long worms can number in the hundreds in the horse's small intestine and can adversely affect its nutrition. (AVMA, 2008)

Colic, coughing, and diarrhea are common clinical signs associated with ascarid infestation. In addition, ascarids may cause blockage of the intestine or migrate through the lungs causing pneumonia.

Common signs of ascarid infestation in young horses include stunted growth, pot belly, and a rough, dull coat. By 9-12 months of age most young horses develop immunity against Large Roundworms and infections in adult horses is only a problem in horses under the stress of hard work or disease, and in old horses where natural immunity is reduced. (Petalia, 2008)


Pinworms have the most efficient life cycle of all the parasites that infect the horse. They don't migrate through any organ tissue, and they have developed a means of reproduction by which the eggs do not leave the herd of horses. (Complete rider, 2008)

Adult pinworms (Oxyuris equi) are most common in horses less than 18 months old, though most herd horses are susceptible. Adult pinworms are found primarily in the terminal portion of the large intestine. (Merck manual, 2008) Horses acquire the parasite by consuming contaminated water, grain or grass.


Tapeworms have emerged as a significant problem in Australia with up to 60% of horses being infected. (Petalia, 2008) There are several species of tapeworms that infect horses with Anoplocephala perfoliata being the most common in Australia. This worm is a yellow-green color and has a riangular 'fluke-shaped' body. They grow to 3-8 cm in length. A horse may be infected with one tapeworm or hundreds. A survey in Victoria found that the average infection intensity was 99 tapeworms per horse. (Oakey vet, 2008)  Other Tapeworm species include: Anoplocephala perfoliata, Anoplocephala magna & Anoplocephaloides mamillana.



Written by Sylvana Miller on 26 April 2010