Parasitic worms

Three main groups of parasitic worms (Helminths) affect horses:

  1. Roundworms (Nematodes)
  2. Tapeworms (Cestodes)
  3. Flukes (Trematodes)

Many of the species of parasitic worms within these groups have evolved to live specifically in the gut of the horse. Many horses will have some worms but as long as the overall burden is low the horse will suffer no ill effects. However, domesticated horses are kept on much smaller areas of concentrated grazing in close proximity to other horses. This change in husbandry has increased the individual horse’s exposure to worms and increased the chances of the development of large infections.

The principles of parasitic control focus on both management and pharmaceutical means.

Tapeworms (Cestodes)

Tapeworms generally live in the small intestine. Their head is attached to the intestine wall by suckers and their flat, ribbon-like body extends along the bowel. Tapeworms do not have an alimentary tract but absorb nutrients across their body wall. The body itself is formed from a large number of segments; one by one the segments become mature and break off from the end of the worm. These segments contain eggs which they now release and which are ultimately shed in the faeces. These eggs, once on the ground are immediately infective to other animals.

The horse tapeworm (Anoplocephala perfoliata) grows to up to 8 cm in length and 1.5cm in width; it is shorter and wider than tapeworms of dogs, cats and humans. It lives at the point of the intestine called the ileo-caecal junction, where the small intestine notionally joins the large intestine.

Diagnosing tapeworm infection by testing the faeces of the horse is very difficult. The tapeworm segments are shed sporadically and in small numbers, so the likelihood of finding eggs is remote. Consequently, results are inconsistent and do not necessarily give an accurate representation of the scale of infection. Recently, Liverpool University has developed a blood test to detect tapeworm infection. It relies on detecting the level of immune system response (antibody) to tapeworms. The full benefit of this test is derived from the fact that high levels of antibody correlate to a high level of infection and vice versa. The test is also consistent.

Roundworms (Nematodes)

The term ’roundworm’ describes many different zoological groups of worms. Most roundworms are free-living, (non- parasitic), others can become parasitic if conditions dictate. The roundworms that affect the horse are purely parasitic.

Small Redworms (Cyathostomes)

Infection is by ingestion of an infectious larva. The larva passes into the intestine and penetrates the lining of the small bowel. The larvae may remain within the mucosa of the intestine for long periods of time before emerging to complete the life cycle. Potentially, large numbers of encysted larvae can build up over time and, if a significant proportion emerges at the same time, severe damage can be caused to the gut wall. Once reactivated the larvae develop into adults and lay eggs that are passed in the faeces.

Large Redworms (Stongylus vulgaris & edentatus)

The beginning lifecycle of the large redworms is similar to that of the small redworms apart from where the infective larvae encyst (lie dormant). Large migratory redworm larvae penetrate the lining of the intestine and, depending on the species, migrate to different areas within the body. One species of redworm larvae rest within the intestinal arteries and the other type migrates through the liver before returning to the intestine as adults. The developing larvae may become dormant within the body over winter; they are described as being ‘encysted’. Once they have become encysted they will not emerge as adults until the spring. When they do reactivate they will all do so at the same time, causing severe damage to the tissues where they are at the time. The encysted redworm larvae in the mesenteric arteries may cause thrombosis, embolism, colic and death.

Large Roundworms (Parascaris equorum)

Roundworm eggs are ingested by grazing horses from the field where they can remain infectious for many years. The eggs hatch inside the intestine and the larvae penetrate the small intestine and migrate within the mesenteric veins. They are transported within the blood circulation to the lungs where they pass from the circulation into the lungs. The horse coughs up the larvae and swallows them. Once back in the small intestine, they mature into adults and lay eggs that are shed in the faeces.

Bots (Gasterophilus intestinalis)

Bots are the larvae of the bot fly. The adult insect lays eggs on the horse’s coat, typically on the chest and front legs. The horse ingests the eggs during grooming. Once in the stomach the eggs hatch and the larvae attach to the lining of the stomach. Ultimately they are passed through the intestines and are expelled in the faeces. Once outside the horse the larvae develop into adult flies and the cycle starts again.

Pinworms (Oxyuris equi)

The most serious complication of Pinworm infection is thought to be irritation around the tail base and perineum. Female worms deposit their eggs around the anus using a sticky material; this is irritating to the horse. The eggs are dropped onto the pasture when the horse rubs or passes droppings where they are potentially infective for other horses.

Other worms that infect horses:

Stronglyoides Westeri Habronema Dictyocaulus Arnfeldi