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Intestinal Parasites of the Horse PARASITE CYCLES | SMALL REDWORMS | LARGE REDWORMS | TAPEWORMS | LARGE ROUNDWORMS | BOTS | PINWORM | |
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The horse has a number of different parasites which have evolved to live in its intestine. These worms are highly specialised to live and reproduce within the horse. Most can not live in other species of animal. Almost every horse alive will have at least one parasitic worm in its body. It is important to realise that often these worms are present in low numbers and cause the horse no ill-effects. Occasionally however, horses develop heavy worm burdens which can be damaging and even fatal. Domestication of the horse, forcing it to graze in enclosed fields, close to other horses has increased the liklihood of heavy burdens developing. The section entitled How worms cause disease describes the harmful effects of heavy worm burdens. |
Horses Grazing |
Parasite Lifecycles Worms are spread between horses by means of the horse's droppings. Adult worms shed eggs into the horse's gut, these eggs fall onto the pasture when the horse produces droppings. Another horse can then inadvertently eat the infective eggs when grazing close to the pile of droppings. Once inside the second horse, the eggs hatch and develop into adult worms capable of shedding more eggs. This is a simplified worm lifecycle. Each species of parasite has its own variation of this basic pattern and there are additional complexities to consider. |
Small Redworms on droppings 
Encysted larvae in the mucosa of the large colon |
Small redworms (cyathostomins)
These worms are the most common horse parasites. There are over 50 different species. Adults living in the large intestine lay eggs that are passed out in the faeces. The worm eggs hatch and develop through 3 stages. The 3rd stage is the infective stage. If a grazing horse eats the infective stage the larvae will migrate to the intestinal lining where it can remain dormant for long periods of time. Vast numbers of larvae can potentially build up in this area. At a later date the larvae will re-activate and erupt out of the gut wall. If large numbers of larvae all do this at the same time it can cause extensive damage to the gut wall. Larvae emerge into the large intestine, develop into adults which lay eggs that are passed out in the droppings thus completing the cycle. 
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Arterial thrombus with Strongylus vulgaris larvae present |
Large Redworm (Strongylus
vulgaris and Strongulus edentatus)
The large redworm differs from small redworms in size and life cycle. Horses become infected by ingesting larvae as they are grazing. The larvae of large redworms are a significant cause of disease. They migrate through the blood vessels to the arteries of the intestinal tract where they can cause severe damage. After about six months they return to the intestine as egg-laying adult worms. Adult worms vary in size between 1.5 and 5 cm and, in large numbers, can cause several different disease problems. |
Adult Equine Tapeworm 
ileo-caecal junction of a horse heavily infected with tapeworms |
Tapeworms (Anoplocephala perfoliata) The equine tapeworm is flat, triangular and relatively short compared to tapeworms found in dogs, cats and humans. It has a small, round head which attaches to the lining of the gut by four suckers. The equine tapeworm can grow to about 8 cm long by about 1.5 cm wide. It lives in the mid-part of the gut at a junction between the small and large intestine known as the ileo-caecal junction. Infected horses pass eggs on to the pasture. These eggs are eaten by tiny oribatid mites which are present in their thousands in every square metre of grass. Once inside the mite, the eggs hatch and develop into an intermediate infective stage. Grazing horses inadvertently eat mites with almost every mouthful. Very few of the mites will be infected but those that are will release the larvae, which continue to develop into adult tapeworms inside the horse. The adults attach in clusters to the lining of the gut at the ileo-caecal junction and release eggs, thereby completing their lifecycle. At one time it was thought that the tapeworm was a relatively harmless inhabitant of the horses' intestine. However, recent research has revealed that tapeworm are associated with certain types of colic in the horse. |
Ascarid impn |
Large Roundworm (Parascaris equorum) Roundworm or ascarids are usually only problematic for foals and younger horses. Roundworm eggs can remain viable in the soil for many years and horses usually ingest as they graze. The eggs hatch inside the intestine and the emerging larvae migrate through the walls of the small intestine and into the veins. Larvae are then transported via the blood to the lungs where they are coughed up, re-swallowed and passed into the small intestine. Once in the small intestine the larvae develop into adult roundworm. The adults, which can grow up to about 50cm long, lay eggs which are passed out in the faeces. |
Bot eggs on the legs of a horse 
Bot larvae attached to the stomach wall 
Pinworm present on the droppings of a horse |
Bots(Gasterophilus intestinalis) Bots are the larvae of the bot fly. Small, sticky, yellow eggs are deposited on the horses coat by the female bot fly. The eggs hatch and the larvae get into the horses mouth when the horse licks its coat. Larvae in the mouth are then swallowed. Once in the gut the bot larvae attach to the stomach lining. They are eventually passed through the horses digestive system and are expelled in the droppings. Once expelled the larvae develop into the adult bot flies and the life cycle begins again. Bots can cause mild damage to the mouth and gums and also to the stomach lining. Pinworm (Oxyuris equi) Pinworms are not thought to be harmful but can provoke irritation around the tail. Pinworms inhabit the large and small colon and have a relatively simple life cycle. The females lay their eggs around the anus of the horse using a sticky substance, which is irritating to the horse. The eggs are dislodged as droppings are passed and fall onto the pasture where they are eaten by horses. |