All horses carry a worm burden many of which cause no damage to the host. However it is essential to control these internal parasites, as severe infestation can result in poor growth, unthriftiness and in extreme cases can prove fatal.
Ascarids Parascaris equarum
The horse swallows ascarid eggs, which hatch into larvae in the intestine. The larvae penetrate the intestinal wall and migrate to the liver and lungs. Their presence in the liver causes the animal to cough, expelling the larvae into the mouth, when they are reswallowed. On this second arrival in the intestine, the larvae mature into adults which can reach a length of 35cm. Foals are usually most affected. The symptoms include poor growth, potbelly, coughing. In extreme cases there may be diarrhoea, lung and liver damage, pneumonia, intestinal blockages and death.
Strongyles Strongylus sp. Redworm or bloodworm
The so-called large strongyles cause the most damage. Adults mate and lay eggs in the large intestine. The eggs are passed out in the faeces and hatch into infective, well-protected larvae in the grass or in the stable. The horse swallows the larvae in food or water. They burrow through various parts of the body, including the arteries of the intestine and the liver, before returning to the large intestine to mature into adults and reproduce.
Small strongyles have a similar life cycle, but cause less damage as they do not migrate through the organs of the body. They are the most common worm found in horses and only cause a problem when infestation is massive.
Symptoms of strongyles infestation can include diarrhoea, colic, decreased food efficiency and performance. There may be organ damage and intestinal blockages in extreme cases.
Pinworm Oxyuris Equi
These small worms do not cause damage of the intestinal tract. The female crawls out of the anus of the horse and lays her eggs in a sticky substance under the horse’s tail. It is this substance which causes extreme irritation to the horse. The animals may rub their tails and hindquarters until an open wound is caused.
Bots are not true helminths, but the larvae of the bot fly. The female fly lays her eggs on the coat of the horse. These eggs are ingested by the horse licking or nosing the site. The larvae hatch in the tongue, migrate down the oesophagus to the stomach where they mature. The maturing larvae attach to the lining of the stomach or small intestine causing erosion and ulcerations at the site of attachment. They develop for 10 – 12 months before being passed out in the faeces. The adult fly emerges after 1 – 2 months and the cycle begins again.
Tapeworm Anoplocephalidae
The intermediate host of the horse tapeworm is a small mite that lives in damp grass and is swallowed by the horse while grazing. The eggs hatch in the intestine of the horse and develop into the adult tapeworm. The tapeworm consists of a number of ‘segments’ each containing male and female reproductive organs, where the eggs are formed. As these egg-bearing segments mature, they become detached from the tapeworm and are passed out through the faeces which are then swallowed by the mites, continuing the cycle.
Horse tapeworms are not common and are difficult to detect as the eggs are not found in faeces samples. Symptoms include unthriftiness, rough coat, lethargy and loss of appetite.

Prevention and Control

•    Remove manure regularly (at least once daily) from stables and dispose of it by composting or burning.
•    Keeping yards clean, removing manure and litter.
•    Routinely discing or harrowing in manure on pastures and paddocks
•    Leaving paddocks empty for a period or rotating with ruminants, which have different parasites.
•    Control of flies to prevent infestation by bots.
•    Prevent horses from grazing wet grass where the mite carrying tapeworm eggs lives.
•    Routine testing of dung samples to establish worm load and then judicious dosing with an approved anthelmintic.