WORMS are parasites which live within, and take nourishment from, the horse. All horses will have some degree of worm infestation which is completely harmless.
It is only cases of very large worm burdens which will have a detrimental effect on the horse’s health. Worming and parasite control is a fundamental part of equine husbandry and recommended worming regimes have changed considerably over recent years.
In order to have the correct worming programme in place, it is vital to understand the life cycle and transmission of the common equine parasites.
Large Redworms (Strongylus Vulgaris)
Horses become infected by ingesting larvae at pasture. Upon entering the intestines of the horse, the larvae migrate through the large intestine into blood vessels causing damage and inflammation along the way. They return to the large intestine as adults about six months later where they lay eggs which are, in turn, excreted in the faeces. In severe cases of large redworm infestation, the larvae cause blood clots to be formed within the vessels through which they are migrating. These clots can block the flow of blood to parts of the intestines causing necrosis of the intestinal wall due to lack of oxygen and vital nutrients. Horses with severe large redworm infections can develop abdominal pain, anaemia and sometimes life-threatening colic from necrotic or ruptured intestines.
Small Redworms (Cyathostomins)
These are the most common worms affecting the adult horse. The adult worm lives in the large intestine where it lays eggs. These eggs get passed on to pasture in the faeces. While on pasture, the eggs hatch and develop through three stages.
It is the third stage of development which is the infective larval stage. If this infective stage is eaten by the grazing horse, they will migrate to the intestinal lining, where they become encysted and can lie dormant for long periods of time.
Small redworms are the most pathogenic parasite affecting the horse. They cause significant intestinal damage both by the invasion of and the emergence from the intestinal wall. The most damaging effect of the encysted larvae is when they emerge ‘en masse’ from the intestinal wall in late winter/early spring to continue their development to an adult worm. This mass emergence causes severe damage to the gut wall resulting in diarrhoea and colic.
Tapeworms (Anoplocephala perfoliata)
Tapeworms live at the junction between the small and large intestines. They have a small round head which attaches onto the lining of the gut. Segments containing eggs are released from the tape worm and are excreted in faeces on to pasture.
The eggs are then eaten by tiny mites, inside which the eggs hatch and develop into the infective stage. These mites are inadvertently eaten by horses grazing at pasture and once ingested the infected mites will release the larvae.
These larvae will continue to develop into adult tapeworms inside the horse. Large tapeworm burdens can cause colic by reducing gut motility and causing impaction.
There are reported cases of gut perforation leading to peritonitis and ultimately death as a result of extreme tapeworm infestations.
Large Roundworms (Parascaris equorum)
Roundworms will usually only affect young horses and foals. Their eggs can remain viable in soil for many years and are ingested by horses as they graze. Once inside the intestines, they hatch and migrate through the intestinal walls, into veins where they are transported to the lungs.
Once in the lungs, they are coughed up and swallowed where they develop into adult roundworms in the small intestines. Roundworm infection is most significant in foals causing ill-thrift and poor growth. The migrating larvae can cause airway inflammation, coughing and nasal discharge. In extreme cases of roundworm infestation, there is a risk of small intestinal impaction which has a poor prognosis for survival.
Pin Worms (Oxyuris equi)
These are less common than other parasites which affect the horse and do not cause serious clinical conditions. They can however cause irritation around the horses’ anus and tail. The adult pinworm lives in the large intestine. They lay eggs around the anus of the horse which can cause intense irritation provoking the horse to scratch and rub the tail area.
The eggs are dislodged from the anus by droppings and eaten by horses at pasture thereby completing their lifecycle. Clinical signs of pinworm infestation include tail rubbing and inflammation of the skin around the hind quarters as a result of excessive rubbing.
Bots (Gasterophilus intestinalis)
Small eggs are deposited on to the horse’s coat by the female bot fly, normally in late summer. These eggs hatch and get into the horses’ mouth when they lick their coat. The larvae are swallowed and attach themselves to the lining of the gut. They will eventually travel through the digestive system and are passed out in the horse’s droppings.
Parasite Control
Traditional parasite control programmes involved rotational treatment with worming at regular intervals. This approach was based on the large redworms being the most significant parasite affecting the horse. This is no longer the case as the small redworms are now recognised as the most important parasite pathogen. Decades of frequent worming has resulted in high levels of drug resistance.
The aim of parasite control is to limit parasite infections so the animal remains healthy and clinical illness does not develop. The key to a successful and effective worming programme is properly timed treatments with appropriate drugs delivered at the correct time of the year.
At MJR, we use a targeted dosing system whereby we periodically test each horse to identify the level of infection in individuals and treat the individual accordingly. This is achieved by performing a faecal egg count on each horse at quarterly intervals.
Horses which have more than 200 worm eggs per gram of faeces are treated. This is supplemented with every horse being treated for tapeworms in autumn. Identification of tapeworm eggs on faecal egg counts is unreliable as the eggs are only passed from the adult worm intermittently.
Therefore, unless a horse is infected with a large dose of tapeworms, identification will only be by chance.
Types of Wormers
Ivermectin is very effective against adult small redworms although it has limited effect against the encysted larvae. It is also effective against large redworms, pinworms and large round worms. It has no effect on tapeworms.
Praziquantel is used for the treatment of tapeworms. There is presently no known resistance to this wormer.
Moxidectin is a very effective treatment against both adult and developing encysted small redworms. It will also kill large redworms, pinworms, roundworms and bots.
Pyrantel will kill adult large and small redworms. It will have no effect on the encysted larval stages of the small red worms. Administering a double dose of pyrantel will also be effective against tapeworms.
Fenbendazole will kill adult and developing small redworms. A five-day course of fenbendazole is effective against the encysted larval stages of the small redworms. It is also effective against large redworms, pinworms and large round worms. There is now widespread resistance of small redworms to this wormer group.
Pasture Management
Correct pasture management will also contribute significantly to effective worm control. Paddocks should not be over-stocked.
Over-stocking of the paddocks will result in a high concentration of droppings and therefore a greater risk of infection.
Ideally droppings should be removed from the paddock a couple of times a week, as this will remove the source of new worm eggs.
Harrowing paddocks in dry conditions will expose the larvae causing them to dry out and die.
If possible it can be beneficial for horses to graze with cattle or sheep. Cattle and sheep will eat grass which horses find unpalatable and in the process will ingest equine worm larvae.
John Martin from Stradbally, Co Laois, graduated from University College Dublin and is in practice at Mark Johnston Racing. The article first appeared in the Kingsley Klarion, published by Mark Johnston Racing.