Valley Veterinary Group

Large Strongyles (Large redworms, bloodworm) 

These are the type of worm which in the past owners and vets were most concerned about. The life cycle of these species involves migration of larvae from the bowel into the abdomen, before returning back to the bowel as adult worms. Eggs are passed out onto the pasture and hatch, developing into infectious larvae which are ingested during grazing. These worms may be associated with general signs of malaise; but one species, Strongylus vulgaris, burrows into the gut wall and migrates specifically to the arteries supplying the bowel. They remain in this migratory phase for around 6 months and can cause constriction of the arteries, reducing the supply of blood to the gut and leading to tissue death. Blood clots may also form which dislodge and travel to other organs where they can have devastating results. The larvae develop into adults which attach to the intestinal wall and suck blood, causing inflammation and ulceration. Modern worming regimes are usually very effective at killing these species and so recently their significance has diminished.

Small strongyles (Cyathastomes, small redworms) 

These worms are a major causal agent of poor performance and loss of condition in the horse. They can also cause subclinical disease which may go unrecognized and untreated. The cyathostomes are highly prolific, with it being possible for a horse to carry many thousands of worms. The mature females inhabiting the large intestine produce eggs which are released into the gut and expelled onto the pasture in the faeces. They can complete their life cycle in only 6-8 weeks, enabling a massive growth in population and subsequent spread through a group of horses.

Eggs that are shed onto the pasture hatch and develop into infective larvae and crawl up stems of grass where they are ingested by the horse while grazing. They pass along the digestive tract, and once in the large intestine, burrow into the walls to complete their development. They then emerge as mature adults capable of starting the cycle again. However, there is a tendency of these worms to ‘hibernate’ where they burrow into the intestine and undergo a period of arrested development whereby they encyst and can remain in this state for over 2 years (even if your horse is being routinely wormed). The larvae eventually emerge, sometimes in very large numbers, leaving a damaged and ulcerated gut wall, which may even prove fatal.

A further problem associated with small strongyles is the development of resistance to certain worming chemicals. This is aided by the short life cycle which, when subjected to doses of worming chemicals, creates a 'selection pressure’ so that worms with some degree of resistance to the chemical are more likely to survive than the others, so they go on to produce more worms with this same trait, and so on until a population of resistant worms has developed. Naturally, this presents certain problems:

• the owner, being unaware of resistance, continues to worm using the chemical to which resistance has developed, believes that his/her animal is free of worms. Numbers of resistant worms build up through successive cycles of re-infection leading to a slow decline in fitness of the horse.
• the choice of chemicals for use in the worming strategy becomes limited so that it becomes difficult or impossible to target certain developmental stages of some parasites.
• the horse carrying resistant worms sheds eggs onto shared grazing where they are picked up by other horses and ponies who then become carriers of resistant worms. The owners of these horses may be worming diligently and believe that their horses are worm free.
• the horse travels to shows or camps both in and out of the district and sheds eggs. Horses grazing these paddocks later pick up the larvae. By this means, resistance spreads and many owners will not even be aware that they have a problem building up.

Tapeworms (Cestodes) 

The tapeworm Anoplocephala perfoliata can grow to about 8cm in length and attaches to the gut using four suckers. It is usually found at the ileo-caecal junction where the large and small intestine join. There is normally no sign of infection although horses mat show some condition loss and if untreated the intestinal wall can be grossly affected leading to more serious problems. It is now believed that tapeworm may be responsible for some cases of spasmodic colic.

In order for the life cycle of the tapeworm to be completed, it is essential for the eggs to enter a secondary host, in this case a small oribatid mite foud in the pasture. Eggs are passed out via the horse’s faeces and eaten by the mites inside which they hatch and mature over 2-4 months. The mites are then eaten by horses as they graze, and the tapeworm continues its development to maturity within the horse over a period of months. Tapeworm eggs are not usually detected in dung samples, but a blood test is available.

Lung Worm (Dictyocaulus arnfieldi) 

Donkeys are the natural host for these worms, and don't usually show any symptoms. However horses grazing in the same field, or in a field previously grazed by donkeys, may also be affected. The worm does not usually complete its life cycle in the horse and so is not often detectable by faecal worm counts. Horses affected with significant numbers of this worm cough due to the presence of the larvae within the lungs.

Roundworms (Ascarids, Parascaris equorum) 

This usually affects young horses only as immunity has usually built up by the time the horse reaches 2 years old. The worms are very long and mature adult females produce vast numbers of thick-walled, tough, sticky eggs which can adhere to anything they come into contact with and remain viable for several years. The life cycle takes approximately 10 weeks and begins when the foal ingest the infective eggs which then hatch and the larvae burrow through the intestinal wall and travel to the liver. Despite this worm being an intestinal parasite, infestation often results in coughing as one week later, the larvae are carried in the blood to the lungs. From here they are coughed up and reswallowed, ending up in the small intestine where they finish their development to adults. Large numbers of roundworms may slow down movement of food thought the gut, potentially leading to a problem.

Threadworm (Strongyloides westeri)

This is a parasite usually associated with young foals and implicated in cases of scouring from 10 days of age.   Whilst this parasite is rarely seen in adults, it does have the ability to lie dormant in the mare and to activate following foaling at which time it is passed to the foal via the mare’s milk. 

Pinworm (Oxyuris equi) 

This is of low importance as an internal parasite but they can cause irritation to the horse around the anus. This may cause the horse to rub its tail area creating damage to the skin and creating the potential for infection of the resulting sores.

Bots (Gastrophilus intestinalis) 

In truth these aren't really anything to do with worms, but are often put into the same category by owners. Bots are the larval stage of a certain species of fly. This fly lays her small, very sticky cream/yellow eggs on the horse's coat, often around the forelegs, abdomen and neck during late summer. Despite being painless, this can often cause extreme agitation. The larvae hatch and are licked off by the horse, and, after burrowing within the oral cavity and passing through 2 maturation stages, eventually travel to the stomach where they attach to the stomach lining. They remain in the stomach for about 9-10 months, growing to around 2cm in length until they are passed in the dung as pupae. They then burrow into loose soil, pupate and emerge as adults 3 weeks later. Untreated stomach bots in large amounts can lead to tremendous discomfort. Bots can also cause digestive problems, weight loss, and poor coat growth. Severe bot infestation can ever lead to ulcers and more serious perforations of the stomach lining leading to peritonitis or rupture of the stomach.

Bots do not reach adulthood and do not shed eggs whilst in the horse, so they cannot be detected using a faecal worm-egg count.