4 Common Equine Parasites Your Horses Might Have

Internal parasites are a common in the lives of most grazing animals. They are often consumed along with grass, feed or water, and then make their way into various internal organs. While many parasites can infect horses, there are only a few that cause, or can cause, significant health problems. Many of these can be identified though faecal egg counts (FEC).

1. Large redworms

Large redworms, or large strongyles are some of the most dangerous parasites a horse can pick up. There are three species that particularly affect horses: Strongylus vulgaris, Strongylus endentatus, and Strongylus equinus. Of these, S. vulgaris has historically been the most common.

The eggs of the worm leave the horse in its dung and then hatch. Once fully developed, the larvae survive on the grass until they are consumed by another horse. The larvae migrate through the blood vessels causing significant internal damage until finally residing in the intestinal wall. They then mature and lay eggs which pass out in the faeces. The movement of the larvae through the horse’s system is by far the most dangerous aspect of infection. This can cause colic, diarrhoea, anaemia, weight loss, blood clots and ulceration.

Most intestinal parasites are picked up when horses ingest infected grass, feed or water.
Most intestinal parasites are picked up when horses ingest infected grass, feed or water.

While these parasites are very dangerous, these days they pose a relatively minor threat to horses in the UK. They are well controlled and are quite rare today. This is likely to be due to their susceptibility to the ivermectin anthelmintics and their long life cycle. They have low resistance1 to anthelmintics so any infections can be treated easily. That said, as reliance moves from frequent anthelmintic treatment to monitoring these infections could be overlooked as their eggs appear very similar to those of small strongyles.

2. Small redworms

Small redworms, or small strongyles, are a group made up of around 50 different species of nematode worms. While they have a similar lifecycle to large strongyles, these worms do not migrate through the body and internal organs of the horse. Their development takes place entirely in and on the intestinal wall.

The vast majority of horses will have some small strongyles, but infections only become symptomatic when parasite burden is very high. In the late autumn, larvae that are ingested tend to hypobiose. This is when they halt in their development, remaining as larvae within the intestinal wall through the winter. In the early spring these worms emerge and in heavy infections may be seen within the dung.

Careful management2 of these parasites is necessary as over-use of wormers has led to resistant strains forming. Instead of wide-spread herd de-worming, a more effective strategy is to target only horses with concerning egg counts for treatment.

3. Roundworms

Horses develop resistance to parasites over their lifetime. As such foals are especially susceptible to roundworms.
Horses develop resistance to parasites over their lifetime. As such foals are especially susceptible to roundworms.

Roundworms, or ascarids, are the largest parasites that commonly infect horses. The worms can grow up to 40cm long.3 Young horses are most commonly affected, but geriatric horses and those with diminished immune systems can also suffer from roundworm infection. High worm burdens in young horses can cause severe symptoms. This can range from poor growth and weight loss, to intestinal obstruction and even death.

Eggs are produced by mature roundworms in the intestines of infected animals and are then excreted in the faeces. The larvae develop, but remain protected inside the egg until they are eaten by the horse. They then hatch, and migrate through the horse’s liver and lungs. While the liver is capable of regenerating damaged tissue, damage to the lungs results in permanent scarring. This can cause increased susceptibility to pneumonia and fitness consequences later in life.

Pinworms irritate the anus of infected horses, causing them to rub. The first sign of these parasites can be a rubbed tail.
Pinworms irritate the anus of infected horses, causing them to rub. The first sign of these parasites can be a rubbed tail.

Roundworms can be treated with specific wormers, but have resistance to some drugs. Good pasture management and regular worm counts can significantly decrease the incidence of these worms in a herd.

4. Pinworms

Pinworms, Oxyuris equi, are common in equine species across the world. While not as dangerous as other parasites, these worms can cause significant irritation in infected animals.4 The main symptom of pinworm infection is excessive tail rubbing.

The adult worms live in the rectum of the horse and the females travel outside to lay their eggs around the horse’s anus. This causes irritation and itching in the horse, so it rubs its tail head and disperses the eggs into the environment. For more information on pinworms, read our article here.

These are just a few of the internal parasites that horses can catch. With the start of spring, more horses are out in the fields, allowing them to pick up any parasites in the grass. So if you have horses, now is the time to consider management strategies and start getting faecal egg counts to keep your horses in peak condition.

References

  1. Matthews, J. B. Anthelmintic resistance in equine nematodes. Int. J. Parasitol. Drugs Drug Resist. 4, 310–315 (2014).
  2. Nielsen, M. K., Monrad, J. & Olsen, S. N. Prescription-only anthelmintics—A questionnaire survey of strategies for surveillance and control of equine strongyles in Denmark. Vet. Parasitol. 135, 47–55 (2006).
  3. Clayton, H. M. Ascarids: Recent Advances. Vet. Clin. North Am. Equine Pract. 2, 313–328 (1986).
  4. Wolf, D., Hermosilla, C. & Taubert, A. Oxyuris equi: Lack of efficacy in treatment with macrocyclic lactones. Vet. Parasitol. 201, 163–168 (2014).