Parasites: four to look out for

By Wendy Talbot on 25 September 2020

Every horse, like any animal, is home to countless microscopic creatures, and most of them are nothing to worry about. The types of tiny creature living in and on your horse mostly have no effect and can even be beneficial, like the intestinal microbes without which your horse could not digest food. However, some of these creatures are truly parasitic and represent a serious risk to the health of your horse. Here we share details of the four most dangerous equine worm parasites.

Large redworms

As the name might suggest, large redworms are usually reddish in colour. Their scientific name is strongyles, and there is more than one type. They look like a long worm and they have a well-defined biting mouth.

Large redworms can get into your horse’s body when he is grazing. The larvae migrate from the gut into the arteries that supply the gastrointestinal tract with blood and then move around the bloodstream and internal organs, returning to the large intestine when it’s time to lay their eggs. The effects of a large redworm infestation range from weight loss, general unthriftiness and mild colic to severe complications caused by the migrating larvae which may result in fatal colic due to disruption of blood supply to the gut.

Fortunately large redworms and the problems they cause have been very much reduced by the advent of modern wormers; however, disease is still seen in horses that have not received appropriate wormers.1

Small redworms

These are also a type of strongyle and have the species name of cyathostomins. The small redworm is a common and potentially more serious danger than the large redworm. It’s smaller than the large redworm, but still visible to the naked eye. Like large redworms, the eggs are released with your horse’s droppings; they hatch into larvae, which then live on grass and are ingested by your horse as he grazes. They move to the large intestine and burrow into the gut wall where a cyst forms around the larvae to protect it (encysted stage). Here they develop through different larval stages before emerging back into the gut to form egg-laying adult redworm. In some horses and at certain times of the year, a large proportion of larvae do not immediately develop in the cysts but instead ‘hibernate’ within the cysts in the gut wall for a prolonged period before recommencing development.

A horse with an infestation of adult redworms may pass hundreds of thousands of eggs in its manure, but, be warned: if the redworms are still in the encysted stage they will not lay eggs, so looking for worm eggs in the faeces will give a misleading result. A specific antibody test can show if encysted small redworm are present.2

Encysted small redworm can mature and emerge in a big batch. This often occurs in late winter or early spring; it is due to a number of factors and has even been associated with recent deworming, especially if the wormer used is not effective against the encysted stages. This mass emergence of small redworms can cause very serious and potentially fatal disease, usually seen as diarrhoea, weight loss and colic.3

Large Roundworms

These are also known as Parascaris equorum, or ascarids. Like the large and small redworms they are nematode worms. They tend to be less of a problem for adult horses because horses over two years old have usually built up an immunity, but they can make foals very ill. Like the redworm family they enter the body when your horse is grazing. The ingested larvae migrate through the gut and enter the circulation to pass through the lungs before being coughed back up and then swallowed to live and mature in the small intestine. A young horse with a roundworm infestation might lose his appetite, develop a cough and look ‘poor’ with a straggly coat and pot belly. Another particularly worrying symptom may be colic. In very bad cases the burden of roundworms can cause a potentially fatal intestinal blockage.4


These are parasitic flatworms, also known as cestodes, which live in the digestive tract at the junction between the small and large intestine. There are several species; the most common is known as anoplocephala perfoliata. Before they reach the horse, they live in a different host, a grass-dwelling forage mite. Your horse can become infected with tapeworm if he ingests these mites while grazing. They can cause digestive upsets which show as mild colic which resolves with medical therapy but may recur. More rarely, tapeworm may cause intestinal blockages and fatal colic.5

To find out how to keep your horse safe from worms visit our seasonal worming blog.


  1. Reinmeyer and Nielsen (2013) Handbook of Equine Parasite Control. Eds. Reinmeyer CR and Nielsen MK. Wiley- Blackwell.
  3. Matthews (2008) Equine Vet. Educ. 552-556
  4. Proudman and Matthews (2000) In Practice 2000 22: 90-97
  5. Proudman (2003) Journal of Veterinary Science. 6-9




Wendy graduated from Bristol University in 1999. She then went on to complete a residency at Liverpool University and holds a European Diploma in Equine Internal Medicine. After working in practice for 13 years, she joined Zoetis in 2012 as the National Equine Veterinary Manager.

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