Worms in Depth Series: Roundworms

What is a Roundworm?

Roundworms are an internal parasite that can affect horses. They have a typical ‘round’ shaped worm body and can vary in length up to a massive 50cm! There are many different roundworm species that can infect horses. We will examine the most common roundworms and their effects:

  • Small redworms (Cyathostomes): Small redworms are the most common internal parasite of the horse and can account for at least 90 percent of a horse’s worm burden. Some horses may be infected with as many as a million of these worms at any one time. The danger with these worms is that they have the ability to hibernate as larvae in the gut wall in small cysts (encysted small redworm). This can be problematic as the emergence of large numbers of larvae at the same time (usually during late winter) can cause huge damage to the gut lining, potentially leading to loss of condition, diarrhoea, colic and sometimes even death.


  • Large redworms (Strongyles): Large redworms are potentially the most dangerous internal parasite of horses. However, due to improved worming regimes and effective treatments, this parasite is not as common as it used to be. The larval stage of the lifecycle is of the most concern, because they migrate through blood vessels to develop within the major artery supplying blood to the intestinal tract. This migration not only damages the blood vessels walls but can also lead to blood clots and a weakening of the blood vessels. Disruption to the blood supply can cause colic, and in rare cases, death.


  • Large roundworms (Ascarids): Adult large roundworms can reach up to 50cm in length. Large roundworms typically only affect foals and young horses. Adult horses are not normally affected as they develop immunity with age. Adult ascarids and migrating larvae can be problematic to foals and young horses, causing poor growth, digestive and respiratory problems and occasional fatalities. The horse will often develop a cough as the larvae migrate from the lungs to the small intestine. The eggs of large roundworms can survive in the soil and stables for many years. Young horses become infected by ingesting these eggs from the pasture and their surroundings.


How do Horses become Infected?

Horses become infected with roundworms by ingesting worm larvae from the pasture. Adult worms live in the horse’s intestine and lay eggs that are passed out with the horse’s droppings. These eggs hatch into larvae on the pasture and the infective 3rd stage larvae are ingested by horses when grazing, starting the whole lifecycle over again.


Roundworm Lifecycle

Roundworm Lifecycle


How to Test for Roundworm

The routine blanket use of wormers is no longer recommended as this can encourage the development of drug resistance, therefore effective worm control adopts a number of strategies including Worm Egg Counts. Worm Eggs Counts (WEC’s) are used to identify horses with high worm burdens. Horses with WEC’s of more than 200epg (eggs per gram of faeces) should be treated with an appropriate anthelmintic. Those with lower WEC’s do not require treatment, preventing the unnecessary and ineffective use of wormers.

You can purchase a Worm Egg Count Kit online, or from your local equine retailer. Many veterinary practices will also offer the service for a small fee. Once you have your kit, performing the WEC is simple. It should be done at 8-10 week intervals after the last treatment or test if no treatment was required. Simply collect a sample of fresh dung and either post it or take it to your veterinary practice. It is important to take a fresh sample, as the older the sample the less reliable the results. The sample will then be analysed and the results sent to you, usually with recommendations for your next course of action.

Unfortunately, there is no diagnostic test as yet for encysted redworm, therefore it is recommended to treat with an appropriate wormer at least once every 12 months.



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