4 Parasites to look out for

Small and large redworms, large roundworms and tapeworms are four of the most dangerous equine worm parasites that can affect your horse. Every horse, like any animal, is home to countless microscopic creatures. 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 be beneficial, such as the intestinal microbes without which your horse could not digest food. On the other hand, some of these creatures are truly parasitic and represent a serious risk to the health of your horse.

Here are some details of the four parasites:

As the name suggests, 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 it 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 this parasite infestation range from weight loss, 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 reduced by the advent of modern wormers. However, disease is still seen in horses that have not received appropriate wormers.

Small redworms, like the large redworms, are a type of strongyle and have the species name of cyathostomins. The small redworm is common and potentially more dangerous than the large redworm. It is smaller than the large redworm but is still visible to the naked eye.

Like large redworms, the eggs are released with your horse’s droppings. They then hatch into larvae, which then live on grass and are ingested by your horse as it 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). It is here where 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 careful, 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.

Encysted small redworm can mature and emerge in big batches. This often occurs in late winter or early spring. This is due to several 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.

Large roundworms are also known as Parascaris equorum, or ascarids. Like the large and small redworms they are nematode worms. These 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 redworms, 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 worrying symptom may be colic. In very bad cases, the burden of roundworms can cause a potentially fatal intestinal blockage.

Tapeworms 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 perfoliate. Before they reach the horse, they live in a different host, a grass-dwelling forage mite. Your horse can become infected with tapeworm if they ingest these mites while grazing. They can cause digestive upsets which show as mild colic which can be resolved with treatment but may recur. More rarely, tapeworm may cause intestinal blockages and fatal colic

To keep your horse safe from worms, consult your vet.