How to control worms in horses

What damage do worms do to horses? What are the best ways to control worms in horses and when should I worm my horse?

What damage do worms do to horses? What are the best ways to control worms in horses and when should I worm my horse?

How do I stop my horse getting worms?

Horses and ponies will come into contact with a variety of internal and external parasites throughout their lifetime. While having a low worm burden will have little or no effect on a horse’s health, a high parasite burden can cause serious health problems if left untreated.

If owners follow a worm control programme and exercise good pasture management (see our top tips below), most horses will not suffer from worm infestation damage. There are five main parasitic worm types (find out more on these below) that affect horses in the UK and these are:

  • Small redworms
  • Large redworms
  • Tapeworms
  • Roundworms
  • Bots

What steps can be taken to prevent worm damage?

To control worms effectively, you need to keep your horse’s pasture clean and use a targeted approach to deworming. This means that you test each horse before making the decision to deworm them. The test provides an estimate of how many parasite eggs the horse is shedding in its droppings to allow treatment of those horses who need it.

It’s important that we move to a targeted approach to worm control because routine or ‘blanket’ deworming has resulted in some of the most serious parasites becoming resistant to many of the drugs that we use. This could mean that in the future we reach a point where all worms are resistant to the dewormers available – which makes it even more important to look at other ways we can control worms in our horses!

How does pasture management help with worm control?

The worms that end up in our horses come from contaminated pasture. A redworm can develop from an egg in a dung pile to the larval stage in just five days. In dry conditions, it can travel a metre in that time but in wet weather they can travel up to three metres.

A harsh winter or extremely hot dry summer can kill off worms on pasture, but UK weather is rarely extreme enough to harm them, which is why collecting droppings is vital. Wet, mild weather just helps worms to spread further, making it even more important to remove droppings regularly in such conditions.

The main focus when controlling worms is on minimising the number of them that horses consume while grazing. There are a number of pasture management strategies that we can use to do this.

Key points for good pasture management
  • DO pick up droppings: this is a very efficient method of controlling parasitic worms.
  • Droppings should be picked up at least twice weekly, particularly during warm weather.
  • The muck heap must be located well away from areas where horses graze.
  • DO cross-graze with cattle or sheep: they act as ‘biological vacuum cleaners’, consuming equine parasite eggs and larvae (most of which can’t survive in cows and sheep) as they graze.
  • DO NOT overstock or overgraze your pasture: allow at least 0.4 to 0.6 hectares (1 to 1.5 acres) of grazing per horse.
  • Overstocking and overgrazing encourages horses to graze close to the ground and to droppings, where the concentration of parasite eggs and larvae will be highest.
  • If you are deliberately overstocking your grazing to aid in weight management, droppings must be picked up more frequently to counteract this.
  • DO NOT harrow to spread droppings, as this just spreads the parasites around the entire pasture.

How do I worm my horse correctly?

Key points to remember:

  • It’s really important to weigh your horse before you worm him or her to make sure they get the correct dose. Your Equine Veterinary Hospital or local weigh bridge will tell you their exact weight but using a weightape will give you a good enough estimate.
  • If you use a weightape, you should add 10% to the weight shown by the tape.
  • Make sure all the wormer goes down the horse’s throat – tilt their head up after you have given the wormer to stop them spitting it out.
  • If your horse spits out even a small amount of the product the dose can be significantly lower than it ought to be, which increases the risk of resistance.
  • If you’re not confident worming your horse, enlist help from someone experienced. You can also practise with a clean, empty syringe to get your horse used to it.

Are some horses more susceptible to worm damage than others?

Key points to remember:

  • Young horses have less natural immunity to worms than older horses, so they tend to carry higher parasite burdens and contribute more eggs to the pasture.
  • Mature horses that are stabled continuously are unlikely to pick up MANY parasites because the worms are unable to complete their life cycle.
  • Even short periods of grazing (e.g. when at a competition) could lead to infection – you should never assume that a horse is free of parasites, whatever its lifestyle.
  • Small redworm larvae can develop and survive in deep litter straw bedding.
  • Roundworm eggs, which mainly infect foals and weanlings, can survive for years in stables and non-pasture environments.
  • The benefits of allowing your horse regular turnout, both for his physical and mental welfare, vastly outweigh the increased risk of your horse developing a parasite infestation.
  • You can significantly reduce the risk of a worm burden through appropriate management of your horse and his environment.

What kind of damage do different worms do and how do you treat a horse for them?

Small redworms: what are they and what do they do to your horse?

Small redworms, also known as cyathostomins, are the most common and most dangerous parasite for horses. They reproduce very quickly and have serious consequences for your horse’s health.

How do small redworms cause damage?

Adult small redworms feed on intestinal tissue, with large numbers causing harm to the gut wall. They are one of the most common causes of spasmodic colic, particularly in young horses.

The term ‘encysted’ means hibernating when we talk about worms. Encysted small redworms are at the larval stage and will tunnel into the gut wall where they hibernate. They then lie dormant, usually over the autumn/winter period although some can remain there for months or years.

Whilst redworm are hibernating in the gut wall they do not cause a problem as such. However, when large numbers emerge in late winter or early spring they can damage the gut wall and cause colic, weight loss, diarrhoea or even death. Young horses less than six years old are most likely to be affected.

If a small redworm infestation is left untreated, in the long term it can cause severe damage to the intestinal wall. This reduces the horse’s natural ability to absorb nutrients and may mean the horse struggles to put on or maintain weight. In the very worst cases, a small redworm infestation can be fatal, with less than 50% of horses who experience damage to the wall of the large intestine surviving.

How are small redworms diagnosed and treated?

Clinical signs of small redworms are variable so they can be tricky to diagnose. A faecal egg count will show up an adult small redworm burden, but encysted small redworm won’t show up as they do not lay eggs. Some horses can also appear healthy while carrying a significant burden of encysted small redworm.

As yet, there is no test for encysted small redworm but this is something which is in development. Until a test is available, a suitable wormer should be given once a year in winter (in the period November to February) to protect your horse against encysted small redworm.

Your vet, faecal egg count provider or another Suitably Qualified Person (SQP) will advise on the best wormer to treat encysted small redworm, which will usually be moxidectin. Do expect to be asked questions about your horse if it’s anyone other than your vet prescribing the product, as the SQP needs the information to be able to advise you properly.

The use of a dewormer for adult redworm can cause any encysted redworm to emerge and trigger an acute response. If you suspect your horse has a redworm infestation, you need to be very careful when treating them. Make sure you consult your vet if you think your horse may be at increased risk of having encysted redworm or if it is showing any clinical signs. Your vet may recommend that your horse is given additional supportive medication to reduce any gut inflammation and aid recovery before a dewormer is given.

It’s important to note that your horse or pony must be more than 6 ½ months old to be treated with moxidectin. He or she also needs to be in good body condition. If you have any doubts or queries, please contact your vet or another SQP.

Small redworms can live on grazing and inside the horse for extended periods of time. Horses do not build up immunity to small redworm and it is becoming more resistant to dewormers. Both these facts make it even more important to control the risk through an appropriate worming and pasture management programme.

Large redworms: what are they and what do they do to your horse?

Large redworms, also known as strongyles, are a lower threat as they have responded well to common worming treatments. The population and prevalence has decreased but they still pose serious health consequences.

How do large redworms cause damage?

Adult large redworms are found in the large intestine and produce eggs which are passed in the horse’s droppings onto the pasture. The eggs are then eaten by horses whilst grazing. The larvae then hatch and burrow into the walls of the arteries that supply the horse’s intestine. They damage the lining of the blood vessels and cause blockages which stop the blood supply to the intestine.
Large redworm can also cause colic and the rupture of blood vessels. Severe damage from large redworm affects the horse’s digestion, causing spasmodic colic. In the very worst cases, the horse may need to have the damaged section of intestine surgically removed.

How are large redworms diagnosed and treated?

Clinical signs of large redworms are colic, anaemia, weight loss, difficulty maintaining or putting on weight, and a dull or lethargic demeanour. Large redworms will be picked up on a faecal egg count and treatment in the form of an ivermectin-based wormer can be prescribed if necessary.

Tapeworms: what are they and what do they do to your horse?

Horses of any age can suffer from tapeworm but the damage caused to the very young and the elderly makes them more vulnerable. Adult tapeworms live at the junction between the small and large intestine and release segments containing eggs into the droppings. These eggs are eaten by forage mites on the grazing land and are then picked up by the horse as they graze.

The presence of tapeworms around this junction of the intestine can cause impaction colic as they block the passage of food. They also irritate the intestine which can lead to spasmodic colic. Adult tapeworms can cause ulcers in the intestinal wall and may even rupture the intestinal tract. Tapeworms in foals can prevent normal growth due to malnutrition.

How is tapeworm diagnosed and treated?

Clinical signs of tapeworm include weight loss, colitis, spasmodic colic and impaction colic. In the worst cases, tapeworms can be fatal. Tapeworm eggs are housed in segments so will not be picked up on faecal egg counts.

The presence of tapeworm can be identified using a saliva test which measures the level of antibodies produced in response to tapeworm parasites. This can accurately detect the level of tapeworms in the horse’s system and will indicate whether treatment is required.

Treatment will be advised by your vet or test provider but will often be with a wormer containing pyrantel or praziquantel.

Roundworms: what are they and what do they do to your horse?

Roundworms, also known as ‘ascarids’, commonly only affect young horses under four years old and are given the name ‘large roundworms’ because they can be up to 30cm in length. Due to their size, roundworms are likely to block the intestine of a small foal, causing impaction colic and rupturing of the intestine. This condition can be fatal and may require emergency surgery to give the foal any chance of survival.

How are roundworms diagnosed and treated?

Clinical signs of roundworms include coughing, nasal discharge, depression, a rough coat, impaction colic, weight loss or a struggle to maintain or put on weight. Faecal egg counts will pick up on roundworm infection. Your vet or test provider can advise on what treatment is needed, which would likely be a wormer containing pyrantel.

Bots: what are they and what do they do to your horse?

Bots, also known as gasterophilus intestinalis, gasterophilus haemorrhoidalis or gasterophilus nasalis, are flies rather than worms and lay pale yellow eggs on the horse’s legs, neck and shoulders, and around his muzzle. Within five days of being deposited, the eggs will hatch into larvae once stimulated by the horse licking or biting them. The larvae will either be ingested by the horse or will crawl to his or her mouth, where they will burrow into the gums and tongue.

After around four weeks the larvae migrate from the mouth to the stomach, where they will attach themselves to the lining of the horse’s stomach and intestinal tracts and dig in.

The larvae will remain in the horse’s digestive system for around eight to ten months, before passing in the manure. They will then pupate in the soil for three to five weeks before emerging as adults, ready to start a new cycle.

How are bots diagnosed and treated?

Preventative measures include use of fly spray and a fly sheet, as well as using a bot fly knife (a flat metal tool used painlessly as a scraper) to remove any eggs from the horse’s skin. Make sure you don’t touch your eyes whilst removing bot eggs and always wash your hands afterwards.

Signs of a bot fly infection include sensitivity of the mouth and dental issues, including problems chewing and loss of appetite. The horse may also develop sinus infections and discharge mucus from their nose.

Bot fly infection can cause gastrointestinal issues including swelling, ulceration and discharge at the attachment site. If large numbers of larvae group in the horse’s stomach they can cause physical blockages which can lead to impaction colic. The larvae also consume nutrients, making it harder for the horse to keep weight on and causing changes in their coat and body condition.

Bot fly larvae can also burrow into the horse’s skin and cause lesions or tears, in which infection can occur.

Your vet or SQP will advise on treatment, probably ivermectin or moxidectin. The treatment should be given in winter after the first frost or in December, whichever is the earlier, to prevent the larvae starting to burrow in the mouth.

Want more advice on worm control?

If you would like further advice on worm control, you can call our Advice Line on +44 (0)1953 497 238 in office hours. For queries about which wormer to use, contact your vet or another SQP. If you suspect your horse may have a significant worm burden or they are showing clinical signs, please call your vet straight away. If you rescue, rehome or buy a horse without knowing their health/worming history, you should keep them isolated until they have been tested, and if necessary treated, for a worm burden.

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