What are intestinal parasites?

Intestinal parasites are one of the most common causes of disease in horses.1 Several types of internal parasites can infest horses, and they fall into two main groups: worms and bots.1

In this section learn more about:

  • Worms
  • Bots
  • Signs of Intestinal Parasites
  • Risk of Intestinal Parasites
  • Encysted Small Strongyles (ESS)
  • How to minimise the risk of Intestinal Parasites
  • Protection with Equest Plus Tape
  • Myths associated with Intestinal Parasites

The main types of worms that affect horses are:

  • Redworms (the Strongylus group) – the most common parasite. These include large redworms (e.g. Strongylus vulgaris) and small redworms1,2
  • Threadworms (Strongyloides westeri)1,2
  • Roundworm (Parascaris equorum)1,2
  • Tapeworms (the most common is Anoplocephala perfoliata)3
  • Pin worms (Oxyuris equi).1

Infestation with bots results in digestive upset and reduced appetite.2 This is because bot flies lay the eggs on the horse’s leg hair or under the jaw, and when the larvae hatch, they find their way to the horse’s tongue and mouth. Once swallowed, the larvae attach to the stomach lining, causing symptoms – in fact, a heavy infestation can result in stomach ulcers or ruptures.1

When the larvae are passed out in the dung, they develop in the dung, resulting in an adult fly, ready to reinfest nearby horses.1

The signs of worm infestation in horses can vary widely: some horses may show barely any signs; others will have severe disease and sometimes die as a result.

Signs of worms include:

  • loss of appetite1
  • poor growth2
  • weight loss2
  • anaemia, caused by redworms, which damage the blood vessels and the gut1
  • tail rubbing, due to irritation as pin worms crawl out to lay their eggs in the tail area1,2
  • coughing in young foals2
  • colic2

Pasture infestation occurs when eggs, larvae or immature worms are passed from an infected horse’s manure into the environment. When the eggs or larvae are accidently ingested, the horse is re-infected.2

In the case of tapeworm, eggs are passed in the manure and eaten by forage mites, with the immature worm developing within the mite.

When a horse accidently ingests the mite while grazing, the mite releases the tapeworm into the horse’s gut, beginning the cycle all over again.3

If there are many horses in a given area, the risk of environmental contamination from dung, and therefore worm infestation, will be increased.2

One of the most important and common causes of parasitic infection in horses are small strongyles (aka “cyathostomins”).1 In a recent major Australian study, 100% of horse properties were infected by this type of worm.2

ESS can be dangerous for your horse

Normally, infection with small strongyles causes signs and symptoms such as:1

  • decreased levels of performance,
  • reduced growth rates
  • weight loss
  • rough hair coat
  • debilitation
  • diarrhoea
  • various types of colic

 This may seem bad enough, but as part of their life-cycle, small strongyles take an “encysted” form (ESS), effectively burying themselves as a cyst in the wall of the large intestine. When the larvae mature and emerge from the cyst, which can be months or years later, this release can sometimes cause a condition called larval cyathostominosis, involving severe diarrhoea, colic or even death.1

Deworming against ESS

There are three main classes of treatment that can help control small strongyles: the benzimidazoles (e.g. fenbendazole and oxibendazole), pyrantel, and the macrocyclic lactones (ivermectin and moxidectin)1,3 These treatments differ in terms of their efficacy and which stages of the lifecycle they control. The encysted form is especially hard to treat, as the worm larvae are effectively “buried” in their cysts, where many drugs cannot reach.

Fenbendazole can kill the adult and developing larval stages at the normal dose, but requires a double daily dose (10 mg/kg liveweight) for 5 consecutive days to treat the encysted larvae. Unfortunately, resistance to benzimidazoles has become widespread and common.1,3

Pyrantel salts can kill sensitive adult worms, but are not effective against encysted larvae.Resistance to pyrantel is also widespread, though not as much as benzimidazole resistance.1,3

Ivermectin is highly effective against most adult and larval stages, but unfortunately has variable and low efficacy against encysted stages, even at high doses.3

In contrast, moxidectin has high efficacy against all stages, including the encysted form, when given as a single standard dose at a rate of 0.4 mg/kg liveweight.3

For the moment, resistance to macrocyclic lactones has been reported in only a few small studies worldwide.1

There are steps you can take to minimise the risk of intestinal parasites.
  • Consider cross-grazing with cattle or sheep if practical to do so
  • Feed horses off the ground
  • Minimise stocking rates
  • Drenching with an effective wormer2
  • Limit pasture reinfestation by removing manure regularly 2
  • Protect young horses by cleaning and disinfecting foaling boxes and stables and keeping young horses in low-worm pastures – segregate your horses by age, if necessary2
  • Monitor the amount of worms in your horse’s dung – your vet can use a worm test to find out if your worming control programme is effective or not2

You can protect your horse against a wide range of intestinal worms with Equest® Plus Tape.

Equest® Plus Tape protects against a broad range of worm species, including tapeworm, bots and roundworms. Since it lasts up to twice as long as other wormers, it saves money and time for horse owners.

It’s supplied in a ready-to-use syringe, calibrated according to the body weight of the horse in intervals of 25 kg body weight. If you have surplus gel after dosing your horse, you can use it to dose another horse, or store it for future use. The gel can be kept until the expiry date provided you close the barrel cap and store it at less than 30°C.

For more details about Equest® Plus Tape and correct dosing information, visit the Products section of this site.

Myth: Worms affect only young horses.

While it is true that foals and young horses are the most vulnerable to disease from worms, any horse can be affected, so all your horses must be protected against intestinal parasites.1,2

With Equest® Plus Tape, you can take a longer break from worming – and give your wallet a rest too.

That’s because Equest Plus Tape lasts up to twice as long as other wormers. So your horse is protected against a broad range of worms – including tapeworm, bots and roundworms – with just a few simple treatments a year.

Lasts up to twice as long as other wormers:

  • Fewer treatments each year
  • Lower cost over time
  • Pastures are free from faecal egg contamination for longer

Equest gives broad spectrum protection against worms:

  • The only wormer effective against encysted small strongyles with just 1 dose
    • Small strongyles are the most prevalent worm in Australia, and the encysted stages can be very dangerous if there is a mass migration into the gut.
  • Effective against benzimidazole-resistant strains of small strongyles
  • No documented resistance to Equest Plus Tape in Australia
  • Safe for use in pregnant mares, breeding stallions, and foals from 4 weeks

No hassles with Equest’s single-dose, low-volume gel: 

  • Save money – and hassles – with just a single dose each time
  • Low-volume gel and narrow barrel syringe make dosing quick and easy

Save time and money with Equest Plus Tape – Available from your favourite retailer or vet 


  1. Queensland Government Department of Agriculture, Fisheries and Forestry. Worms in horses, January 2012.
  2. Robinson S. Worm control in horses. PrimeFact 976, February 2010. NSW Government Department of Industry and Investment.
  3. American Association of Equine Practitioners. Tapeworms. April 2012.

Encysted Small Strongyles References

  1. Peregrine AS et al. Vet Parasitol 2014; 201(1-2): 1–8.
  2. Beasley A. Prevalence of cyathostomins, Strongylus vulgaris, Parascaris equorum and Anoplocephala perfoliata on Australian horse properties. Data on file. 2015.
  3. Corning S. Parasit Vectors 2009; 2 Suppl 2: S1.

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