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Horse looking over fence


Roundworms are the most common class of parasites to affect your horse. A wide range of species fall within this class, with the small redworm being the most common parasite to affect horses today.

An overview of all species important for horses:



Brief information

Small Redworm

Large intestine

Larvae burrow into gut wall and re-emergence can be fatal

Large Redworm

Larvae migrate in horses’ blood vessels

Adults in large intestine


Live in colon

Adults migrate to rectum to lay eggs

Large Roundworm

Larvae migrate through horse’s bloodstream to the liver and lungs

Adults in small intestine



Stomach Worm


Larvae in skin wounds prevent healing: ‘summer sores’

Intestinal Threadworm

Larvae migrate via lungs

Adults in small intestine

Neck Threadworm




Expand All
    • For many worms: a bit of diarrhoea until rapid and severe weight loss, colic and even death
    • Large redworm infection: also can be fatal by blocking blood vessels.
    • Pinworm: itchy anus and fly strike
    • Large Roundworm: coughing, poor growth rate and a dull coat. Because of the worms size, they can cause a fatal blockage or rupture of the gut.
    • Stomach Worm: gastritis
    • Intestinal Threadworm: diarrhoea, anorexia and dullness as well as a reduced growth rate and loss of weight, which in foals can result in potential complications in later life.
    • Neck Threadworm: occasional skin irritation and also painless swelling of the tendons and ligaments.
    • Lungworm: Symptoms include a persistent cough and increased respiratory rate. May cause secondary pneumonia.
  • Although adult worms may sometimes be seen in faeces, specific diagnostic tests are necessary to determine the nature of the worm burden. The Faecal Worm Egg Counts (FWEC) is a useful tool to measure the effectiveness of your worming programme, parasite status of new horses, and also to identify and treat just those horses with a significant worm burden. It should be noted that there are currently no practical diagnostic tests to identify encysted cyathostomins, being a larval stage which does not produce eggs. This worm must therefore be targeted with anthelmintic treatment which specifically affects this larval stage.

  • Treatment of the horse with a modern wormer that reduces also the number of inhibited encysted larvae (EL3).

    • Prevention should include a routine roundworm control throughout the grazing season. However with milder and wetter winters, horses are at risk of picking up infective larvae late in the year, as such routine roundworm control should be undertaken throughout the year. Small redworm are now the most common and harmful parasite to affect horse. Ingested while the horse grazes, the larvae can cause life-threatening illness due to the invasion and build up of larvae in the horse’s gut wall. In addition, although the widespread use of modern wormers has reduced the number of large redworms, the migratory larval stage of the large redworm remains potentially fatal.
    • ENCYSTED SMALL REDWORMS - Hidden in the gut wall the encysted small redworm if left untreated typically emerge en-masse in late winter/early spring. These should be treated at least in November and February.
    • Pasture Management:
      • Remove horse droppings from your pasture regularly – at least twice weekly during the grazing season, and once a week between November and March. Twice weekly removal
      • Don’t overstock pasture - ideally fields should contain no more than one or two horses per acre.
      • Alternate pasture with cattle / sheep if possible
      • Rest and manage the fields to ensure even grazing and good grass coverage
  • What is the main parasite that can affect my horse?
    The most common parasite to affect horses today is the small redworm (small strongyle/cyathostomin), which can cause diarrhoea, rapid and severe weight loss, as well as life-threatening colic.

    I understand that encysted small redworms are the biggest threat to my horse – what treatments are available to control this parasitic stage?
    Traditionally, the treatment to control encysted small redworm was a 5-day course of a fenbendazole–based wormer. In 2003 a moxidectin-based wormer was licensed for the control of all stages of encysted small redworm with a unique single standard dose.

    It is important to note that if there is widespread small redworm resistance to benzimidazole-based wormers, use of fenbendazole is not advisable where resistance is present.

    When should I worm my horse?
    Horses should be wormed routinely throughout the year for the control of roundworms, with strategic treatments given at specific times for the control of tapeworms, bots and most importantly encysted small redworm.

    My horse has a heavy worm burden? How do I start its treatment?
    If you are concerned that your horse has a very high worm burden it is advisable to contact your veterinary surgeon to establish a treatment specific to your horse’s needs.

    What happens if I overdose my horse?
    Although a slight overdose of an anthelmintic is unlikely to have harmful effects, as with all drugs, it is important to dose accurately and in accordance with the manufacturer’s recommendations or those given by your vet, especially in low bodyweight foals or pony foals. If in doubt please contact your vet.

    What about underdosing?
    Underdosing will not do the horse any harm, but you may be administering a sub therapeutic level of the drug and thereby exposing the worms to the drug but perhaps not at a sufficient dosage to kill them. Worms that survive treatment may pass on their “resistance” to subsequent generations and so causing the potential for resistance to develop to that drug.

    How important is it to weigh horses? Can’t I just guess his weight or give him a whole syringe?
    Establishing a horse’s weight by guessing and not using a weigh tape or weigh bridge, means you risk giving your horse an insufficient dose of wormer. By not giving your horse enough treatment for its body weight you risk giving a sub-lethal dose of the drug which will not be enough to kill all resident worms. These will not only be left to continue their life cycle and breed in your horse, but are also given the opportunity to build up immunity and resistance by being exposed to, but not killed by the drug.

    Do I need to rotate wormers each grazing season to combat resistance?
    There are a number of factors which may be involved in the development of resistance. It has been suggested that an ideal anthelmintic is one to which there is no resistance, and which can be given as infrequently as possible.

    Rotation has long suggested as method of slowing resistance development, although it is now widely recognized that it is not as effective a strategy as it was first thought.

    Pasture management and faecal worm egg counts (FWEC) play a major role in delaying the onset of resistance. Removing dung from the pasture removes most sources of re-infection, including resistant worms and so breaking their lifecycle and reducing the need for worming. It is possible to extend dosing intervals by using FWEC: only treating animals when they are producing eggs above a specific level. It is important to note, however, that FWEC do not give an indication of larval burden as eggs are only produced by adult worms, therefore FWEC will not give an indication of encysted small redworm larvae burden inside your horse.

    How frequently do I need to pick up the dung from the pasture?
    Dung should be removed regularly, at least twice weekly. Although harsh frosts may kill infective larvae on the pasture, high levels of infective larvae can occur at any time of the year. Thus, there is an increased likelihood of grazing animals acquiring parasite burdens throughout the year.

    Should I worm my pregnant mare?
    It is very important to worm pregnant mares, not only for their own health, but also that of the foal when it is born. Always check that the product you want to use is licensed to treat pregnant and lactating mares and try to worm her before foaling and throughout lactation according to manufacturer’s instructions.

    From what age should I start worming my foal?
    Foals have a low tolerance to worms so they can quickly acquire massive worm burdens. Seek advise from your Vet or SQP and always check ‘the age from use’ of any wormer prior to use.