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Horse looking over fence

Small redworm or Cyathostominae or Small strongyles

The small redworm is now the most common worm found in horses today - the majority of eggs in faeces or larvae on the pasture being those of the small redworm. The life cycle of the small redworm is from 6 weeks, but can last as long as 2 years. The small redworm is up to 2.5cm long, thin and reddish in colour.

The L3 infective larvae are ingested by the horse from the pasture; they then migrate to the large intestine where they burrow into the gut wall and become encysted. They may develop quickly into L4 larvae and emerge into the gut to become adults. However, up to 90% of the encysted larvae may become dormant, known as inhibited encysted larvae (EL3)(1). Tens of thousands of these encysted larvae can line the gut wall, where they impair absorption of nutrients, possibly resulting in weight loss and life-threatening illness. These inhibited encysted larvae can emerge ‘en-masse’ without warning. Potentially fatal, this emergence typically occurs during early spring and is known as Larval Cyathostominosis. Severe cases can result in a 50% chance of death(2). Young horses (less than 6 years of age) can often be at higher risk of the disease, but small redworm can cause life-threatening illness at any time of year and in any age of horse. 

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  • Diarrhoea
    Rapid and severe weight loss
    Colic and even death

  • 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 horses. 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.
    • Against 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 potentially again in early spring depending on grazing strategy.
    • Pasture Management:
      • Remove horse droppings from your pasture regularly – at least twice weekly.
      • 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 single 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 is the difference between strategic and routine worming?
    A strategic worming programme involves the use of Faecal Worm Egg Counts (FWEC) throughout the year with extended dosing interval periods between treatments. Horses on this programme are treated only when FWEC results are above a specified level (eg >200 eggs per gram (epg)). Care must be taken with this strategy, as FWEC may not give an accurate measure of the worm burden; not accounting for larval stages including encysted small redworm larvae present in the horse. Routine worming is the use of equine anthelmintics throughout the year in accordance with dosing intervals recommended by the manufacturer to treat for roundworms, tapeworms and bots.

    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 been suggested as method of slowing resistance development, although it is now widely recognised that it is not as effective a strategy as was first thought.

    In addition, pasture management and faecal worm egg counts (FWEC) also play a 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 inside your horse.