THRUSH IN HORSES

What is thrush?

  • Thrush is an infection which causes degeneration of the frog of the horse’s feet. The central and lateral grooves of the frog become softened and subsequently damaged.
  • Thrush is the equine equivalent of athlete’s foot in humans.
  • A healthy frog has a regular outline and is smooth, while a thrushy frog is soft, pitted and irregular.

    • What causes thrush?

  • It is caused by an anaerobic organism named Fusiformis Necrophoris (same as the sheep foot rot organism), which thrives in most moist environments.
  • An anaerobic organism can survive without oxygen.
  • Keratonolytic (keratin-loving) bacteria can attack the softened tissues of the frog causing it to rot away.
  • These damaged areas multiply and increase when the frog area becomes under-run.

    • What are the usual predisposing factors in thrush?

    Inadequate or poor standards in:

  • Stable management.
  • Conformation.
  • Farriery or the lack of it.

    • Stable management

  • Poor stable management: The primary cause is considered to be poor hygiene which can cause thrush to flourish. For example unhygienic environmental conditions which require the animal to stand in mud, urine-soaked or faeces-soaked bedding, is one of the main causes.
  • The frog is made from soft horn and is susceptible to penetration by bacteria especially when stabled horses are left standing in wet and dirty bedding. The invasive organisms that cause thrush thrive in moist conditions. They burrow through the horn and can cause irritation of the underlying sensitive tissues.
  • However, while stabling a horse for prolonged periods in foul, underfoot conditions can cause thrush, some horses kept in these conditions are unaffected, while others in perfect accommodation can develop thrush.
  • The individual susceptibility of the horse seems to be a major factor.
  • When thrush is confined to the frog it rarely causes lameness, but once the deeper tissues are affected, intermittent tenderness can result.
  • Turnout on constantly damp, swampy or marshy pasture may also cause thrush.

    • Conformation – poor foot conformation (especially of the frog)

  • Hoof and frog conformation play a significant role in a horse’s susceptibility to thrush. Long, forward-sloping heels tend to encourage the central frog cleft to deepen and continue into the bulbs of the heel. This area can then get packed with debris, leading to inflammation and infection. Therefore some horses are more susceptible to this condition, as their foot conformation can lead to a predisposition to thrush.
  • For example, a deep cleft in the frog may become packed with sand after working in an arena. If not carefully cleaned, this could lead to irritation and allow bacteria to enter.
  • Long narrow feet, prone to contracted heels, with associated small, narrow frog and compressed involuted central groove.
  • Sheared heels, where a gap develops between the bulbs of the heels due to a chronic foot imbalance.
  • An acquired frog deformity, perhaps as the result of an injury.

    • Farriery

  • Poor foot care: Failure to clean out the hooves regularly. This is one of the commonest causes of thrush. If not cleaned out regularly the hoof does not get the opportunity to dry and remains moist, making perfect conditions for thrush to develop.
  • Poor or incorrect foot trimming/shoeing: Lack of frog pressure due to bad shoeing or poor standards of foot trimming (e.g. sheared heels). Shoeing with full pads, allowing dirt and moisture to collect and fester. A badly shaped shoe, nailed too far back, can prevent expansion and contraction of the heels, leading to shrinkage and possible “rotting away” of the frog. A badly fitting frog plate of a heart-bar shoe can damage the frog, leading to infection.

    • Is thrush a painful condition?

  • Despite the very strong nasty smell and discharge, the horse often appears completely unperturbed by the problem, with no apparent discomfort or lameness evident.
  • However, pain and lameness can occur in some more serious long-standing cases.
  • This occurs where the infection is severe and it can under-run the adjacent sole and spread to involve the deeper structures, such as the digital cushion, hoof wall and heel bulb. Then there may be some pain on palpation around the frog and bulbs of the heel, together with filling of the limbs and a varying degree of unsoundness. This is due to infection getting into the deeper tissues and then travelling up the horse’s leg.

    • What are the symptoms of thrush?

  • The most obvious signs of thrush is the foul smelling black discharge from the lateral and central clefts of the frog which itself may have softer spots and appear irregular in shape.
  • The hind feet are more commonly affected but perhaps this is because the hind feet are not cleaned out as regularly or as often as the front feet.
  • The horse may show lameness. In some cases the cleft of the frog becomes deeply under-run and infection gets into the sensitive laminae.
  • When the central cleft of the frog splits, the infection proliferates and either on its own account or along with the independent medial and later movement at the back of the foot, pain and lameness result.

    • NEXT WEEK Treatment & prevention