Thrush in horses is a degenerative infection of the central and collateral grooves of the frog. In classic cases it results from the softening and damage to these soft tissue structures of the horse’s foot through standing on wet, dirty bedding.

Keratonolytic (keratin-loving) bacteria, including the sheep foot rot organism fusobacterium necrophorum, attack the softened tissues of the frog, causing it to rot away.

The most obvious sign of thrush is a foul-smelling, black discharge from the frog, which itself may have softer spots and appear irregular in shape.

Despite the grisly smell and discharge, the horse often appears completely unperturbed by its unsociable affliction, with no apparent discomfort or lameness evident.

If the infection is severe it can underrun 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.

It should not be confused with canker, which is an altogether more serious infection (see below). Fortunately, canker is rare as it is a difficult condition to cure, whereas thrush usually resolves with correct management.

What causes thrush in horses?

  • Unhygienic environmental conditions: Stabling for prolonged periods on soiled, sodden bedding; turnout on constantly damp, swampy or marshy pasture. The damp conditions of a dirty stable provide the perfect environment for the anaerobic bacteria, (those needing a low-oxygen environment) which cause thrush to flourish.
  • Poor foot conformation (especially of the frog): 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. 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.
  • Poor or incorrect foot trimming/shoeing: A badly fitting frog plate of a heart-bar shoe can damage the frog, leading to a secondary infection; 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; shoeing with full pads, allowing dirt and moisture to collect and fester.
  • Poor foot care: Not regularly picking out and cleaning the feet

Traditionally, thrush has suffered bad press and is commonly considered a product of bad stable management. This is definitely not always the case. Some horses kept in foul underfoot conditions are unaffected, while others in perfect accommodation are. The individual susceptibility of the horse seems to be a major factor.

Treatment of thrush in horses

If a horse has thrush the underlying cause needs to be identified and removed. The horse should be moved to a clean, dry environment and the feet cleaned daily.

Treatment may need to be carried out by a vet or farrier and can be very time consuming. All dead and/or damaged tissue needs to be pared away on at least one occasion until healthy tissue is reached. After paring, topical treatment with a caustic material such as 10% formalin, chlorine bleach, phenol or providone iodine follows. An antibiotic solution or spray should then be applied and if the trimming has been extensive, bandaging may be necessary.

The hoof and its environment should be kept as dry and clean as possible. Keep stables clean with plenty of good-quality, dry bedding.

Paper or a shaving bed may be preferable and if the horse is turned out it should only be on a well draining paddock or it should be brought in to stand in the dry for several hours each day. If horses are in for long periods, bank the beds during the day to allow them to stand on an area of clean, dry concrete.

Antibiotics may be needed if the infection has spread. Ensure the feet are properly trimmed and shod, especially if there are any contributory foot issues.

As the bacteria are killed by oxygen, regular use of the hoof pick will allow air to the foot and reduce the ability of the bacteria to take hold.

The following homeopathic remedies can be useful in treating this condition: Merc Sol; Hepar Sulph; Silica and Lycopodium (for boosting general immunity).

Contined below…

Related feature from H&H

Thrush will never resolve unless the hoof hygiene is. A damaged frog is the perfect entry point for the bacteria that cause tetanus, so ensure that the horse has adequate protection against this.

Vet Karen Coumbe says: “Thrush should be relatively easy to sort out. If it lingers, review your management or consider whether it could be the more serious condition of canker. Draught horses tend to carry a higher likelihood of being canker candidates.”

Don’t confuse thrush with wounds, cysts and canker

  • Wounds – Sometimes the frog is sliced or punctured by flints or other sharp objects. These cuts can become infected if they’re deep, causing lameness.
  • Cysts – These are common on the frog and usually go unnoticed. They’re small pockets of white fluid which burst and develop into dirty cavities.
  • Canker – This is a more serious condition, where the horse is made lame and the frog becomes really soft, smelly and stringy. Canker can spread to the sole and is hard to treat,often requiring surgery. Thankfully these days canker is relatively rare.