Thrush: Causes and Treatment

Definition and causes

The Merck Veterinary Manual describes thrush as “a degeneration of the frog with secondary bacterial infection that begins in the central and collateral sulci”, the sulci being the valleys on either side of the frog and the small groove in the middle of the frog. The bacterium that causes thrush is found in the horse’s environment, and grows very well in wet, unsanitary conditions such as dirty stalls or muddy paddocks. It is an anaerobic bacterium that prefers a low-oxygen environment, and finds the clefts of the frog a perfect place to multiply. The infection can extend into the sensitive tissue all the way into the digital plantar cushion and, if left unchecked, into the navicular and coffin bones themselves. Although a number of bacterial species may be involved in causing thrush, Fusobacterium necrophorum seems to be the most common.


A hoof infected with thrush will have an offensive odor, the surface of the frog being white and crumbly, and a thick, dark colored discharge will be found in the cracks, depressions, or fissures in the horn of the frog. Although a horse with thrush is not necessarily lame, if the infection is allowed to invade the sensitive tissues of the frog lameness will ensue, especially if thrush has found its way to the central sulcus.


The best treatment is good foot care. When thrush is found in a horse’s hoof the farrier or veterinarian will trim away the flaps of the frog to expose the infected area. From then on the following treatment recommended is as follows:

  1. Cleanse the foot, the clefts in particular, with warm water and disinfectant, using a wire brush.
  2. Clean up the horse’s living conditions, if necessary
  3. Treat with a germ-killing product. Some farriers recommend using iodine or a bleach-and-water mix, diluted 50-50. Of course, the product used will depend on how much sensitive tissue is exposed. If the infection was caught before it got very advanced and it’s only superficial, the diluted bleach would be fine. However, if the sensitive tissue of the frog has been compromised, the bleach would be much too caustic and would burn. If this case, the use of an iodine solution or commercial thrush lotion would be in order. Be careful not to use medications containing formalin if the frog is tender when cleaned out.
  4. Be careful when using a hoof pick to clean a thrush-infected hoof since you may be dealing with exposed sensitive tissue. Rather than a hoof pick, use a strip of gauze to clean a deep central sulcus, pulling it through the groove as you would dental floss.
  5. Bandage the hoof to prevent contamination.
  6. Treat the infection once a day for the first week, every other day for the second week, every third day for the third week, every fourth day for the fourth week, and then leave it alone for two weeks. Thrush usually disappears quickly and completely if the above steps are taken and seriously followed.
  7. A bar shoe may be used to promote frog regeneration

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