Photo: The Horse Staff

A horse's hoof travels through some seriously nasty places. As a result it is bombarded with bacteria. So what keeps horses from continually oozing pus from infected feet? According to Stephen O'Grady, DVM, MRCVS, of Northern Virginia Equine, horses have a self-cleaning mechanism that helps maintain a healthy balance of bacteria in the hoof. However, if this mechanism is hindered, infection (such as thrush) can occur.

Thrush is an infection of the horse's frog. Bacteria invade the frog tissue adjacent to the sulci (the grooves on either side of the frog) and produce an offensive odor and a black discharge. If the infection progresses long enough, sensitive tissue might become involved, which can cause serious and sometimes permanent lameness.


Many types of bacterium can cause thrush, although Fusobacterium necrophorum is a common player. This bacterium is an anaerobic (survives without oxygen) organism that has the ability to invade and infect the frog's sensitive horny tissue.

It is a common perception that thrush is caused by poor hygiene and unclean living conditions. However, O'Grady explains, "this may not be totally accurate. The horse possesses a natural hoof-cleaning mechanism. In the nomad foot, as weight is borne on the limb, the third phalanx (coffin bone) will descend, causing the sole to flatten. Descent of the coffin joint occurs as the navicular bone gives in a distopalmar direction (from the coffin bone toward the ground), pushing against the navicular bursa and the deep flexor tendon, causing expansion of the frog as it approaches the ground surface. This continuous change in structure prevents the accumulation of material in the bottom of the foot.

"Impairment of this hoof-cleaning mechanism appears to be the outstanding cause of thrush, as thrush is seen in a large percentage of animals that are kept in immaculate conditions, whereas other horses that live in a filthy environment never contract the problem," he explains.

O'Grady says chronic lameness, improper hoof trimming, and insufficient exercise can reduce the effectiveness of this mechanism and increase the risk of infection.

He explains, "Chronic lameness, especially when involving the heel area, causes decreased weight bearing, which, in turn, causes inadequate heel expansion and decreased wear on the horny wall."

This causes the heels to grow longer, which causes the frog to recess. A horse is unable to naturally clean these deep, narrow cavities, and the bacteria are trapped.

"Improper and irregular hoof trimming also leads to improper balance and increased length of the heels, causing impairment of the natural hoof-cleaning mechanism," O'Grady adds. "Normal exercise is vitally important to promote normal physiology of the foot structures and to prevent organic material from packing into the sulci of the frog."


Generally, thrush is a mild disease that is easy to treat. There are several commercial products available that are effective in treating thrush. However, if the causative factors (chronic lameness, improper hoof balance, or insufficient exercise) are not corrected, the infection will likely return.

O'Grady says, "Treatment is directed at restoring the normal physiology of the foot through proper balance and trimming, adequate exercise to encourage frog stimulation, and debridement of all devitalized (dead) tissue."

Once the necrotic (dead) tissue has been removed, you can treat the area with an antiseptic (such as 2% iodine) or astringent (drying agent) preparation. Be sure to discuss the appropriate course of treatment with your veterinarian because it is possible to overtreat the infection and cause chemical burns or other damage to the area.

Take-Home Message

Thrush in a horse used to be a stigma for the owner; the condition was considered an indication of poor horsekeeping. However, even well-managed horses can get thrush if they have hoof imbalances. By cleaning your horse's hooves daily, you can help prevent this infection from occurring even in a horse with poor hoof conformation.