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.


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 im