Thrush is a moist exudative dermatitis that specifically involved the central and lateral sulci of the frog. This disease has always been thought to be associated with unhygienic conditions, but this may not be totally accurate. The horse possesses a natural hoof-cleaning mechanism. In the nomad foot, as weight is home on the limb, the third phalanx will descend, causing the sole to flatten. Descent of the coffin joint occurs as the navicular bone gives in a distopalmar direction, pushing against the navicular bursa and the deep flexor tendon, and finally causes 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 it is seen in a large percentage of animals that are kept in immaculate conditions, whereas other horses who live in a filthy environment never contract the problem.

Impairment of the hoof-cleaning mechanism can be due to three problems: 1) chronic lameness; 2) improper hoof trimming, or 3) insufficient exercise.

Chronic lameness, especially when involving the heel area, causes decreased weightbearing, which in turn causes inadequate heel expansion and decreased wear on the horny wall. This causes increased length of heels, with a resultant deepening of the hoof sulci of either side of the frog.

Improper and irregular hoof trimming also leads to improper balance and increased length of the heels, causing impairment of the natural hoof-cleaning mechanism.

Third, 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.

Accumulation of moist saw-dust, manure, and other organic material creates an environent of increased moisture and decreased oxygen that is conducive to rapid bacterial development. A commonly isolated bacteria is F.necrophorum, which is responsible for the characteristic black, watery discharge associated with thrush. This bacteria can create a tremendous tissue response and associated pain when it invades normal sensitive tissue.

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 tissue. Application of any number of antiseptic or astringent preparations will resolve the associated bacterial infection. If a chronic lameness is present, the underlying cause must be diagnosed and corrected.
 

Stephen E. O'Grady, DVM, MRCVS

Dr. Stephen E. O'Grady is an equine practitioner and farrier practicing in the Northern Virginia area. He is also a member of the Farrier's Liaison Committee of the American Association of Equine Practitioners.