Background on Thrush and the Hoofs Self-Cleaning Mechanism.
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)
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.