Thrush is a degenerative condition of the epidermal tissue of the
frog generally caused by a bacterial
infection. The disease begins when bacteria
penetrate the outer horn or epidermis of the
frog and is characterized by deterioration of
the frog and the presence of black necrotic
exudate with a foul odor.1,2,3 In severe cases,
the bacteria can reach the dermis, the sensitive
tissue beneath the frog, causing pain and
lameness. Blood may be encountered when
the frog is cleaned with a brush or on the end
of the hoof pick during routine grooming.
Severe cases of thrush must be differentiated
from canker, which is a proliferative
disease where abnormal frog tissue increases
in comparison to thrush, which is a degenerative
disease where the horn of the frog
deteriorates (Figures 1A & 1B).
Figure 1A&1B: Figure 1a is thrush. Note the deterioration of the frog and
the recession below ground surface of the foot. Figure 1b is severe canker.
Note the proliferation of the diseased horn.
Etiology Of Thrush
The overall health of the frog is an
important component of the etiology of
thrush. A healthy, well-formed frog is
broad, firm and deforms with thumb pressure.
The frog should fill the space between
the heels of the hoof capsule and, when
measured at its base (the widest part), the
frog width should be at least 70% of its
A large, healthy frog acts as an
expansion joint that holds the heels apart,
shares the load-bearing function with
the other structures of the palmar/plantar
hoof and helps to absorb concussion.
Under normal conditions, weight bearing
stimulates the normal physiology of
the frog, ensuring its continued health.
A well-shaped frog that fills the space
between the heels also promotes a natural
self-cleaning mechanism such that when
the foot is loaded, the frog expands, expelling
any accumulated dirt or debris from
the frog sulci.
An unhealthy frog is markedly smaller
in size and recessed below the level of the
ground surface of the hoof, thus creating a
void in which debris can accumulate. This
can occur for a variety of reasons, including
many of the recognized hoof capsule
distortions such as the low heel, clubfoot or
sheared heels along with inappropriate farriery
practices. Other contributing factors
for thrush include wet, unhygienic stable
conditions, especially when horses continually
stand in urine and manure-soiled
bedding; neglecting daily routine foot care;
and lack of exercise.1,2,3,5 The fundamental
problem with palmar/plantar conformation
usually involves the frog not being near or
on the same plane as the heels, thus not
contacting with the ground, which reduces
the stimulation from the ground, causing
the frog to atrophy.
As the frog recedes within the hoof
capsule, the frog loses its “self-cleaning
mechanism,” which allows material (dirt,
manure, etc.) to accumulate over the frog,
creating excessive pressure. Over time,
this constant pressure on a compromised
frog leads to increased deterioration and
atrophy. Weakened by the reduced protective
horn of the epidermis, the frog tissue
becomes susceptible to penetration by bacteria
and, consequently, development of the
disease. Furthermore, the accumulation of
debris over the frog creates an environment
conducive to bacterial growth, especially
for anaerobic bacteria. The recessed frog
loses its ability to act as an expansion joint and the heels begin to contract. Thediseased frog can no longer share in supportingthe horse’s weight, which shifts most of the load-bearing function solely onto the heels of the hoof capsule.
No specific organism has been identified
as the sole cause of thrush. Two
anaerobic bacteria species, Bacteroides
sp. and Fusobacterium necrophorum
are opportunistic organisms found in the
soil and are commonly isolated from the
bottom of the horse’s foot. On the positive
side, when the necrotic tissue of the frog
is debrided and the conformation of the
structures in the palmar/plantar foot are
improved, the anaerobic conditions will
be improved and bacterial growth will be
decreased. Furthermore, if antimicrobials
are required, these organisms are quite susceptible
to most topical preparations.
Figure 2: This picture shows an
untrimmed foot with thrush, the frog
recess below the ground surface of
the foot, sheared heels and a fissure
present in the central sulci of the
frog, extending to the hairline.
And Diagnosis Of Thrush
The diagnosis of thrush is usually
straightforward and based on examination
of the frog and surrounding structures.
There is usually an increased amount of
moisture on the bottom of the foot and a
black exudate in the sulci of the frog. This
exudate, which varies in quantity, usually
has an offensive odor.2,5 The affected sulci
of the frog are often deeper than usual and
may extend into the sensitive tissues of the
foot, causing them to be painful.
The frog will generally be small,
recessed within the hoof capsule, lack
a firm consistency and have a necrotic
appearance. There may be a fissure present
in the central sulci extending from the
base of the frog to the hairline at the bulbs
of the heels (Figure 2). The frog may also
be undermined and areas of necrotic horn
can often be detached from the underlying
Lameness is present in severe cases
that involve the dermis, and swelling of the
distal limb may be seen.
Figure 3A&3B: Figures 3A and 3B both present foot conformation showing
the frog receded and asymmetric foot landing, leading to sheared heels. The
difference in force on each heel is exerted in opposite directions and results
in the fissure in the base of the frog. Figure 3b is a severe clubfoot combined
with a sheared heel. Note the loss of frog mass between the heels.
Fissure In The
Central Sulci Of The Frog
There will often be a fissure in the
central sulcus of the frog that may be
associated with thrush. The author believes
the fissure is a primary structural defect
in the soft tissue structures in the palmar/
plantar section of the foot that secondarily
becomes infected, leading to thrush. The
fissure is often seen in horses with a hoof
capsule distortion (such as low heels with
instability and sheared heels), a recessed
frog, compromised soft tissue structures
and an abnormal strike pattern.
The asymmetrical landing pattern results
in an unequal force placed on each heel,
causing the heels to move in opposite
directions. This movement in opposite
directions causes a fissure to form in the
weakest part of the soft tissue, which is the
central sulcus of the frog (Figures 3A and
3B). Significant movement in the fissure
can often be demonstrated by moving the
heel bulb in opposite directions.
Treatment Of Thrush
Farriery. Treatment for thrush begins
with improving the hoof capsule, the conformation
of the palmar/plantar foot and
the health of the frog. Thrush will not
be resolved as long as the frog remains
recessed below the ground surface of
the foot. The amount of improvement in
the frog is correlated with the amount of
chronic damage, especially involving the
frog dermis. If the horse can be given
time off from work, the shoes should be
removed, the heels trimmed to the same
plane as the frog, the perimeter of the hoof
wall rounded and the horse left barefoot.
When barefoot, load is shared by all the
structures of the palmar foot and the integrity
of structures improves rapidly and it appears
to promote regrowth.6
Treatment of the
thrush as described below is performed daily.
Figure 4: Palmar section of foot showing heels trimmed to
the base of the frog with the heels of the hoof capsule and
the frog on the same plane.
Figure 5: A straight bar shoe used to stabilize the
hoof capsule and the vertical movement of the heels.
If shod, the basic trim begins by drawing
a line across the widest part of the foot
and trimming the heels of the hoof capsule
such that they are on the same plane
as the frog (Figure 4). This can generally
be accomplished in most cases. The toe
length is reduced dorsally to the line drawn across the foot, resulting in approximate
proportions on either side of the line.6,7 If
significant heel is removed from an upright
or club foot, heel elevation may be necessary
to accommodate the resultant stresses
placed on the deep digital flexor tendon.
When a fissure is present in the central frog
sulcus, it should be stabilized to decrease
the vertical movement of the heels that
will inhibit healing. This can readily be
accomplished by applying a straight bar
shoe (Figure 5)
Medical. Any detached areas of necrotic
horn on the surface of the frog should be
debrided. The frog is cleaned initially by
soaking the foot in a saturated (add salt
until it no longer dissolves) solution of
Epsom salts (MgSO4). Topical antiseptics/
astringents including 2% iodine, merthiolate,
chlorohexidine and various copper
sulfate solutions are applied on a daily
basis. Topical antibiotic ointments/solutions
appear to be unnecessary and caustic
preparations that contain phenol, formalin
or formaldehyde should be discouraged.
Daily aftercare. Horses should have
their feet cleaned and the frogs brushed
vigorously with a stiff brush, after which
an astringent is applied on a daily basis.1,2,3
If there is a fissure present in the central
sulcus, the area above the fissure should be
flooded with the antiseptic/astringent solution
to allow penetration into the fissure,
and then packed with a folded gauze pad.
The horse should be kept in a dry, clean
stall and bedded on wood shavings or sawdust.
Turn-out should be in a dry paddock.
Exercise should be encouraged to improve
the foot’s normal physiology.
Repeated trimming of any detached
horn on the frog may
be required until the
infection is controlled.
Routine farriery should
be scheduled every 4 to
5 weeks, paying close
attention to the trim, the
foot conformation, the
frog and the integrity of
the soft tissue structures of the foot.
There is a plethora of commercial products
purported to treat and resolve thrush.
However, unless the health and structural
integrity of the frog is improved, these
products are of limited value. It is rare to
see thrush in a foot with a healthy frog
and support structures in the palmar/
A review of the literature published on
the cause and treatment of thrush does not
consider the role of farriery in the etiology of
this disease process. Acquired hoof capsule
distortions such as a low heel, club foot and
sheared heels all affect the foot’s palmar/
plantar soft tissue structures and the frog’s
health. Therefore, the health of the frog can’t
be improved and, consequently, the thrush
can’t be resolved without using appropriate
farriery. Remembering that prevention
is always superior to treatment, emphasis
should be placed on good farriery as the primary
means for prevention of this disease.
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