Introduction
Hoof abscesses are probably the most
common cause of acute severe lameness
in horses encountered by veterinarians and
farriers. A hoof abscess can be defined
as a localized accumulation of purulent
exudate located between the germinal
and keratinized layers of the epithelium,
most commonly subsolar (beneath the
sole) or submural (beneath the hoof wall).
Organisms that are responsible for a hoof
abscess gain entry through the hoof capsule
(epidermis) into the inner subsolar / submural
tissue (dermis) where the organisms
propagate and initiate the formation of an
abscess. Foreign matter (such as gravel,
dirt, sand and manure coupled with infectious
agents such as bacteria or fungal
elements) generally gain entry into the hoof
capsule through a break or fissure in the
sole-wall junction somewhere on the solar
surface of the foot.
Anatomical Review
A brief anatomical review of hoof capsule
structures may be helpful before
discussing hoof abscesses. The foot is
composed of the hoof, the skin between
the bulbs of the heels and all the structures
within. The structures of the hoof complex
comprise the hoof capsule, distal phalanx,
digital cushion, ungual cartilages and deepf
digital flexor tendon 1. These biological
structures are susceptible to trauma and are
prone to various disease processes including
infections (hoof abscesses, puncture
wounds) and keratomas. The equine foot
is designed to perform numerous functions
including bearing the weight of the horse
at all gaits, protecting the structures contained
within the hoof capsule, absorbing
concussion as the hoof strikes the ground
along with providing traction 2. The unique
interrelationship of the structures working
in concert and the viscoelastic nature of
the hoof capsule allow the hoof to perform
these functions. The hoof wall, sole, frog
and bulbs of the heels comprise the hoof
capsule which, through the unique continuous
bond between its components, forms
a casing on the ground surface of the foot
which affords protection to the dermal and
osseous structures enclosed within the capsule
2. Furthermore, optimal protection is
reliant on overall hoof health and a strong
hoof capsule which can be influenced by
genetics, environment, exercise, nutrition
and farriery practices.
The dermis or corium lines the entire
inner surface of the hoof capsule and connects
the hoof capsule to the underlying
distal phalanx. The dermal tissue that
appears most susceptible to injury is the
laminar and solar corium. The bond or
junction between the hoof wall and sole
is especially important as it becomes
susceptible to damage when subjected to
the continuous repetitive stress of weight
bearing. Damage at the sole wall junction
may allow a portal for entry of pathogens
to invade the interior of the hoof capsule.
Distally at the sole wall junction, the
dermal lamellae end in the formation of
terminal papillae. These papillae are lined
by stratum germinativum, which produces
a flexible intertubular horn that fills the
spaces between the non-pigmented horny
wall and the horny sole. This association
forms the bond between the hoof wall and
the sole known as the white line or zona
alba. The sole wall junction extends around
the circumference of the solar surface of
the foot and the heels running forward on
the abaxial surface of the bars. Pathogens
or debris that may lead to infection commonly
penetrate the hoof capsule in one
of three ways.
- A separation or defect in the hoof wall
- A fissure or tract in the sole wall junction
- A puncture wound through the sole or
the frog.
A misplaced horseshoe nail could be
considered a puncture wound. However,
in this case, infection is introduced by a
nail entering the inner part of the sole wall
junction versus a puncture wound, where
infection gains entry by direct penetration
through the other soft tissue structures on
the solar surface of the foot. This article
will describe the most current information
and the most practical approach to
treatment for disease processes that occur
within the hoof capsule such as hoof
abscesses or puncture wounds.
Mechanism of a hoof abscess
It may be easier to understand how to
treat an abscess with a brief look at the
mechanism by which an abscess will form.
Foreign debris will gain entry and accumulate
in a small separation or fissure
located in the sole-wall junction anywhere
around the perimeter of the foot including
the abaxial surface of the bars adjacent to
the sole (Figure 1A & 1B). As the animal
bears weight, the pressure will cause foreign
matter to migrate through the
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Figure 1A&1B: Foreign debris will gain entry and accumulate in a small separation (red arrows) or fissure
located in the sole-wall junction (white line) anywhere around the perimeter of the foot (A) including a fissure
(circle)on the abaxial surface of the bars adjacent to the sole (B). |
fissure, forming a tract, which eventually reaches
the subsolar or submural tissue (dermis).
Once the debris reaches the dermis inside
the hoof capsule, the foreign material
activates the animal’s immune system
(defense mechanism) inciting an inflammatory
response within the dermal tissue.
The bacterium contained within the
debris propagate, further accentuating
the inflammatory response which draws
inflammatory cells into the area. Enzymes
released from the bacteria and from the
invading inflammatory cells (white cells)
lead to liquefaction tissue necrosis and the
development of the grey/black exudate. The
infection is quickly walled off with a thin
layer of fibrous tissue to form an abscess.
The inflammation and the pressure from
the accumulation of the exudate exerted on
the surrounding dermal tissue lead to the
pain and clinical signs associated with a
hoof abscess.
Clinical Signs
Most affected horses show a sudden
onset of (acute) severe lameness. The degree of lameness varies from being
subtle in the early stages to non-weight
bearing. The digital pulse felt at the level
of the fetlock is increased, usually bounding
and the involved foot will be warmer
than the opposite foot. With careful observation,
unless the abscess is in the middle
of the toe, the intensity of the digital pulse
will be much stronger on the side of the
foot where the infection is located. If the
abscess is long standing, there may be soft
tissue swelling in the pastern up to or even
above the fetlock on the side of the limb
corresponding to the side of the foot where
the abscess is located. The site of pain can
be localized to a small focal area through
the careful use of hoof testers. Sometimes
with acute lameness, the pain will be noted
over the entire foot with hoof testers and,
in this case, it is necessary to rule out laminitis,
a severe bruise or even a possible
fracture of the distal phalanx (P3).
 |
|
Figure 2: When a tract or fissure is found, it can be followed within the
sole-wall junction (white line) using a small thin loop knife, a 2 mm bone
curette or another suitable probe. A horseshoe nail makes an excellent tool
for drainage. |
Treatment
There is still debate between the veterinary
and farrier professions as to who
should treat a hoof abscess and the best
method in which to resolve the abscess.
Considering that a walled off hoof abscess
is an extension of the epidermis, it is the
author’s opinion that the infection could be
treated by either the veterinarian or the farrier.
The most important aspect of treating
a subsolar / submural hoof abscess is to
establish drainage. The opening should be
of sufficient size to allow drainage but not
so extensive as to create further damage.
When pain is localized to a small focal area
with hoof testers, a small tract or fissure
will commonly be found in the sole wall
junction. The tract or point of entry may
not always be visible as the sole wall junction
is somewhat elastic and tracts in this
area tend to close. In this case, a suitable
poultice should be applied to the foot daily
in an attempt to soften the affected area
and eventually a tract will become obvious.
 |
|
Figure 3A, 3B & 3C: (A) red arrow shows approach to abscess through the sole wall junction. (B) shows the tract
is open into the cavity of the abscess. A small opening is all that is necessary to obtain proper drainage. Pressure
from hoof testers are used to promote drainage. (C) shows drainage at the heel using a horseshoe nail. |
When a tract or fissure is found, it can
be followed / explored within the white line
using a small thin loop knife, a 2 mm bone
curette or another suitable probe such as
a horseshoe nail (Figure 2). The tract is
slowly followed until a grey/black exudate
(pus) is released and the probe enters the
“belly” of the abscess. At this point, the
tract is open into the cavity of the abscess.
A small opening is all that is necessary
to obtain proper drainage. This can be
determined by using thumb pressure on
the solar side of the tract just behind the
opening or by placing hoof testers on the
sole next to the tract and observing if more
exudate is expressed or a bubble is forming
at the opening of the tract when pressure is
applied (Figure 3A & 3B). Care should
be taken to avoid exposing any corium,
as it will invariably prolapse through the
opening, preventing closure of the tract
and possibly creating an ongoing source
of pain.Under no circumstances should
a routine hoof abscess be approached
through the sole!
The draining tract can be kept soft and
drainage promoted in several ways. The
author will generally apply a commercial
medicated poultice a for the first 24-48
hours. The poultice is immersed in hot
water, the water is squeezed out, the poultice
is placed on the foot and attached with
a roll of wide brown gauze, a cohesive bandage
and waterproof tape (Figure 4a, 4b
& 4c). The sheet version of this poultice is
preferred rather than the poultice pad that is
distributed by the company. The whole foot
including the coronet should be enveloped
 |
|
Figure 4: : Application of a medicated poultice. Submerge in hot water, squeeze out excess water, envelope the foot
with the plastic side of the poultice on the outside. Secure to the foot with a wide brown gauze, cohesive bandage
and duct tape. |
in the poultice. Another method to encourage
drainage is to apply what is termed a
‘soak bandage’. Here layers of soft practical
cotton (available in rolls) are folded
together and then used to envelope the foot
to form a thick bandage. MgSo4 (Epson
Salts) is placed on the inner foot surface
of the bandage and the bandage is attached
to the foot as described above. The bandage
is now saturated with hot water and
then saturated periodically over the next
24-48 hours. Using either of these methods
eliminates the necessity for continued
foot soaking which can be cumbersome and
possibly less effective.
Ichthammol ointment, which is a coal tar
derivative with mild antiseptic properties,
has been described for treating skin disease
in both humans and animals. The use of
an Ichthammol bandage for treating hoof
abscesses, both before and after drainage,
has become another traditional treatment
used by veterinarians, farriers and horse
owners with reportedly good results.
However, the author has no experience
using this product. There are numerous
commercial products marketed to treat foot
abscesses but these products will only be
helpful if they complement the principles
of drainage described above.
Once drainage is established the horse
should show marked improvement within
24 hours. Once drainage has ceased, the
hoof is kept bandaged with an appropriate
antiseptic such as Betadine b solution/ointment
or 2% iodine applied over the tract
until the wound is dry and sealed. At this
point, the opening of the tract is filled with
a medicated hoof putty c which keeps the
affected area clean and prevents further
debris from entering the tract or wound.
The shoe is replaced when the horse is
completely sound
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|
Figure 5: Small channel created in the hoof wall with a small pair of half round
nippers so tract can now be approached in a horizontal plane. |
Often, a painful tract can be located but
drainage cannot be established at the sole
wall junction. In this case, the infection is
deep and may have migrated under the sole
or wall away from the sole wall junction
or white line. Again, under no circumstances
should an opening be created in
the adjacent sole! This seldom leads to
the abscess and often leads to hemorrhage
and may create a persistent, non-healing
wound with increased potential for infection
or osteomyelitis of the distal phalanx.
Instead, a small channel can be created on
the abaxial or hoof wall side of the solewall
junction using a small narrow pair of
half-round nippers. The channel is made
in a vertical direction following the tract to
the point where it courses inward. Drainage
can usually be established using a small
probe in a horizontal plane (Figure 5).
Preferably, this is done at an early stage of
the lameness before the infection ruptures
at the coronet.
If left untreated, a hoof abscess will
follow the path of least resistance along
the outer margin of the dermal tissue and
eventually rupture at the coronet forming a
draining tract. Many horse owners actually
consider this to be an acceptable practice
and elect to wait for this to take place.
This practice often extends the time the
animal experiences severe pain. Rupture
at the coronet also leads to a permanent
scar or tract under the hoof wall. This
tract leading to the coronet may result in
a prolonged recovery from the abscess, a
chronic draining tract, repeated abscesses
and a full thickness hoof wall crack. Every
effort should be made to establish drainage
of the abscess on the solar surface of the
foot prior to a rupture at the coronet.
Infection from a
misplaced horseshoe nail
Dermal tissue can be inoculated by bacteria
from a misplaced nail in two ways. The
nail can be driven directly into the laminar
corium. When the nail enters dermal tissue,
the horse will generally show discomfort
as the nail is driven into the foot and there
will be hemorrhage present where the nail
exits the outer hoof wall. Blood observed
at the exit of the offending nail will alert
the farrier of the misplaced nail. The blood
actually has a beneficial function as it acts
as a “physiologic rinse” to dilute or eliminate
bacterial contamination. Removal of
the nail and application of an appropriate
antiseptic will usually prevent infection.
Another scenario that occurs frequently is
while the farrier is driving a nail, the horse
shows significant discomfort indicating the
nail is invading dermal or sensitive tissue.
Often the farrier will remove the nail, place
it in another spot / direction and again drive
it into the foot. However, when this scenario
occurs, the farrier should remove
the shoe and examine the spot where the
nail entered the foot. If a nail has entered
dermal tissue (even if removed), it causes
trauma to the dermal tissue and can seed
the area with organisms which may lead to
abscess formation. If the nail has entered
the foot inside the sole-wall junction, the
owner/trainer should be alerted as to the
potential problems and the horse can be
placed on an oral broad-spectrum antibiotic
for 3 – 5 days as a prophylactic measure.
Lastly, we have the condition described
as a “close nail” where the nail is placed
such that it lies against the border of the
dermis just axial to the hoof wall. Pressure
against the corium combined with constant
movement of the nail against the corium
as the horse moves and bears weight may
cause an inflammatory response and allow
any bacteria that were introduced with the
nail to form an abscess as described above.
There is generally a lag period of 7-14 days
or even longer before clinical symptoms
or discomfort is observed following the
placement of a “close nail”. Treatment
again would be to establish and promote
drainage.
Penetrating
Injuries to the Foot
Puncture wounds to the solar surface of
the foot most often occur when the horse
steps on a sharp object(s) such as a fixed
narrow solid object, a sharp rock, a nail or
piece of glass which penetrates the horny
sole. Superficial puncture wounds penetrate
only the cornified tissue while deep
wounds penetrate the germinal epithelium.
Wounds to the sole need only penetrate 1
cm or less to invade germinal epithelium
and seed the site with bacteria that leads
to infection. Puncture wounds of the sole
will be discussed here as puncture wounds
that involve the soft tissue structures of the
palmar / plantar foot are beyond the scope
of this article.
Farriers are often asked to treat puncture
wounds to the solar surface of the foot.
Medications such as antibiotics and anti-inflammatory
drugs may be indicated and
will need a veterinarian’s prescription. If
a farrier were to treat an established infection
in the hoof, it would be perceived as
practicing veterinary medicine and the farrier
could be held liable. Farriers are often
asked to place a shoe with a removable
treatment plate on the foot with a puncture
wound for protection but at the same
time allowing access for daily treatment.
As drainage ceases and the puncture wound
begins to cornify, the farrier will be asked
to place a pad between the hoof and shoe
for protection until healing is complete.
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|
Figure 6: Small channel created in the hoof wall with a small pair of half round
nippers so tract can now be approached in a horizontal plane. |
Clinical Presentation
and Diagnosis
Puncture wounds frequently create
marked lameness. The degree of lameness
may vary considerably depending
on the depth, location and duration of the
wound. Superficial wounds may initially
have minimal lameness but can progress
to severe lameness within several days
with the development of an infection. In
general, puncture wounds that invade the
corium become quite painful soon after the
injury as the corium above the horny sole
seals, thus preventing any further drainage
and creating a medium for infection.
Progression to severe non-weight bearing
lameness can occur as the rigid hoof capsule
restricts the swelling associated with
not only the inflammatory response from
the trauma of the puncture wound but
the resultant infection within the dermis.
Wounds that involve damage to deeper
structures such as the distal phalanx (fracture)
are painful from the onset. Depth
of penetration can be difficult to ascertain
when the wound progressives past the horny
sole and the severity of the clinical signs
do little to help define which structures
are involved. An increased digital pulse
is common and, in some cases, the digital
pulse may be increased only on the affected
side (often with a recent wound). Increased
heat may be palpable at the coronary band
and/or over the hoof capsule in the affected
limb. Longstanding wounds may lead to
a diffuse swelling in the soft tissue of the
pastern and above. Early hoof tester application
after the injury may reveal focal
sensitivity but over time a painful reaction
may be elicited over the entire sole region.
Visual inspection of the solar surface of
the foot may reveal the source of the lameness
but often the defect in the sole is not
apparent. If the offending object such as a
nail is noted, an attempt to obtain a radiograph
is essential in order to measure
the direction and depth of penetration
and to evaluate which structures may be
involved (Figure 6). Examination should
begin by cleaning the solar surface of the
foot with a wire brush and paring the sole
lightly with a hoof knife; this may reveal
the site of the puncture wound, a black tract
or a crack in the horny sole. Due to the
invasive nature and serious complications
that can occur following puncture wounds,
it is the author’s opinion that when dermal
tissue is involved and requires debridement,
a veterinarian should become involved in
the case. Any delay in the initiation of the
appropriate treatment can have serious consequences.
Debridement may be painful
and necessitates the use of local analgesia
at the level of the palmar digital or abaxial
sesamoid nerves. Once the surface
of the foot is cleaned and the wound or
tract is identified, a sterile prep of the foot
with an antiseptic scrub and alcohol rinse
should be performed. This will allow further
exploration of the wound without fear
of contaminating the surrounding normal
tissue. Probing the wound with a blunt sterile
probe or teat canula can help determine
the depth and direction of the wound tract.
Radiographs taken with the probe or canula
in place is another option to accurately
assess depth and direction of the wound
along with any gas shadows, debris or any
radio-opaque foreign bodies that may be
present. If an obvious crack or black tract is
found, exploration may lead to a pocket of
infection and subsequent drainage. A small
looped hoof knife or a bone curette (# 2) is
useful to explore these areas.
Penetrating objects are contaminated
with dirt, rust and manure and this material
may be driven deep into the wound. Without
adequate drainage an anaerobic environment
develops that promotes the growth
of anaerobic bacteria. Contamination with
the organism Clostridium tetani is of particular
concern because of the potential
threat of tetanus. This disease is difficult
to treat successfully and an inquiry into
the animal’s history of appropriate vaccination
is important. Although adequate
immune protection may exist from a previous
vaccination with tetanus toxoid, a
booster of tetanus toxoid should be given
in the event of a puncture wound to the
foot. Superficial wounds carry a good
prognosis and can have dramatic resolution
of lameness within 24-48 hours following
drainage while deeper wounds require surgical
debridement. Superficial wounds and
infections are effectively treated by establishing
drainage, applying an antiseptic
bandage until drainage has ceased and
protecting the foot until the hoof capsule
defect has healed.
Long standing and deeper puncture
wounds require more extensive debridement.
Surgical drainage and debridement
of necrotic soft tissue or possibly infected
bone is necessary for the wound to heal.
Wounds to the sole can be safely explored
and debrided with the horse standing using
local analgesia. The horse should be placed
on systemic antibiotics and anti-inflammatory
medication before surgery. An area of
sole 1–2 cm in diameter should be removed
around the puncture site in a conical fashion
so that the tract can be completely
explored. The surgical approach should
follow the draining tract and allow adequate
exposure for removal of any diseased
tissue and to establish sufficient drainage.
Creating the wound in a conical manner
prevents the corium from prolapsing into
the drainages site which is both painful and
prevents healing.
REFERENCES
- Parks, A.H. Form and function of the
equine foot. In: O’Grady SE, ed. The
Veterinary Clinics of North America,
Vol. 19:2. Philadelphia: W.B. Saunders
Co, 2003;296-298.
- 2O’Grady, S.E. Strategies for Shoeing
the Horse with Palmar Foot Pain, in
Proceedings. 52nd Annu Conv Am
Assoc of Equine Pract 2006; 209-212.
- Redding WR, O’Grady SE. Septic
Diseases Associated with the Hoof
Complex: Abscesses and Punctures
Wounds. In O’Grady, SE, Parks, AH,
ed. The veterinary clinics of North
America: Equine Practice. 2012 vol.
28:2. Philadelphia: W.B. Saunders, pp
423-440.
- Richardson GL, Pascoe LR, Meagher D.
Puncture wounds of the foot in horses:
Diagnosis & treatment. Comp Cont
Educ Pract Vet 1986; 8: S379-S387.
- Stashak TS. In: Adams’ Lameness
in horses. 4th edn. Lea and Febiger,
Philadelphia. 1987; pp. 703-710.
- Redding WR, Pathological Conditions
Involving the Internal Structures of
the Foot, Equine Podiatry: Saunders/
Elsevier, 2007.
FOOTNOTE
- Animalintex® 3M Animal Care
Products, St. Paul, MN 55144
- Betadine® Purdue Pharma L.P.,
Stamford, CT 06901
- Keratex Medicated Hoof Putty® P.O.
Box 2 Brookville, MD 2083
