A fresh look at white line disease
Reprinted with permission from Equine Veterinary Education (EVE). Original published in Equine Veterinary Education Vol 23 Oct 2011.
S. E. O'Grady
Northern Virginia Equine, Marshall, Virginia, USA.
Keywords: horse; white line disease; hoof wall separation; resection; therapeutic farriery
White line disease occurs secondary to a hoof wall
separation. Clinical signs may vary from not being lame to
severe lameness with rotation of the distal phalanx
depending on the extent of the disease affecting the inner
hoof wall. The author has found that removal of the hoof wall
overlying the diseased area combined with the appropriate
farriery is the most important aspect of therapy.
|Fig 1: Classic white line disease with separation at junction of stratum medium and stratum internum.
White line disease (WLD) is a disorder localised to the equine
foot. The problem is widespread, the aetiology and
mechanism of the disease are poorly understood and
treatment is often controversial. White line disease is a term
used to describe a keratinolytic process that originates on
the solar surface of the hoof characterised by a progressive
separation of the inner zone of the hoof wall (O'Grady 2001,
2006; Moyer 2003; Pleasant and O'Grady 2009). The
separation occurs in the nonpigmented horn at the junction
between the stratum medium and stratum internum (Fig 1).
The destruction that occurs in the separation as a
consequence of WLD remains superficial to the stratum
internum and does not invade the dermis.
A separation in the hoof wall is considered to be a
delaminating process potentially thought to originate from
genetic factors, mechanical stress, inappropriate farriery
and environmental conditions affecting the inner hoof wall
attachment (Moyer 2003). The separation, which can
originate at the toe, the quarter and/or heel, appears to
be invaded by opportunistic bacteria/fungi leading to a
type of infection where the organisms digest the horn
allowing the separation to progress to varying heights and
configurations proximally toward the coronet.
The disease has been termed seedy toe, hoof wall
disease, yeast infection, Candida and onychomycosis.
Onychomycosis is a mycotic disease that originates in the
nail bed of man and the dog. By contrast, in WLD the
infection appears to have originated at the solar surface of
the hoof and migrates proximally, approaching the
coronet but never invading it. Keratinophilic fungi are often
isolated from separated areas of the hoof wall; however, in
many cases of WLD, the pathogens cultured are purely
bacterial or a mixture of bacterial and fungal organisms
(Turner 1998). Therefore, until proven otherwise,
onychomycosis may not be the appropriate term when
referring to white line disease in the horse (O'Grady 2006).
|Fig 2: Inner section of the stratum medium (hoof wall) that is nonpigmented (reproduced by permission of Dr Chris Pollitt).
Anatomy of the hoof wall
The hoof wall consists of 3 layers which are the:
- Stratum externum (external layer)
- Stratum medium (the middle layer)
- Stratum internum (the inner layer)
The stratum externum arises from the perioplic
epidermis and forms the thin outer layer of keratinised cells
that give the wall its smooth glossy appearance. The
stratum medium, which arises from the coronary epidermis,
forms the bulk of the hoof wall and is the densest part of
the horny wall. It consists of cornified epidermal cells
arranged in parallel horny tubules surrounded by
intertubular horn, which grow distally from the coronary
groove to the basal border. In all hooves the stratum
medium is always nonpigmented in the deepest inner layer
(Fig 2). The stratum internum arises from the lamellar
epidermis, is nonpigmented and, when combined with the
dermal lamellae, is responsible for attaching the hoof wall
to the distal phalanx. Distally at the sole wall junction, the
dermal lamellae end in terminal papillae. These terminal
papillae are lined by a germinal epidermis which
generates keratinised epidermal cells which fills the space
between the nonpigmented horny laminae as they grow
toward the ground surface (Pollitt 2010). This association
forms the bond between the hoof wall and sole known as
the white line or zone (Parks 2003). When observed from
the solar surface, this white line or zona alba is actually
yellow in colour and has a plastic consistency when compared to the dorsal hoof wall.
The aetiology of WLD remains undetermined. The problem
has been described in horses worldwide. WLD can affect a
horse of any age, sex or breed. One or multiple hoofs may
be involved and affected hooves can be barefoot or
shod. One or multiple horses on the same farm may be
affected. It is generally agreed that WLD is a multifactorial
condition that develops secondary to an initial separation
or hoof wall defect (O'Grady 2006; Pleasant and O'Grady
2009). It must be remembered that multiple causes for
white line disease have been proposed but none have
been scientifically proven.
Moisture may play a role as WLD is seen more in wet
humid areas, but it is also seen in hot arid conditions.
Excessive moisture may soften the foot, allowing easier
entry of dirt and debris into an existing separation.
Continual bathing of competition horses, especially during
the warmer months, may contribute to the incidence of
WLD in this population of horses. Excessively dry hooves on the other hand may form cracks or separations in the hoof
wall, allowing pathogens to invade.
Poor hygiene has been blamed but this is questionable
since WLD often appears in well managed stables.
Keratinopathogenic fungi and bacteria are commonly
isolated from the hoof wall defects of horses with WLD,
particularly those with more extensive lesions. It is generally
believed that these microorganisms are opportunistic,
secondary invaders that enter the hoof wall through a
separation or compromised area and then exacerbate
hoof wall separation by the production of proteases that
degrade keratin. Farms that experience a large number of
cases of white line disease may have predisposing
environmental or management conditions and/or
a ubiquitous population of keratinopathogenic
microorganisms (Pleasant and O'Grady 2009). The fact
that WLD can be resolved with debridement alone further
detracts from infection as a primary cause (O'Grady 2006;
Pleasant and O'Grady 2009).
Mechanical stress placed on the inner hoof wall from
less than ideal hoof conformation may encourage a
separation. Types of abnormal hoof conformation would
include excessive toe length, long toe-low heel, club feet
or sheared heels. Separation at the stratum medium/
stratum internum junction of the inner hoof wall increases
the stress in the intact stratum medium/stratum internum
junction of the adjacent wall. Weightbearing coupled with
the force of the deep digital flexor tendon becomes cyclic
and will increase the distractive forces placed on this area
further weakening the bond (Turner 1998). Routine hoof
care is important because when feet are left unattended,
dirt and debris packs into a hoof defect or separation and
may result in progressive mechanical separation of the
Vascular damage to the dermal lamellae associated
with chronic laminitis results in a compromised bond
between the epidermal and dermal lamellae and a loss of
integrity (separation) dorsal to the sole/wall junction. White
line disease can also be noted to be a sequel to tracts
created by extensive subsolar or submural abscesses.
White line disease is a threat to the soundness of the horse
if damage is extensive enough to allow mechanical loss of
the attachment between the stratum medium and
epidermal lamellae, resulting in displacement of the distal
phalanx in a distal direction (rotation). Most commonly,
WLD is noted as an incidental hoof wall separation found
by the farrier during routine hoof care. In the early stages of
white line disease, the only noticeable changes on the
solar surface of the foot maybe a widening of the sole/wall
junction and small powdery areas located just in front of
this junction. The change may remain focal or it may
progress to involve a larger area of the hoof wall. Other
early warning signs of white line disease may be thin, tender soles as noted with hoof testers, occasional heat in
the feet, and the sole will become increasingly flat. If
separation becomes more extensive and involves the toe
and a quarter, a concavity ('dish') may be seen forming
along one side of the hoof and a bulge will be present on
the contralateral side directly above the affected area at
the coronary band. The distal phalanx is suspended
circumferentially within the hoof capsule in the state of
equilibrium. When a substantial separation affecting the
epidermal lamellae is present and the laminar attachment
is compromised, the equilibrium is disrupted and the distal
phalanx will shift toward the separation causing a
concavity in the hoof wall on the opposing side of the foot
thus explaining the change in the hoof wall shape. White
line disease often goes undetected until the horse begins
to show signs of lameness.
|Fig 3: Extensive hoof wall separation due to white line disease.
Lameness may not be observed in the early stages of the
disease. Hoof tester examination does not always elicit a
response. The clinical signs along with a thorough
examination of the solar surface of the hoof will confirmthe
diagnosis. On the solar surface of the hoof, the sole/wall
junction (white line) will be wider, softer and have a chalky
or waxy texture. Exploring the inner hoof wall, which lies
dorsal to the sole/wall junction, will generally reveal a
separation filled with white/grey powdery horn material.
Further exploration with a blunt probe will give the depth
and extent of the cavitation (Fig 3). There may be a black
serous drainage from the separation. A hollow sound will
be noted when the outer hoof wall over the separation is
percussed with a hammer. If lameness is present, a
thorough lameness examination should be performed
including diagnostic analgesia to localise and confirm the
suspected area followed by radiographs. With extensive
hoof wall damage, WLD accompanied by pain can mimic
laminitis both clinically and radiographically.
|Fig 4: a) A radiograph of WLD where separation begins at the ground surface of foot. b) Radiograph of laminitis where separation extends distally to lamellae papilla.
Radiology can be very informative and should be
considered necessary. Radiographs will show the extent of
the hoof wall separation and whether displacement of the
distal phalanx within the hoof capsule has occurred.
Radiographs allow the clinician to differentiate between
white line disease and laminitis (Figs 4a and b).
Radiographically, the separation in the epidermal
lamellae will originate at the ground surface of the foot
and extend dorsally in white line disease, whereas in
laminitis, the lucency will originate in the dermal lamellae
and extend distally to the terminal laminar papillae. Pedal
osteitis may be noted in some chronic cases of white line
disease. Finally, radiographs can be used as a guide for
applying the appropriate farriery treatments.
Laboratory findings have been unrewarding and exert
minimal influence with regards to treating this disease.
Cultures are of little value since the samples taken from the
separations are contaminated with dirt and multiple opportunistic organisms. Aerobic cultures usually reveal a
mixed bacterial flora while anaerobic cultures are
generally negative (Turner 1998). Fungal cultures require a
special media and time. The most common fungal species
cultured are Pseudoallsheria, Scopulariopsis and
Aspergillus. A biopsy taken at the junction between the
normal and affected hoof wall shows a mixed population
of microorganisms. These will generally include
coccobacilli, yeast organisms and fungal spores.
Inflammation in the laminar dermis will be seen deep to the
affected area (Turner 1998).
Improving hoof conformation and correcting any hoof
capsule distortion that may have contributed to the hoof
wall separation is essential. If left untreated, WLD will allow
the separations to become extensive and displacement of
the distal phalanx is a likely sequel. In order to prevent small
lesions from becoming extensive, farriers are encouraged
to examine each foot carefully during routine trimming.
Abnormal areas or separations involving the inner hoof wall
should be explored and debrided down to solid horn
whenever possible. Ignoring or incompletely debriding
early lesions is likely to lead to progression of the
separation. Any cavity that is left after debridement should
be filled with a medicated hoof putty (Keratex)1 before
being covered with a shoe.
Treatment of white line disease is directed toward
protecting and unloading the damaged section of the
foot with therapeutic shoeing combined with resection of
the hoof capsule overlying the affected area. As a
resection disrupts the continuity and weightbearing
strength of the hoof wall, some type of shoe should be
applied for protection, to stabilise the hoof capsule and to
prevent the horse from utilising the sole for weightbearing.
If the separated area of the foot is determined to be
extensive, it is important to plan and perform the farriery
prior to the outer hoof wall being resected. The type of
shoe used and the method of attachment depend on the
extent of the damaged hoof wall. If the defect is small, the
hoof can be shod with an open shoe paying strict
attention to any abnormal hoof conformation. If the
defect is large and the overlying segment of hoof wall
needs to be resected, some type of bar shoe is indicated
to stabilise the hoof capsule. If the separation at the toe
and often a quarter become extensive, it is useful to
redistribute the weight to the palmar/plantar section of the
foot and also move the breakover in a palmar/plantar
direction. A line is drawn across the widest part of the foot
and the foot trimmed from this line palmarly/plantarly in a
tapered fashion. Any excessive toe length is reduced from
the dorsal hoof wall using a rasp. This method of trimming
will create 2 planes on the solar surface of the foot and
thus unload the toe. The shoe is fitted so breakover is placed just dorsal to the margin of the distal phalanx in an
attempt to remove the 'lever arm' effect at the toe. This
will also stop the 'pinching' effect that often occurs at the
junction of normal hoof wall and the resection.
|Fig 5: Shoes used to share the weight from the hoof wall. a) Heart bar shoe and b) heel plate shoe.
If the resection is to be extensive and/or if rotation of
the distal phalanx is present, the foot should be trimmed
according to the radiographs and some form of bar shoe
(heart bar) or shoe with a heel plate should be used. This
type of shoe allows some weightbearing to be transferred
from the hoof wall to the frog (heart bar) or frog, sole and
bars (heel plate) (Figs 5a and b). Alternatively, the foot
may be shod with an open shoe and the solar surface of
the foot between the branches of the shoe is filled with
some type of silastic material. If there is limited hoof wall
available in which to place nails or shoes cannot be nailed
on safely, glue-on shoes may be used. The author attaches
an aluminum shoe directly to healthy horn on the ground
surface of the foot and the outer hoof wall at the heels
using an acrylic composite (Equilox)2, thus leaving the
resected area open to be observed, cleaned and debrided regularly (O'Grady and Watson 1999). In severe
WLD cases where there is marked rotation of the distal
phalanx, the author has been successful using a wooden
shoe (O'Grady and Steward 2009). Foot casts and various
types of boots have become popular in treating WLD
especially after a resection has been performed but, in the
author's opinion, should be avoided as casts tend to cover
the affected section of the foot and boots create a
continuous moist environment. Foot casts and boots should
not be used as a substitute for skilled farriery as this author
has not encountered a case of WLD where a shoe of some
form could not be attached to the foot.
|Fig 6: Illustrating hoof wall resections. Note the solid margins around the perimeter of both resections. Figure 6b has a wooden shoe attached to the foot to address rotation of the distal phalanx.
|Fig 7: The application of a dye marker (2% iodine) will reveal any remaining tracts or fissures in the stratum internum.
Complete hoof wall resection (removal of outer hoof wall to
expose diseased horn) and debridement of all tracts and
fissures in the affected area is necessary. This can be readily
accomplished using a loop hoof knife and half-round hoof
nippers. The debridement should be continued proximally
and marginally until there isasolid attachment between the
hoof wall and external lamellae (Fig 6). Hemorrhage should
not be encountered by the veterinarian or farrier if the
debridement is performed properly.
Medical or topical treatment in any form is of no value
without resection of the affected hoof wall in the author's
opinion. A plethora of topical medications have been
described for treatment following hoof wall resection but
there have been no controlled studies on any product and
none in the author's opinion have been proven effective.
Disinfectants/astringents such as methiolate or 2% iodine
are commonly used butmayhave the most benefit asadye
marker to outline the remaining tracts in the stratum
internum (Fig 7). The dye marker will serve as an aid in
making the remaining tracts more visible at subsequent
examinations and as a guideline during debridement. Either
preparation should not be applied more than weekly so as
not to make the exposed lamina excessively hard and
brittle. After thorough hoof wall resection, the affected area
can be left open to grow out with debridement at frequent
intervals. A wire brush is used daily to keep the resected
area clean. Thorough exploration and debridement of any
remaining tracts should take place at 2 week intervals.
When the resection has grown out, a thorough examination
of the sole wall junction is imperative at reshoeing intervals
every 4-5 weeks.
Acrylic repair of the resected area should be avoided if
possible. It should only be considered in selected cases
where the client is unable to treat the resected area and
where cosmetics are a necessity. The composite may hide
and/or foster infection and it tends to weaken the
surrounding solid hoof wall, all of which can encourage
reinfection. Combining an antibiotic with the acrylic has
been described but has not proved to be consistently
effective in the long term (Turner and Anderson 1996). If
repair is performed, there should be an interface such as
clay or some type of foam inserted between the acrylic
composite and surface of the resection.
A change in environment is important. The feet should be
kept as dry as possible throughout the recovery period.
Sawdust or wood shavings appear to dehydrate the feet making them the bedding of choice and bedding should
always be kept clean and dry. Limited turnout in rain or
wet weather is helpful. Turnout can be delayed in the
morning until the sun has dried the dew from the pasture.
Commitment from the owner with regards to a
continuous treatment schedule is necessary until all signs of
disease have been eliminated and then the foot/feet must
be monitored monthly until the hoof wall grows out. The
extent of the damage will determine the approximate
amount of time required to complete the treatment
process. However, it is not always necessary for the horse to
be out of work for this treatment period. The amount of
exercise permissible while treating WLD is contingent on
the extent of the damage and presence of sufficient hoof
wall necessary for weightbearing.
Prevention of WLD is difficult because the exact cause is
unknown. Discussing the problem with the farrier and
having him/her examine each foot when the horse is shod
is extremely important. Any small abnormal area involving
the sole/wall junction should be noted, explored and
debrided down to solid horn. Proper physiological trimming
and shoeing is essential for creating a strong sole/wall
junction that may prevent separations and offer protection
(O'Grady and Poupard 2003). Equally important is the
necessity to carefully monitor horses that have previously
had white line disease as it may suddenly reappear in
some horses with strong hoof walls that show no previous
signs of a hoof wall separation.
White line disease involves the inner, nonpigmented
section of the stratum medium of the hoof wall, not the
sole-wall junction (zona alba, or 'white line'). Thus, 'white
line disease' is somewhat of a misnomer. Nevertheless, it
has become the accepted term used by the majority of
farriers and veterinarians. Certainly it is a more useful term
than onychomycosis, as it does not limit the primary
aetiological organism to a fungal agent.
Treating WLD has created a dilemma for owners,
veterinarians and farriers. Owners have been deluged with
many different proposed causes of WLD and a variety of
treatment protocols. Numerous commercially available
preparations have been marketed for treating WLD, all
claiming success. The internet describes a multitude of
products and methods guaranteed to provide miraculous
improvement. At present, there is no convincing scientific
evidence as to the efficacy of any given product.
Veterinarians are often unaware of the magnitude of this
problem as they only see the severe cases that present for lameness evaluation and/or when radiographic changes
become apparent. White line disease may be a subtle
contributor to other causes of lameness within the foot.
Farriers are very aware of this disease as they are often
confronted with nailing a shoe on limited or compromised
hoof wall and keeping the shoe on between resets. They
continually search for topical treatments since owners are
reluctant to have resections performed and farriers are
often reluctant to recommend resections that can be a
daunting procedure. Following a hoof wall resection,
farriers have traditionally performed a composite repair,
often to appease the horse owner or allow the horse to
continue performing. This practice should be discouraged
as it prevents careful monitoring of the resection, appears
to harbour organisms under the repair and may impede
resolution of the disease (Pleasant and O'Grady 2009).
Research, owner education and continued farrier
awareness of WLD appears to be the most promising
direction for the future.
Author's declaration of interests
No conflicts of interest have been declared.
- Keratex PO Box 2, Brookeville, Maryland, USA.
- Equilox Int'l, Pine Island, Minnesota, USA.
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