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
White line disease occurs secondary to a hoof wallseparation. Clinical signs may vary from not being lame tosevere lameness with rotation of the distal phalanxdepending on the extent of the disease affecting the innerhoof wall. The author has found that removal of the hoof walloverlying the diseased area combined with the appropriatefarriery is the most important aspect of therapy.
White line disease (WLD) is a disorder localised to the equinefoot. The problem is widespread, the aetiology andmechanism of the disease are poorly understood andtreatment is often controversial. White line disease is a termused to describe a keratinolytic process that originates onthe solar surface of the hoof characterised by a progressiveseparation of the inner zone of the hoof wall (O'Grady 2001,2006; Moyer 2003; Pleasant and O'Grady 2009). Theseparation occurs in the nonpigmented horn at the junctionbetween the stratum medium and stratum internum (Fig 1).The destruction that occurs in the separation as aconsequence of WLD remains superficial to the stratuminternum and does not invade the dermis.
A separation in the hoof wall is considered to be adelaminating process potentially thought to originate fromgenetic factors, mechanical stress, inappropriate farrieryand environmental conditions affecting the inner hoof wallattachment (Moyer 2003). The separation, which canoriginate at the toe, the quarter and/or heel, appears tobe invaded by opportunistic bacteria/fungi leading to atype of infection where the organisms digest the hornallowing the separation to progress to varying heights andconfigurations proximally toward the coronet.
The disease has been termed seedy toe, hoof walldisease, yeast infection, Candida and onychomycosis.Onychomycosis is a mycotic disease that originates in thenail bed of man and the dog. By contrast, in WLD theinfection appears to have originated at the solar surface ofthe hoof and migrates proximally, approaching thecoronet but never invading it. Keratinophilic fungi are oftenisolated from separated areas of the hoof wall; however, inmany cases of WLD, the pathogens cultured are purelybacterial or a mixture of bacterial and fungal organisms(Turner 1998). Therefore, until proven otherwise,onychomycosis may not be the appropriate term whenreferring to white line disease in the horse (O'Grady 2006).
Anatomy of the hoof wall
The hoof wall consists of 3 layers which are the:
The stratum externum arises from the perioplicepidermis and forms the thin outer layer of keratinised cellsthat give the wall its smooth glossy appearance. Thestratum medium, which arises from the coronary epidermis,forms the bulk of the hoof wall and is the densest part ofthe horny wall. It consists of cornified epidermal cellsarranged in parallel horny tubules surrounded byintertubular horn, which grow distally from the coronarygroove to the basal border. In all hooves the stratummedium is always nonpigmented in the deepest inner layer(Fig 2). The stratum internum arises from the lamellarepidermis, is nonpigmented and, when combined with thedermal lamellae, is responsible for attaching the hoof wallto the distal phalanx. Distally at the sole wall junction, thedermal lamellae end in terminal papillae. These terminalpapillae are lined by a germinal epidermis whichgenerates keratinised epidermal cells which fills the spacebetween the nonpigmented horny laminae as they growtoward the ground surface (Pollitt 2010). This associationforms the bond between the hoof wall and sole known asthe white line or zone (Parks 2003). When observed fromthe solar surface, this white line or zona alba is actuallyyellow in colour and has a plastic consistency when compared to the dorsal hoof wall.
The aetiology of WLD remains undetermined. The problemhas been described in horses worldwide. WLD can affect ahorse of any age, sex or breed. One or multiple hoofs maybe involved and affected hooves can be barefoot orshod. One or multiple horses on the same farm may beaffected. It is generally agreed that WLD is a multifactorialcondition that develops secondary to an initial separationor hoof wall defect (O'Grady 2006; Pleasant and O'Grady2009). It must be remembered that multiple causes forwhite line disease have been proposed but none havebeen scientifically proven.
Moisture may play a role as WLD is seen more in wethumid areas, but it is also seen in hot arid conditions.Excessive moisture may soften the foot, allowing easierentry of dirt and debris into an existing separation.Continual bathing of competition horses, especially duringthe warmer months, may contribute to the incidence ofWLD in this population of horses. Excessively dry hooves on the other hand may form cracks or separations in the hoofwall, allowing pathogens to invade.
Poor hygiene has been blamed but this is questionablesince WLD often appears in well managed stables.Keratinopathogenic fungi and bacteria are commonlyisolated from the hoof wall defects of horses with WLD,particularly those with more extensive lesions. It is generallybelieved that these microorganisms are opportunistic,secondary invaders that enter the hoof wall through aseparation or compromised area and then exacerbatehoof wall separation by the production of proteases thatdegrade keratin. Farms that experience a large number ofcases of white line disease may have predisposingenvironmental or management conditions and/ora ubiquitous population of keratinopathogenicmicroorganisms (Pleasant and O'Grady 2009). The factthat WLD can be resolved with debridement alone furtherdetracts from infection as a primary cause (O'Grady 2006;Pleasant and O'Grady 2009).
Mechanical stress placed on the inner hoof wall fromless than ideal hoof conformation may encourage aseparation. Types of abnormal hoof conformation wouldinclude excessive toe length, long toe-low heel, club feetor sheared heels. Separation at the stratum medium/stratum internum junction of the inner hoof wall increasesthe stress in the intact stratum medium/stratum internumjunction of the adjacent wall. Weightbearing coupled withthe force of the deep digital flexor tendon becomes cyclicand will increase the distractive forces placed on this areafurther weakening the bond (Turner 1998). Routine hoofcare is important because when feet are left unattended,dirt and debris packs into a hoof defect or separation andmay result in progressive mechanical separation of thehoof wall.
Vascular damage to the dermal lamellae associatedwith chronic laminitis results in a compromised bondbetween the epidermal and dermal lamellae and a loss ofintegrity (separation) dorsal to the sole/wall junction. Whiteline disease can also be noted to be a sequel to tractscreated by extensive subsolar or submural abscesses.
White line disease is a threat to the soundness of the horseif damage is extensive enough to allow mechanical loss ofthe attachment between the stratum medium andepidermal lamellae, resulting in displacement of the distalphalanx in a distal direction (rotation). Most commonly,WLD is noted as an incidental hoof wall separation foundby the farrier during routine hoof care. In the early stages ofwhite line disease, the only noticeable changes on thesolar surface of the foot maybe a widening of the sole/walljunction and small powdery areas located just in front ofthis junction. The change may remain focal or it mayprogress to involve a larger area of the hoof wall. Otherearly warning signs of white line disease may be thin, tender soles as noted with hoof testers, occasional heat inthe feet, and the sole will become increasingly flat. Ifseparation becomes more extensive and involves the toeand a quarter, a concavity ('dish') may be seen formingalong one side of the hoof and a bulge will be present onthe contralateral side directly above the affected area atthe coronary band. The distal phalanx is suspendedcircumferentially within the hoof capsule in the state ofequilibrium. When a substantial separation affecting theepidermal lamellae is present and the laminar attachmentis compromised, the equilibrium is disrupted and the distalphalanx will shift toward the separation causing aconcavity in the hoof wall on the opposing side of the footthus explaining the change in the hoof wall shape. Whiteline disease often goes undetected until the horse beginsto show signs of lameness.
Lameness may not be observed in the early stages of thedisease. Hoof tester examination does not always elicit aresponse. The clinical signs along with a thoroughexamination of the solar surface of the hoof will confirmthediagnosis. On the solar surface of the hoof, the sole/walljunction (white line) will be wider, softer and have a chalkyor waxy texture. Exploring the inner hoof wall, which liesdorsal to the sole/wall junction, will generally reveal aseparation filled with white/grey powdery horn material.Further exploration with a blunt probe will give the depthand extent of the cavitation (Fig 3). There may be a blackserous drainage from the separation. A hollow sound willbe noted when the outer hoof wall over the separation ispercussed with a hammer. If lameness is present, athorough lameness examination should be performedincluding diagnostic analgesia to localise and confirm thesuspected area followed by radiographs. With extensivehoof wall damage, WLD accompanied by pain can mimiclaminitis both clinically and radiographically.
Radiology can be very informative and should beconsidered necessary. Radiographs will show the extent ofthe hoof wall separation and whether displacement of thedistal phalanx within the hoof capsule has occurred.Radiographs allow the clinician to differentiate betweenwhite line disease and laminitis (Figs 4a and b).Radiographically, the separation in the epidermallamellae will originate at the ground surface of the footand extend dorsally in white line disease, whereas inlaminitis, the lucency will originate in the dermal lamellaeand extend distally to the terminal laminar papillae. Pedalosteitis may be noted in some chronic cases of white linedisease. Finally, radiographs can be used as a guide forapplying the appropriate farriery treatments.
Laboratory findings have been unrewarding and exertminimal influence with regards to treating this disease.Cultures are of little value since the samples taken from theseparations are contaminated with dirt and multiple opportunistic organisms. Aerobic cultures usually reveal amixed bacterial flora while anaerobic cultures aregenerally negative (Turner 1998). Fungal cultures require aspecial media and time. The most common fungal speciescultured are Pseudoallsheria, Scopulariopsis andAspergillus. A biopsy taken at the junction between thenormal and affected hoof wall shows a mixed populationof microorganisms. These will generally includecoccobacilli, yeast organisms and fungal spores.Inflammation in the laminar dermis will be seen deep to theaffected area (Turner 1998).
Improving hoof conformation and correcting any hoofcapsule distortion that may have contributed to the hoofwall separation is essential. If left untreated, WLD will allowthe separations to become extensive and displacement ofthe distal phalanx is a likely sequel. In order to prevent smalllesions from becoming extensive, farriers are encouragedto examine each foot carefully during routine trimming.Abnormal areas or separations involving the inner hoof wallshould be explored and debrided down to solid hornwhenever possible. Ignoring or incompletely debridingearly lesions is likely to lead to progression of theseparation. Any cavity that is left after debridement shouldbe filled with a medicated hoof putty (Keratex)1 beforebeing covered with a shoe.
Treatment of white line disease is directed towardprotecting and unloading the damaged section of thefoot with therapeutic shoeing combined with resection ofthe hoof capsule overlying the affected area. As aresection disrupts the continuity and weightbearingstrength of the hoof wall, some type of shoe should beapplied for protection, to stabilise the hoof capsule and toprevent the horse from utilising the sole for weightbearing.If the separated area of the foot is determined to beextensive, it is important to plan and perform the farrieryprior to the outer hoof wall being resected. The type ofshoe used and the method of attachment depend on theextent of the damaged hoof wall. If the defect is small, thehoof can be shod with an open shoe paying strictattention to any abnormal hoof conformation. If thedefect is large and the overlying segment of hoof wallneeds to be resected, some type of bar shoe is indicatedto stabilise the hoof capsule. If the separation at the toeand often a quarter become extensive, it is useful toredistribute the weight to the palmar/plantar section of thefoot and also move the breakover in a palmar/plantardirection. A line is drawn across the widest part of the footand the foot trimmed from this line palmarly/plantarly in atapered fashion. Any excessive toe length is reduced fromthe dorsal hoof wall using a rasp. This method of trimmingwill create 2 planes on the solar surface of the foot andthus unload the toe. The shoe is fitted so breakover is placed just dorsal to the margin of the distal phalanx in anattempt to remove the 'lever arm' effect at the toe. Thiswill also stop the 'pinching' effect that often occurs at thejunction of normal hoof wall and the resection.
If the resection is to be extensive and/or if rotation ofthe distal phalanx is present, the foot should be trimmedaccording to the radiographs and some form of bar shoe(heart bar) or shoe with a heel plate should be used. Thistype of shoe allows some weightbearing to be transferredfrom the hoof wall to the frog (heart bar) or frog, sole andbars (heel plate) (Figs 5a and b). Alternatively, the footmay be shod with an open shoe and the solar surface ofthe foot between the branches of the shoe is filled withsome type of silastic material. If there is limited hoof wallavailable in which to place nails or shoes cannot be nailedon safely, glue-on shoes may be used. The author attachesan aluminum shoe directly to healthy horn on the groundsurface of the foot and the outer hoof wall at the heelsusing an acrylic composite (Equilox)2, thus leaving theresected area open to be observed, cleaned and debrided regularly (O'Grady and Watson 1999). In severeWLD cases where there is marked rotation of the distalphalanx, the author has been successful using a woodenshoe (O'Grady and Steward 2009). Foot casts and varioustypes of boots have become popular in treating WLDespecially after a resection has been performed but, in theauthor's opinion, should be avoided as casts tend to coverthe affected section of the foot and boots create acontinuous moist environment. Foot casts and boots shouldnot be used as a substitute for skilled farriery as this authorhas not encountered a case of WLD where a shoe of someform could not be attached to the foot.
Complete hoof wall resection (removal of outer hoof wall toexpose diseased horn) and debridement of all tracts andfissures in the affected area is necessary. This can be readilyaccomplished using a loop hoof knife and half-round hoofnippers. The debridement should be continued proximallyand marginally until there isasolid attachment between thehoof wall and external lamellae (Fig 6). Hemorrhage shouldnot be encountered by the veterinarian or farrier if thedebridement is performed properly.
Medical or topical treatment in any form is of no valuewithout resection of the affected hoof wall in the author'sopinion. A plethora of topical medications have beendescribed for treatment following hoof wall resection butthere have been no controlled studies on any product andnone in the author's opinion have been proven effective.Disinfectants/astringents such as methiolate or 2% iodineare commonly used butmayhave the most benefit asadyemarker to outline the remaining tracts in the stratuminternum (Fig 7). The dye marker will serve as an aid inmaking the remaining tracts more visible at subsequentexaminations and as a guideline during debridement. Eitherpreparation should not be applied more than weekly so asnot to make the exposed lamina excessively hard andbrittle. After thorough hoof wall resection, the affected areacan be left open to grow out with debridement at frequentintervals. A wire brush is used daily to keep the resectedarea clean. Thorough exploration and debridement of anyremaining tracts should take place at 2 week intervals.When the resection has grown out, a thorough examinationof the sole wall junction is imperative at reshoeing intervalsevery 4-5 weeks.
Acrylic repair of the resected area should be avoided ifpossible. It should only be considered in selected caseswhere the client is unable to treat the resected area andwhere cosmetics are a necessity. The composite may hideand/or foster infection and it tends to weaken thesurrounding solid hoof wall, all of which can encouragereinfection. Combining an antibiotic with the acrylic hasbeen described but has not proved to be consistentlyeffective in the long term (Turner and Anderson 1996). Ifrepair is performed, there should be an interface such asclay or some type of foam inserted between the acryliccomposite and surface of the resection.
A change in environment is important. The feet should bekept 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 shouldalways be kept clean and dry. Limited turnout in rain orwet weather is helpful. Turnout can be delayed in themorning until the sun has dried the dew from the pasture.
Commitment from the owner with regards to acontinuous treatment schedule is necessary until all signs ofdisease have been eliminated and then the foot/feet mustbe monitored monthly until the hoof wall grows out. Theextent of the damage will determine the approximateamount of time required to complete the treatmentprocess. However, it is not always necessary for the horse tobe out of work for this treatment period. The amount ofexercise permissible while treating WLD is contingent onthe extent of the damage and presence of sufficient hoofwall necessary for weightbearing.
Prevention of WLD is difficult because the exact cause isunknown. Discussing the problem with the farrier andhaving him/her examine each foot when the horse is shodis extremely important. Any small abnormal area involvingthe sole/wall junction should be noted, explored anddebrided down to solid horn. Proper physiological trimmingand shoeing is essential for creating a strong sole/walljunction that may prevent separations and offer protection(O'Grady and Poupard 2003). Equally important is thenecessity to carefully monitor horses that have previouslyhad white line disease as it may suddenly reappear insome horses with strong hoof walls that show no previoussigns of a hoof wall separation.
White line disease involves the inner, nonpigmentedsection of the stratum medium of the hoof wall, not thesole-wall junction (zona alba, or 'white line'). Thus, 'whiteline disease' is somewhat of a misnomer. Nevertheless, ithas become the accepted term used by the majority offarriers and veterinarians. Certainly it is a more useful termthan onychomycosis, as it does not limit the primaryaetiological organism to a fungal agent.
Treating WLD has created a dilemma for owners,veterinarians and farriers. Owners have been deluged withmany different proposed causes of WLD and a variety oftreatment protocols. Numerous commercially availablepreparations have been marketed for treating WLD, allclaiming success. The internet describes a multitude ofproducts and methods guaranteed to provide miraculousimprovement. At present, there is no convincing scientificevidence as to the efficacy of any given product.Veterinarians are often unaware of the magnitude of thisproblem as they only see the severe cases that present for lameness evaluation and/or when radiographic changesbecome apparent. White line disease may be a subtlecontributor to other causes of lameness within the foot.Farriers are very aware of this disease as they are oftenconfronted with nailing a shoe on limited or compromisedhoof wall and keeping the shoe on between resets. Theycontinually search for topical treatments since owners arereluctant to have resections performed and farriers areoften reluctant to recommend resections that can be adaunting procedure. Following a hoof wall resection,farriers have traditionally performed a composite repair,often to appease the horse owner or allow the horse tocontinue performing. This practice should be discouragedas it prevents careful monitoring of the resection, appearsto harbour organisms under the repair and may impederesolution of the disease (Pleasant and O'Grady 2009).Research, owner education and continued farrierawareness of WLD appears to be the most promisingdirection for the future.
Author's declaration of interests
No conflicts of interest have been declared.