Virginia Therapeutic Farriery

Hoof Abscesses:A Practical Approach

Reprinted with permission from the American Farriers Journal.
Originally printed in The American Farriers Journal, 2018 Volume 44

Stephen E. O'Grady DVM, MRCVS

Introduction
Hoof abscesses are probably the mostcommon cause of acute severe lamenessin horses encountered by veterinarians andfarriers. A hoof abscess can be definedas a localized accumulation of purulentexudate located between the germinaland keratinized layers of the epithelium,most commonly subsolar (beneath thesole) or submural (beneath the hoof wall).Organisms that are responsible for a hoofabscess gain entry through the hoof capsule(epidermis) into the inner subsolar / submuraltissue (dermis) where the organismspropagate and initiate the formation of anabscess. Foreign matter (such as gravel,dirt, sand and manure coupled with infectiousagents such as bacteria or fungalelements) generally gain entry into the hoofcapsule through a break or fissure in thesole-wall junction somewhere on the solarsurface of the foot.

Anatomical Review
A brief anatomical review of hoof capsulestructures may be helpful beforediscussing hoof abscesses. The foot iscomposed of the hoof, the skin betweenthe bulbs of the heels and all the structureswithin. The structures of the hoof complexcomprise the hoof capsule, distal phalanx,digital cushion, ungual cartilages and deepfdigital flexor tendon 1. These biologicalstructures are susceptible to trauma and areprone to various disease processes includinginfections (hoof abscesses, puncturewounds) and keratomas. The equine footis designed to perform numerous functionsincluding bearing the weight of the horseat all gaits, protecting the structures containedwithin the hoof capsule, absorbingconcussion as the hoof strikes the groundalong with providing traction 2. The uniqueinterrelationship of the structures workingin concert and the viscoelastic nature ofthe hoof capsule allow the hoof to performthese functions. The hoof wall, sole, frogand bulbs of the heels comprise the hoofcapsule which, through the unique continuousbond between its components, formsa casing on the ground surface of the footwhich affords protection to the dermal andosseous structures enclosed within the capsule2. Furthermore, optimal protection isreliant on overall hoof health and a stronghoof capsule which can be influenced bygenetics, environment, exercise, nutritionand farriery practices.

The dermis or corium lines the entireinner surface of the hoof capsule and connectsthe hoof capsule to the underlyingdistal phalanx. The dermal tissue thatappears most susceptible to injury is thelaminar and solar corium. The bond orjunction between the hoof wall and soleis especially important as it becomessusceptible to damage when subjected tothe continuous repetitive stress of weightbearing. Damage at the sole wall junctionmay allow a portal for entry of pathogensto invade the interior of the hoof capsule.Distally at the sole wall junction, thedermal lamellae end in the formation ofterminal papillae. These papillae are linedby stratum germinativum, which producesa flexible intertubular horn that fills thespaces between the non-pigmented hornywall and the horny sole. This associationforms the bond between the hoof wall andthe sole known as the white line or zonaalba. The sole wall junction extends aroundthe circumference of the solar surface ofthe foot and the heels running forward onthe abaxial surface of the bars. Pathogensor debris that may lead to infection commonlypenetrate the hoof capsule in oneof 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 orthe frog.

A misplaced horseshoe nail could beconsidered a puncture wound. However,in this case, infection is introduced by anail entering the inner part of the sole walljunction versus a puncture wound, whereinfection gains entry by direct penetrationthrough the other soft tissue structures onthe solar surface of the foot. This articlewill describe the most current informationand the most practical approach totreatment for disease processes that occurwithin the hoof capsule such as hoofabscesses or puncture wounds.

Mechanism of a hoof abscess
It may be easier to understand how totreat an abscess with a brief look at themechanism by which an abscess will form.Foreign debris will gain entry and accumulatein a small separation or fissurelocated in the sole-wall junction anywherearound the perimeter of the foot includingthe abaxial surface of the bars adjacent tothe sole (Figure 1A & 1B). As the animal bears weight, the pressure will cause foreignmatter to migrate through the fissure, forming a tract, which eventually reachesthe subsolar or submural tissue (dermis).

Figure 1A&1B: Foreign debris will gain entry and accumulate in a small separation (red arrows) or fissurelocated 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).

Once the debris reaches the dermis insidethe hoof capsule, the foreign materialactivates the animal’s immune system(defense mechanism) inciting an inflammatoryresponse within the dermal tissue.The bacterium contained within thedebris propagate, further accentuatingthe inflammatory response which drawsinflammatory cells into the area. Enzymesreleased from the bacteria and from the invading inflammatory cells (white cells)lead to liquefaction tissue necrosis and thedevelopment of the grey/black exudate. Theinfection is quickly walled off with a thinlayer of fibrous tissue to form an abscess.The inflammation and the pressure fromthe accumulation of the exudate exerted onthe surrounding dermal tissue lead to thepain and clinical signs associated with ahoof abscess.

Clinical Signs
Most affected horses show a suddenonset of (acute) severe lameness. The degree of lameness varies from beingsubtle in the early stages to non-weightbearing. The digital pulse felt at the levelof the fetlock is increased, usually boundingand the involved foot will be warmerthan the opposite foot. With careful observation,unless the abscess is in the middleof the toe, the intensity of the digital pulsewill be much stronger on the side of thefoot where the infection is located. If theabscess is long standing, there may be softtissue swelling in the pastern up to or evenabove the fetlock on the side of the limbcorresponding to the side of the foot wherethe abscess is located. The site of pain canbe localized to a small focal area throughthe careful use of hoof testers. Sometimeswith acute lameness, the pain will be notedover the entire foot with hoof testers and,in this case, it is necessary to rule out laminitis,a severe bruise or even a possiblefracture of the distal phalanx (P3).

Figure 2: When a tract or fissure is found, it can be followed within thesole-wall junction (white line) using a small thin loop knife, a 2 mm bonecurette or another suitable probe. A horseshoe nail makes an excellent toolfor drainage.

Treatment
There is still debate between the veterinaryand farrier professions as to whoshould treat a hoof abscess and the bestmethod in which to resolve the abscess.Considering that a walled off hoof abscessis an extension of the epidermis, it is theauthor’s opinion that the infection could betreated by either the veterinarian or the farrier.The most important aspect of treatinga subsolar / submural hoof abscess is toestablish drainage. The opening should beof sufficient size to allow drainage but notso extensive as to create further damage.When pain is localized to a small focal areawith hoof testers, a small tract or fissurewill commonly be found in the sole walljunction. The tract or point of entry maynot always be visible as the sole wall junctionis somewhat elastic and tracts in thisarea tend to close. In this case, a suitablepoultice should be applied to the foot dailyin an attempt to soften the affected areaand 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 tractis open into the cavity of the abscess. A small opening is all that is necessary to obtain proper drainage. Pressurefrom 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 canbe followed / explored within the white lineusing a small thin loop knife, a 2 mm bonecurette or another suitable probe such asa horseshoe nail (Figure 2). The tract isslowly followed until a grey/black exudate(pus) is released and the probe enters the“belly” of the abscess. At this point, thetract is open into the cavity of the abscess.A small opening is all that is necessaryto obtain proper drainage. This can bedetermined by using thumb pressure onthe solar side of the tract just behind theopening or by placing hoof testers on thesole next to the tract and observing if moreexudate is expressed or a bubble is formingat the opening of the tract when pressure isapplied (Figure 3A & 3B). Care shouldbe taken to avoid exposing any corium, as it will invariably prolapse through theopening, preventing closure of the tractand possibly creating an ongoing sourceof pain.Under no circumstances shoulda routine hoof abscess be approachedthrough the sole!

The draining tract can be kept soft anddrainage promoted in several ways. Theauthor will generally apply a commercialmedicated poultice a for the first 24-48hours. The poultice is immersed in hotwater, the water is squeezed out, the poulticeis placed on the foot and attached witha roll of wide brown gauze, a cohesive bandageand waterproof tape (Figure 4a, 4b& 4c). The sheet version of this poultice ispreferred rather than the poultice pad that isdistributed by the company. The whole footincluding the coronet should be enveloped in the poultice.

Figure 4: : Application of a medicated poultice. Submerge in hot water, squeeze out excess water, envelope the footwith the plastic side of the poultice on the outside. Secure to the foot with a wide brown gauze, cohesive bandageand duct tape.

Another method to encouragedrainage is to apply what is termed a‘soak bandage’. Here layers of soft practicalcotton (available in rolls) are foldedtogether and then used to envelope the footto form a thick bandage. MgSo4 (EpsonSalts) is placed on the inner foot surfaceof the bandage and the bandage is attachedto the foot as described above. The bandageis now saturated with hot water andthen saturated periodically over the next24-48 hours. Using either of these methodseliminates the necessity for continuedfoot soaking which can be cumbersome andpossibly less effective.

Ichthammol ointment, which is a coal tarderivative with mild antiseptic properties,has been described for treating skin diseasein both humans and animals. The use ofan Ichthammol bandage for treating hoofabscesses, both before and after drainage,has become another traditional treatmentused by veterinarians, farriers and horseowners with reportedly good results.However, the author has no experienceusing this product. There are numerouscommercial products marketed to treat footabscesses but these products will only behelpful if they complement the principlesof drainage described above.

Once drainage is established the horseshould show marked improvement within24 hours. Once drainage has ceased, thehoof is kept bandaged with an appropriateantiseptic such as Betadine b solution/ointmentor 2% iodine applied over the tractuntil the wound is dry and sealed. At thispoint, the opening of the tract is filled witha medicated hoof putty c which keeps theaffected area clean and prevents furtherdebris from entering the tract or wound.The shoe is replaced when the horse is completely sound.

Figure 5: Small channel created in the hoof wall with a small pair of half roundnippers so tract can now be approached in a horizontal plane.

Often, a painful tract can be located butdrainage cannot be established at the solewall junction. In this case, the infection isdeep and may have migrated under the soleor wall away from the sole wall junctionor white line. Again, under no circumstancesshould an opening be created inthe adjacent sole! This seldom leads tothe abscess and often leads to hemorrhageand may create a persistent, non-healingwound with increased potential for infectionor osteomyelitis of the distal phalanx.Instead, a small channel can be created onthe abaxial or hoof wall side of the solewalljunction using a small narrow pair ofhalf-round nippers. The channel is madein a vertical direction following the tract tothe point where it courses inward. Drainagecan usually be established using a smallprobe in a horizontal plane (Figure 5).Preferably, this is done at an early stage ofthe lameness before the infection rupturesat the coronet.

If left untreated, a hoof abscess willfollow the path of least resistance alongthe outer margin of the dermal tissue andeventually rupture at the coronet forming adraining tract. Many horse owners actuallyconsider this to be an acceptable practiceand elect to wait for this to take place.This practice often extends the time theanimal experiences severe pain. Ruptureat the coronet also leads to a permanentscar or tract under the hoof wall. Thistract leading to the coronet may result ina prolonged recovery from the abscess, achronic draining tract, repeated abscessesand a full thickness hoof wall crack. Everyeffort should be made to establish drainageof the abscess on the solar surface of thefoot prior to a rupture at the coronet.

Infection from amisplaced horseshoe nail
Dermal tissue can be inoculated by bacteriafrom a misplaced nail in two ways. Thenail can be driven directly into the laminarcorium. When the nail enters dermal tissue,the horse will generally show discomfortas the nail is driven into the foot and therewill be hemorrhage present where the nailexits the outer hoof wall. Blood observedat the exit of the offending nail will alertthe farrier of the misplaced nail. The bloodactually has a beneficial function as it actsas a “physiologic rinse” to dilute or eliminatebacterial contamination. Removal ofthe nail and application of an appropriateantiseptic will usually prevent infection.Another scenario that occurs frequently iswhile the farrier is driving a nail, the horseshows significant discomfort indicating thenail is invading dermal or sensitive tissue.Often the farrier will remove the nail, placeit in another spot / direction and again driveit into the foot. However, when this scenariooccurs, the farrier should removethe shoe and examine the spot where thenail entered the foot. If a nail has entereddermal tissue (even if removed), it causestrauma to the dermal tissue and can seedthe area with organisms which may lead toabscess formation. If the nail has enteredthe foot inside the sole-wall junction, theowner/trainer should be alerted as to thepotential problems and the horse can beplaced on an oral broad-spectrum antibioticfor 3 – 5 days as a prophylactic measure.Lastly, we have the condition describedas a “close nail” where the nail is placedsuch that it lies against the border of thedermis just axial to the hoof wall. Pressureagainst the corium combined with constantmovement of the nail against the coriumas the horse moves and bears weight maycause an inflammatory response and allowany bacteria that were introduced with thenail to form an abscess as described above.There is generally a lag period of 7-14 daysor even longer before clinical symptomsor discomfort is observed following theplacement of a “close nail”. Treatmentagain would be to establish and promotedrainage.

PenetratingInjuries to the Foot
Puncture wounds to the solar surface ofthe foot most often occur when the horsesteps on a sharp object(s) such as a fixednarrow solid object, a sharp rock, a nail orpiece of glass which penetrates the hornysole. Superficial puncture wounds penetrateonly the cornified tissue while deepwounds penetrate the germinal epithelium.Wounds to the sole need only penetrate 1cm or less to invade germinal epitheliumand seed the site with bacteria that leadsto infection. Puncture wounds of the solewill be discussed here as puncture woundsthat involve the soft tissue structures of thepalmar / plantar foot are beyond the scopeof this article.

Farriers are often asked to treat puncturewounds to the solar surface of the foot.Medications such as antibiotics and anti-inflammatorydrugs may be indicated andwill need a veterinarian’s prescription. Ifa farrier were to treat an established infectionin the hoof, it would be perceived aspracticing veterinary medicine and the farriercould be held liable. Farriers are oftenasked to place a shoe with a removabletreatment plate on the foot with a puncturewound for protection but at the sametime allowing access for daily treatment.As drainage ceases and the puncture woundbegins to cornify, the farrier will be askedto place a pad between the hoof and shoefor protection until healing is complete.

Figure 6: Small channel created in the hoof wall with a small pair of half roundnippers so tract can now be approached in a horizontal plane.

Clinical Presentationand Diagnosis
Puncture wounds frequently createmarked lameness. The degree of lamenessmay vary considerably dependingon the depth, location and duration of thewound. Superficial wounds may initiallyhave minimal lameness but can progressto severe lameness within several dayswith the development of an infection. Ingeneral, puncture wounds that invade thecorium become quite painful soon after theinjury as the corium above the horny soleseals, thus preventing any further drainageand creating a medium for infection.Progression to severe non-weight bearinglameness can occur as the rigid hoof capsulerestricts the swelling associated withnot only the inflammatory response fromthe trauma of the puncture wound butthe resultant infection within the dermis.Wounds that involve damage to deeperstructures such as the distal phalanx (fracture)are painful from the onset. Depthof penetration can be difficult to ascertainwhen the wound progressives past the hornysole and the severity of the clinical signsdo little to help define which structuresare involved. An increased digital pulseis common and, in some cases, the digitalpulse may be increased only on the affectedside (often with a recent wound). Increasedheat may be palpable at the coronary bandand/or over the hoof capsule in the affectedlimb. Longstanding wounds may lead toa diffuse swelling in the soft tissue of thepastern and above. Early hoof tester applicationafter the injury may reveal focalsensitivity but over time a painful reactionmay be elicited over the entire sole region.Visual inspection of the solar surface ofthe foot may reveal the source of the lamenessbut often the defect in the sole is notapparent. If the offending object such as anail is noted, an attempt to obtain a radiographis essential in order to measurethe direction and depth of penetrationand to evaluate which structures may beinvolved (Figure 6). Examination shouldbegin by cleaning the solar surface of thefoot with a wire brush and paring the solelightly with a hoof knife; this may revealthe site of the puncture wound, a black tractor a crack in the horny sole. Due to theinvasive nature and serious complicationsthat can occur following puncture wounds,it is the author’s opinion that when dermaltissue is involved and requires debridement,a veterinarian should become involved inthe case. Any delay in the initiation of theappropriate treatment can have serious consequences.Debridement may be painfuland necessitates the use of local analgesiaat the level of the palmar digital or abaxialsesamoid nerves. Once the surfaceof the foot is cleaned and the wound ortract is identified, a sterile prep of the footwith an antiseptic scrub and alcohol rinseshould be performed. This will allow furtherexploration of the wound without fearof contaminating the surrounding normaltissue. Probing the wound with a blunt sterileprobe or teat canula can help determinethe depth and direction of the wound tract.Radiographs taken with the probe or canulain place is another option to accuratelyassess depth and direction of the woundalong with any gas shadows, debris or anyradio-opaque foreign bodies that may bepresent. If an obvious crack or black tract isfound, exploration may lead to a pocket ofinfection and subsequent drainage. A smalllooped hoof knife or a bone curette (# 2) isuseful to explore these areas.

Penetrating objects are contaminatedwith dirt, rust and manure and this materialmay be driven deep into the wound. Withoutadequate drainage an anaerobic environmentdevelops that promotes the growthof anaerobic bacteria. Contamination withthe organism Clostridium tetani is of particularconcern because of the potentialthreat of tetanus. This disease is difficultto treat successfully and an inquiry intothe animal’s history of appropriate vaccinationis important. Although adequateimmune protection may exist from a previousvaccination with tetanus toxoid, abooster of tetanus toxoid should be givenin the event of a puncture wound to thefoot. Superficial wounds carry a goodprognosis and can have dramatic resolutionof lameness within 24-48 hours followingdrainage while deeper wounds require surgicaldebridement. Superficial wounds andinfections are effectively treated by establishingdrainage, applying an antisepticbandage until drainage has ceased andprotecting the foot until the hoof capsuledefect has healed.

Long standing and deeper puncturewounds require more extensive debridement.Surgical drainage and debridementof necrotic soft tissue or possibly infectedbone is necessary for the wound to heal.Wounds to the sole can be safely exploredand debrided with the horse standing usinglocal analgesia. The horse should be placedon systemic antibiotics and anti-inflammatorymedication before surgery. An area ofsole 1–2 cm in diameter should be removedaround the puncture site in a conical fashionso that the tract can be completelyexplored. The surgical approach shouldfollow the draining tract and allow adequateexposure for removal of any diseasedtissue and to establish sufficient drainage.Creating the wound in a conical mannerprevents the corium from prolapsing intothe drainages site which is both painful andprevents healing.

REFERENCES
  1. Parks, A.H. Form and function of theequine foot. In: O’Grady SE, ed. TheVeterinary Clinics of North America,Vol. 19:2. Philadelphia: W.B. SaundersCo, 2003;296-298.
  2. 2O’Grady, S.E. Strategies for Shoeingthe Horse with Palmar Foot Pain, inProceedings. 52nd Annu Conv AmAssoc of Equine Pract 2006; 209-212.
  3. Redding WR, O’Grady SE. SepticDiseases Associated with the HoofComplex: Abscesses and PuncturesWounds. In O’Grady, SE, Parks, AH,ed. The veterinary clinics of NorthAmerica: Equine Practice. 2012 vol.28:2. Philadelphia: W.B. Saunders, pp423-440.
  4. Richardson GL, Pascoe LR, Meagher D.Puncture wounds of the foot in horses:Diagnosis & treatment. Comp ContEduc Pract Vet 1986; 8: S379-S387.
  5. Stashak TS. In: Adams’ Lamenessin horses. 4th edn. Lea and Febiger,Philadelphia. 1987; pp. 703-710.
  6. Redding WR, Pathological ConditionsInvolving the Internal Structures ofthe Foot, Equine Podiatry: Saunders/Elsevier, 2007.

FOOTNOTE
  1. Animalintex® 3M Animal CareProducts, St. Paul, MN 55144
  2. Betadine® Purdue Pharma L.P.,Stamford, CT 06901
  3. Keratex Medicated Hoof Putty® P.O.Box 2 Brookville, MD 2083