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Foot: subsolar abscess/infection

ISSN 2398-2977


Introduction

  • Very common.
  • Focal accumulation of purulent exudates, usually between the germinal and keratinized epithelium of the hoof.
  • Usually due to defects in the sole or white line, or less commonly, a penetrating foreign body which leads to ingress of bacteria through the insensitive sole or frog of the foot; the pathology is in the deeper sensitive tissue where there is an abscess formed. Rarely, infection may extend to the deeper structures when pedal bone periosteum, the digital cushion, navicular bursa, palmar/plantar partition of the coffin joint and insertion of the deep digital flexor tendon (DDFT). When such deeper tissues are involved the condition can become potentially life-threatening. 
  • Signs: lameness of varying severity, but generally unilateral, acute and severe Musculoskeletal: gait evaluation, forelimb > hindlimb.
  • Diagnosis: foot examination Foot / shoe: examination, foot trimming may reveal point of abscess; in some cases may be difficult even with radiography Digit: radiography.
  • Treatment: local drainage, and control of infection, pain and inflammation.
  • Prognosis: dependent on extent of initial damage, adequate treatment and rest, but generally good.
Print off the Owner factsheets on Caring for your horse's feet and Subsolar abscess - pus in the foot to give to your clients.

Presenting signs

  • Lameness is initially acute in most cases but a protracted history is possible if there is deep-seated infection, inadequate drainage or pedal osteitis Distal phalanx: pedal osteitis - septic.
  • Pain in the sole is usually detectable using hoof testers Farriery: tools - hoof testerthough it may be of a diffuse nature and difficult to pinpoint. In a naturally very hard hoof capsule this is less likely.
  • Area of discoloration or a small defect in the hoof of the sole, frog or white line.
  • Pus exiting a damaged tract in the hoof or found after exploring such tracts.
  • Foreign body penetrating the sole or frog.
  • Exploring the sole may reveal bruising in the deeper sensitive sole Foot: sole bruising.
  • Demineralization or loss/necrosis of the pedal bone on radiography Forelimb: radiography is seen in severe and chronic cases Distal phalanx: pedal osteitis - non-septic Distal phalanx: pedal osteitis - septic.

Acute presentation

  • Acute, severe, unilateral lameness, sometimes non-weight-bearing.

Geographic incidence

  • Wet or very dry climate conditions can increase the incidence of this disease.

Cost considerations

  • Most cases will resolve quickly and without the use of antibiotics where drainage can be established early, and therefore treatment costs are limited.
  • Treatment can be long and protracted in complicated cases because of stabling/hospitalization, bandaging, specialized shoeing and antibiotic costs.

Special risks

  • The animal's temperament should be assessed before attempting any potentially painful procedures, such as trimming the hoof to expose the abscess.
  • Ensure that there is adequate and competent assistance available.

Pathogenesis

Etiology

  • The cause is not always identifiable.
  • Most commonly, the bacteria appear to enter the hoof capsule through small defects in the horn, such as microfractures or separation of the white line, or through hoof cracks.
  • Less commonly a penetrating foreign body, most likely through the sole or frog, eg nails, screws, wire, stones, glass or even wood.
  • May be iatrogenic, associated with nail misplacement during shoeing - nail prick or nail bind.

Predisposing factors

General

  • Exercise in poor conditions such as wet muddy areas and hard rough ground can increase the incidence.
  • Poor foot shape, structure and balance are common predisposing causes, most particularly the long toe/low heel conformation and poor mediolateral foot balance Foot: trimming and balancing.
  • Chronic laminitic foot Foot: laminitis.

Pathophysiology

  • Infection into the foot → inflammation → migration of white cells and fluid from the dermal vessels to form an abscess which leads to increased pressure on sensitive hoof laminae → pain and lameness. This is exacerbated by the low compliance of the hoof capsule restricting any swelling and increasing the pressure within the sensitive laminae.
  • As an abscess develops it separates the germinal layer of the epithelium from the hoof capsule extending further under the sole, frog or proximally under the wall.
  • Abscessation may develop from previous bruise or hematoma Foot: sole bruising.
  • Pus develops and may discharge through the penetrating site, elsewhere through the sole or proximally around the coronet (gravel), typically opposite to the site of the infection.
  • Lameness diminishes with either natural or induced drainage.
  • If drainage occurs at the coronet (the junction of the hairline and the wall), short horizontal defects (cracks) may appear in the hoof wall and slowly move distally as the hoof grows out.
  • Forelimbs are more commonly affected than hindlimbs.
  • Chronic discharge may lead to sinus and fistula development.
  • Infection may extend deeper to any or all of the structures in the region, eg pedal bone, coffin joint, navicular bursa, DDFT.
  • Chronic abscessation may occur in chronic laminitis Foot: laminitis, immunocompromise, keratoma Foot: keratoma or septic osteitis Bone: osteitis - septic.

Timecourse

  • If infection becomes established the disease is likely to be protracted.

Diagnosis

Presenting problems

  • History of lameness may be acute or chronic.
  • Fistulous tracts may be seen at the coronet or at the sole.

Client history

  • Commonly a variable period of lameness which may be severe.
  • There may or may not be known history of a penetrating foreign body.
  • May be history of recent shoeing - nail bind, nail prick.
  • Response to analgesics and or antibiotics in the past with relapses.
  • Recurrent foot problems with shoeing, loss of shoes, hoof cracks, abscesses and solar bruising.

Clinical signs

'Pus in the foot' is such a common cause of lameness that it is always prudent to eliminate it from the differential diagnosis of foot lameness at an early stage in any clinical examination.

  • Usually unilateral, moderate to severe lameness.
  • Often acute onset, sometimes following exercise or turnout, but in some cases, there is a more low-grade lameness that suddenly changes to more severe.
  • Penetrating foreign body detected in frog Foot: foreign body 01 - frogor sole.
  • Small defects in the sole or white line which may be black or discolored.
  • Increased heat and digital pulse.
  • Discharging purulent tracts from areas of the sole, frog or from the coronet (there may be swelling and palpable pain here before a sinus breaks open).
  • Location of point of abscess by foot trimming Sole: defect 01 - subsolar abscess opened.
  • Detection of areas of sole/frog bruising Foot: sole bruisingand inflammation on examination of the sole/frog Sole: discharge 02 - subsolar abscess.
  • Pain is detectable in the sole area using hoof testers Farriery: tools - hoof tester, it may be localized or diffuse Musculoskeletal: physical examination - adult.
  • Positive response to palmar digital and/or abaxial sesamoid nerve block Forelimb: perineural analgesia.
  • Clinical findings may be vague and non-specific other than localizing the lameness to the foot.
  • Occasionally there may be pastern swelling and cellulitis extending proximally.

Diagnostic investigation

  • Remove the shoe, carefully clean out the foot and carry out a thorough visual inspection first Foot / shoe: examination.
  • Sedation Anesthesia: standing chemical restraint and/or local analgesia Anesthesia: analgesia - foot may be required to conduct a thorough examination when the foot is painful.
  • Explore all defects and discolorations in the horn and follow any tracts until the location(s) of the abscess(es) is/are determined - gray/black pus will discharge, usually accompanied by instant relief of pain.
  • Where no tract is visible, excessive paring of the hoof is contraindicated. Repeat examinations 24-48 h later is recommended, after poulticing the hoof capsule to soften the horn. The abscess frequently spontaneously bursts out from the coronary band during this period. If the clinical signs persist without the abscess being identified, then radiography of the foot Digit: radiography is indicated.

Radiography

  • Radiography Forelimb: radiography.
  • Areas of demineralization around the pedal bone, sequestrum Distal phalanx: osteitis 04 - sequestrum - DPrPaDiO radiograph.
  • Evidence of pedal osteitis Distal phalanx: osteitis 01 - DPrPaDiO radiographSole: collapsed 02 - pedal osteitis.
  • Evidence of foreign body in the structures of the foot.
  • Gas shadows on lateral or dorsopalmar/plantar foot radiographs Foot: abscess - LM radiographFoot: gas shadow 01 - LM radiograph.
  • Allows other pathologies such as fracture to be eliminated.

Magnetic resonance imaging (MRI)

  • See .

Confirmation of diagnosis

Discriminatory diagnostic features

  • History.
  • Clinical signs.
  • Radiography.

Definitive diagnostic features

  • Detection of pus in the foot.

Differential diagnosis

Treatment

Standard treatment

  • Remove all necrotic or underrun horn with a hoof knife Farriery: tools - hoof knife. May be facilitated in difficult horses by using regional nerve blocks Forelimb: perineural analgesia Hindlimb: perineural analgesia and/or sedation Anesthesia: standing chemical restraint.
  • Establish adequate drainage from the area Foot: streetnail procedure 01 - incisions. There is some dispute as to how large a drainage hole should be, but at least 1 cm diameter is essential. In some cases, it may be necessary to enlarge that further, but at all stages, only underrun and separated horse should be removed. Damaging sensitive healthy laminae is counter productive.
  • Hot tub 1-2 times daily in hypertonic solutions of sodium chloride Sodium chloride and magnesium sulfate Magnesium salts until the abscess stops discharging pus (1-2 days).
  • Application of a hot damp poultice may be necessary after drainage is established in some cases for a maximum of 24 h:
    • Hold in place with cohesive or adhesive bandage ensuring that poultice is completely covered.
    • Change poultice every 8 h.
    • Prolonged poulticing should be avoided because it softens the horn tissue and may lead to hoof weakening.
  • Dress wound with povidone-iodine soaked swabs held in position with appropriate bandaging.
  • Protect dressing with suitable external bandage Farriery: foot protection.
  • Tetanus anti-toxin should be administered in unvaccinated or not recently vaccinated horses Tetanus antitoxin.
  • Analgesics and anti-inflammatories Therapeutics: anti-inflammatory drugs, eg phenylbutazone Phenylbutazone, flunixin meglumine Flunixin meglumine, vedaprofen Ketoprofen.
  • Use of antibiotics Therapeutics: antimicrobials is only advocated in severe infection and where adequate drainage is present.
  • Rest.
  • Use of a hospital plate shoe to protect wound and add pressure and pressure to aid healing in cases where wound is extensive Farriery: foot protection. Pads and plates may allow earlier return to exercise.
  • In chronic cases, culture and sensitivity testing should be performed to identify the correct antibiotic to use. Surgical debridement and post-operative intravenous regional perfusion Therapeutics: intravenous regional antibiosis or use of impregnated beads may be required in serious cases where deeper tissues are involved.

Monitoring

  • Resolution of purulent discharge and wound becomes dry in a few days.
  • Rapid improved use of the limb.
  • Failure to improve rapidly, or drainage and abscess recurs, warrants further diagnostic investigation, such as radiography.

Subsequent management

Treatment

  • Ensure adequate drainage is maintained.
  • To prevent contamination, bandaging should continue until discharge ceases and the horn is sufficiently dry to allow shoe replacement.
  • A sole pad may be required to protect the sole in cases where there has been extensive removal of tissue Farriery: shoe pad.
  • A bar shoe may be required to provide added stability to the foot if large resection of tissue Sole: defect 02 - post-drainage of abscess.
  • Radiography to ensure no ongoing bone lysis and complete removal of sequestrum.

Prevention

Control

Prophylaxis

Outcomes

Prognosis

  • Depends on the speed of diagnosis and establishment of adequate drainage: good with early intervention.
  • If the abscess is associated with a predisposing cause, then these problems may determine the long-term prognosis and also increase the likelihood of recurrence.
  • The extent of damage to the underlying structures by the inciting trauma and the depth or extent of the resulting infection may subsequently deteriorate the prognosis.
  • Non-septic Distal phalanx: pedal osteitis - non-septic and septic pedal osteitis Distal phalanx: pedal osteitis - septic, infection of the coffin joint Joint: septic arthritis - adult or navicular bursa Navicular bursa: puncture, and infection involving the DDFT will all deteriorate the prognosis.
  • For the majority of cases once diagnosis is confirmed, drainage established together with rest +/- antibiotics, complete resolution is likely.

Expected response to treatment

  • Resolution of lameness.

Reasons for treatment failure

  • Failure to establish and maintain drainage may only see temporary improvement.
  • Inappropriate use of antibiotics may inhibit resolution and lead to recurrence.
  • Tetanus Tetanus.
  • Infection of deeper structures (pedal bone and digital cushion) and synovial structures such as navicular bursa and coffin joint.

Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Cole S D et al (2019) Factors associated with prolonged treatment days, increased veterinary visits and complications in horses with subsolar abscesses. Vet Rec 184 (8), 251 PubMed.
  • Redding W R & O’Grady S E (2012) Septic diseases associated with the hoof complex: abscesses, punctures wounds, and infection of the lateral cartilage. Vet Clin North Am Equine Pract 28 (2), 423-440 PubMed.
  • Milner P I (2011) Diagnosis and management of solar penetrations. Equine Vet J 23 (3), 142-147 VetMedResource.
  • Stephenson R (2011) Presenting signs of foot abscessation - A practice based survey of 150 cases. UK Vet 16, 4-7 VetMedResource.
  • Carmalt J L (2009) What is your diagnosis? Subsolar abscess with secondary distal interphalangeal joint synovitis. JAVMA 235 (4), 377-378 PubMed.
  • Cullimore A (2009) Clinical aspects of the equine foot. Part 4: Sole penetrations. UK Vet 14 (5), 8-13 VetMedResource.
  • Leonard J M et al (1990) What is your diagnosis? Hoof abscesses and cellulitis extending along the palmar aspect of the pastern of the left forelimb. JAVMA 196 (1), 1791-1794 PubMed.
  • DeBowes R M et al (1989) Penetrating wounds, abscesses, gravel and bruising of the equine foot. Vet Clin North Am Equine Pract 5 (1), 179-194 PubMed.
  • Fessler J F (1989) Hoof injuries. Vet Clin North Am Equine Pract 5 (3), 643-664 PubMed.
  • Jamison J M et al (1983) What is your diagnosis? Sole abscess involving the lateral and plantar aspects of the foot. JAVMA 182 (6), 625-626 PubMed.