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Skin: bacterial disease – overview

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Podcast: Skin: bacterial disease - overview


  • Cause: break in normal skin defences and/or immunosuppression.
  • Signs: pustules, papules, folliculitis/furunculosis, cellulitis, abscesses, crusts, nodules, scaling, alopecia and pain/pruritus.
  • Diganosis: history, clinical signs, lesion distribution and culture.
  • Treatment: resolve any underlying problems. Topical and systemic antimicrobials; immunosuppressnat drugs should generally be avoided but may be required to manage an underlying condition.
  • Prognosis: good to poor; varies depending on diagnosis and/or management of any underlying condition.

Common bacterial infections

Staphylococcal infections

Streptococcal infections

  • Gram-positive cocci.
  •  S. equiS. zooepidemicusand S. equisimilusare most common isolates   Streptococcus spp  .

Corynebacterial infections

  • Gram-positive pleomorphic bacteria.
  •  Corynebacterium pseudotuberculosisis the common isolate, spread by biting flies.
  •  Rhodococcus equi  Rhodococcus equi  is another common isolate.

Actinomycetic infections

  • Gram-positive filamentous bacteria.
  •  Dermatophilus congolensis  Dermatophilus spp  is the most common isolate.

Actinobacillosis infections

  • Gram-positive.
  •  Actinobacillosis lignieresiiis rare but reported.

Clostridial infections

  • Gram-positive rods.
  •  Clostridium septicumClostridium chauvoeiand Clostridium perfringens  Clostridium perfringens  are the most common isolates.


  • Infection relies on failure of normal skin barrier via trauma (ectoparasites, wounds or inflammatory skin diseases), maceration (prolonged wetting or skin folding preventing ventilation) or immunosuppression. 

Clinical findings

Staphylococcal infections

  • Folliculitis   Skin: staphylococcal folliculitis  /furunculosis, papules and pustules.
  • May develop nodules, draining tracts and crusts.
  • Intact pustules are uncommon.

Streptococcal infections

  • Ulcerative lymphangitis   Lymphangitis  .
  • Folliculitis.
  • Furunculosis.
  • Abscesses.

Corynebacterial infections

  •  Corynebacterium pseudotuberculosiscauses:
    • Deep-seated subcutaneous abscesses.
    • Folliculitis.
    • Furunculosis.
    • Ulcerative lymphangitis   Lymphangitis  .

 Corynebacterium pseudotuberculosisis zoonotic.

  •  Rhodococcus equi  Rhodococcus equi   causes single subcutaneous abscesses, multiple abscesses, draining tracts or ulcerated nodules.
  • Ulcerative lymphangitis   Lymphangitis   is rare but recognized.

Actinomycetic infections

  • Actinomycotic mycetomas ('fungal' tumors), draining tracts and grains ('sulfur granules' = microorganism colonies coated with fibrin/host immunoglobulin).
  •  Dermatophilusspp    Dermatophilus spp   causes tufted papules and crusts giving the characteristic paintbrush appearance of the coat.

Actinobacillosis infections  Actinobacillosis 

  • Suppurative to pyogranulomatous lesions with swelling, nodules, ulcers and draining sinus tracts.

Clostridial infections  Clostridia spp 

  • Warm, painful swelling with pitting edema.
  • Crepitus may be present.
  • Skin often sloughs.

Disease associated with bacterial infections

Staphylococcal infections

Streptococcal infections

Corynebacterial infections

  •  Corynebacterium pseudotuberculosiscauses:
    • Deep-seated subcutaneous abscesses.
    • Folliculitis.
    • Furunculosis.
    • Ulcerative lymphangitis   Lymphangitis  .

Actinomycetic infections

  • Dermatophilosis   Dermatophilosis    (rain scald/mud fever).
  • Actinomycetes - rare but has been reported to cause mandibular lymphadenitis, abscesses, fistulous withers   Spine: supraspinous bursa - bursitis  and poll evil.
  • Nocardiosis - rare but can cause suppurative to pyogranulomatous lesions, especially in immunosuppressed horses.

Actinobacillosis infections

  • Actinobacillosis   Actinobacillosis   is a rare cause of suppurative to pyogranulomatous lesions.

Clostridial infections

  • Injection site abscesses.

Diagnosis and treatment

Differential diagnosis


  • Resolution of the underlying problem is important, eg ectoparasites, immunosuppression, wet/macerated skin, etc, to prevent recurrence.
  • Clipping of hair to facilitate removing debris, drainage and application of topical products.
  • Topical treatments include disinfectants (chlorhexidine   Chlorhexidine  , povidone-iodine   Povidone-iodine  and benzoyl peroxide   Benzoyl peroxide  ), antibiotics (fusidic acid, mupirocin, bacitracin, neomycin   Neomycin  , gentamicin   Gentamicin  , polymixin B   Polymixin B Sulfate  and thiostrepton), and antibiotic-steroid combinations. Most suited to small, discrete lesions, but should be used wherever possible - topical therapy achieves high local disinfectant/antibiotic concentrations and reduces the selection pressure on commensal organisms at mucosal sites and in the gut.
  • Antibacterial shampoos, especially chlorhexidine   Chlorhexidine   containing products, can be effective and also aid in physical debris removal.
  • After shampooing, thorough drying to reduce maceration of skin should be undertaken.
  • Systemic antibiotics (penicillin   Penicillin G  , potentiated sulfonamides   Therapeutics: sulfonamides  , macrolides   Therapeutics: macrolides lincosamides  and fluoroquinolones   Therapeutics: nitrofurans / nitroimidazoles / quinolones  are commonly used in skin disease) may be appropriate depending on diagnosis, type and/or extent of lesions. The choice of antibiotic should be based on culture and sensitivity results, aiming to use first tier drugs (penicillin, potentiated sulfonamides and macrolides) wherever possible.
  • Abscesses may require lancing to facilitate drainage.



  • Varies depending on diagnosis.
  • Generally good for bacterial skin disease.

Reasons for treatment failure

  • Incorrect diagnosis.
  • Poor owner compliance, eg keeping skin dry, infrequent shampooing, missing doses and/or ceasing treatment too soon.
  • Selection of inappropriate antimicrobial, eg inactivated in presence of pus, poor distribution to the target lesions, lack of activity against the target organism.
  • Failure to use topical therapy.
  • Inappropriate concurrent use of immunosuppressive treatment.
  • Underdosing or inadequate duration of antimicrobial therapy.
  • Resistance to antimicrobial therapy, eg MRSA or other multi-drug resistant bacteria.

Further Reading


Refereed papers

Other sources of information

  • BEVA (2012) BEVA Antimicrobial Policy Template 2012.Practice Policy: Dose and Routes of Administration of Common Antimicrobials. Website: (PDF).
  • Reed S M, Bayly W M & Sellon D C (2010) Equine Internal Medicine. 3rd edn. Saunders. ISBN: 978-1-4160-5670-6.
  • Scott D W & Miller H (2003) Equine Dermatology. Saunders. ISBN: 0-7216-2571-1.