Lecture 3

• Infections of the skin - I
• (Lecture #3)
• Dr. Ashutosh Verma, BVSc, MVSc, PhD, DipACVM, Professor of Microbiology, Richard A. Gillespie College of Veterinary Medicine, Lincoln Memorial University, Harrogate, TN
• The material used in this presentation has been obtained from different sources including books, published papers and online resources. It is intended only for educational purposes.
• Books referenced

  • Veterinary Microbiology and Microbial Diseases, Quinn et al., 2nd ed.
  • Veterinary Microbiology, Songer and Post
  • Clinical Veterinary Microbiology, Markey et al., 2nd ed.
  • Infectious Diseases of the Dog and Cat, Greene, 4th ed.
  • Veterinary Microbiology, Hirsh et al., 2nd ed.
• Learning Objectives

  • Discuss skin defenses and how they work to protect
  • Explain, with examples, the differences between primary and secondary skin infections
  • Explain differences between resident and transient skin flora and give some examples for each category
  • Explain the differences between colonization and infection with reference to skin
  • Describe predisposing factors for secondary bacterial infections of skin
  • Discuss considerations for collection and diagnosis of clinical samples for skin infections
  • Describe classification of pyodermas

  
• Skin – a natural defense
• Physical barriers

  • Hair/fur – prevent direct skin contact
  • Stratum corneum – inert and impermeable layer
  • Temperature and pH – too cold/unfavorable pH for optimal growth of many
• Chemical barriers

  • Fatty acids – bacteriostatic
  • Inorganic salts – High salt conc.
  • Transferrins – bind iron needed for bacterial growth
• Immunological defense

  • Site for interactions between immune cells and antigens
  • Prevent deeper invasion
• Normal flora

  • Bacteria and fungi
  • Protective
• Factors affecting number and species of normal flora

  • Hydration
  • General health
  • Physical and chemical environment – salt, transferrin
  • Other bacteria – nutrients, antibiotic production
• Types of normal flora

  • Resident flora
  • Transient flora
• Resident flora


  • Live and multiply on the skin (obligate parasites)
  • Form a permanent population
  • Cannot be eliminated
  • Normally are harmless
  • Examples – Coagulase-negative Staphylococcus, alpha-hemolytic Streptococcus, Micrococcus spp., etc.
• Transient flora


  • Acquired from environment or mucous membranes
  • Most do NOT multiply efficiently on skin - just there
  • Are transient - CAN be removed or eliminated
  • May be involved in pathologic processes
  • Examples – Coagulase-positive Staphylococcus, E. coli, Proteus, etc.
• Types of bacterial skin infections

  • Primary infections
  • Secondary infections
• Primary skin infections


  • Bacteria initiate and cause most of the pathology
  • Occur in otherwise "healthy" skin - predisposing factor, if any, is minor
  • Single bacterial species is dominant
  • Characteristic disease pattern evident - consistent disease syndrome
  • Antibacterial therapy is effective - underlying problem is less significant
• Primary bacterial skin infections

  • Greasy pig disease caused by Staphylococcus hyicus
  • Dermatophilosis caused by Dermatophilus congolensis
• Secondary skin infections


  • Bacterial infections associated with other infection(s) and/or condition(s)
  • Predisposing conditions for secondary skin infections: Skin infected with parasites, viruses, fungi, etc.; Other local predisposing factors - moisture, skin folds, etc.; Systemic disease resulting in skin pathology - hypothyroidism, Cushing's disease; Physical or chemical trauma - surgical incision, catheter placement, etc.; Immunosuppression - e.g., corticosteroid therapy
• Colonization


  Present, but not causing disease
• Infection


  Present, associated with pathology
• Identification of bacteria in primary and secondary infection is useful if also consider the presence of normal flora
• Isolation of Staphylococcus pseudintermedius alone is not enough to determine if it is a bacterial infection (and is significant)
• Diagnosis of bacterial skin infections

  1. Correlate history, clinical signs, and bacterial isolates

  2. Bacterial culture to distinguish colonization and infection

  3. Direct smears: Gram's and Wright's stains

  4. Histopathology and surgical biopsy

  
• Sampling for bacterial culture

  • Do NOT sample open tracts or erosions (use unopened pustule)
  • Punch biopsy of lesion- NOT into formalin for culture
• Direct smears: Gram's and Wright's stains

  • Gram's stain (numbers and types bacteria; presumptive ID; initiation of therapy)
  • Wright's stain (e.g., Diff-Quik)- presence/type inflammatory cell; degenerate neutrophils with bacteria within the cells is indicative of infection
• Histopathology and surgical biopsy


  • When to biopsy- Any dermatosis that is unresponsive to the treatment for 3 weeks or any recurrent dermatosis
  • Advantages of early biopsy- Avoids nonspecific, masking, and misleading changes of chronicity; allows rapid institution of specific therapy
  • What to biopsy- Intact pustules; take multiple biopsies; try to obtain primary lesions (avoid those marred by excoriation and chronicity)
• Classification of Skin infections (pyodermas)


  • Surface pyoderma
  • Superficial pyoderma
  • Deep pyoderma

  
• Surface pyoderma


  • Involves epidermis only
  • Often sequelae to self-trauma or allergic skin disease
  • Examples include hot spots and early skin fold dermatitis

  
• Superficial pyoderma


  • Involves skin down to and including intact hair follicles (folliculitis)
  • Pustules are present – mini abscesses
  • Often secondary to other diseases (hormonal, metabolic or parasitic)
  • Recurrence is common and long-term management may be difficult
• Deep pyoderma


  • Involves tissues deeper than hair follicles
  • Almost ALWAYS secondary to other contributing factors
  • Not common, but VERY difficult to treat
• Clinical case from the first lecture
• 10 yr. old Spayed Pekingese, 4.8 Kg, 6-day history of Pyrexia, Anorexia, Depression, Multiple non-pruritic erythematous skin nodules
• The skin lesions began on the abdomen and extended to the neck, axilla and perianal region
• Atopic dermatitis at 2 years of age, pruritus, responded to corticosteroid (prednisone) therapy, owner continued to treat pruritus with prednisone intermittently
• Prior to referral, dog was treated with antibiotic enrofloxacin and corticosteroid prednisone, but clinical signs did not resolve
• Physical exam findings: Temp: 105˚F, Heart rate: 162 bpm, Resp: 51 bpm, Pale mucous membrane

  
• Clinical Case – what's next / how to approach this

  1. Create a problem list

  2. Identify differentials - infectious/non-infectious, likely causative agents

  3. Diagnostic plan to diagnose and make treatment decision and plan for prevention long term
• CBC, Serum chemistry: Albumin (26 g/L, reference range: 29 to 42 g/L), Globulins (46 g/L, reference range: 28 to 42 g/L)
• Clinical Case – diagnostic approach

  1. FNA, biopsy from skin nodules & blood: Culture and histology results pending

  2. Cytological exam on FNA