Bacterial skin infections - Pathology

()Map INTEGUMENTARY SYSTEM (Map) SKIN INFECTIOUS

Cutaneous bacterial infections tend to be called pyodermas. They are superficial, deep and are common in dogs, but less common in other species.

Superficial pyoderma

Superficial pyoderma in a dog (Courtesy of Bristol BioMed Image Archive)

• Affects epidermis and upper infundibulum of hair follicles
• No scarring when healed
• Grossly:
  • Erythema
  • Alopecia
  • Papules and pustules
  • Crusts
  • Epidermal collarettes
• Microscopically:
  • Intraepidermal pustular dermatitis
  • Superficial suppurative folliculitis
  • Bacteria commonly not seen

Deep pyoderma

• Less common than superficial pyoderma
• Occurs mainly in dogs
• Affects infundibulum, isthmic portion of hair follicles and surrounding dermis and subcutis
• Heals with scarring
• Local lymph nodes are often affected
• Often secondary to immunosuppression, follicular hyperkeratosis or demodicosis
• May also be a sequele to superficial pyoderma
• Grossly:
  • Crusted papules, pustules, alopecia, abscesses, nodules, fistulas, ulcers, haemorrhagic bullae
  • In dogs may look similar to pyotraumatic dermatitis
• Microscopically:
  • Pyogranulomatous folliculitis and furunculosis
  • Nodular or diffuse dermatitis
  • Panniculitis
• May involve a foreign bodey reaction to follicular contents and draining sinuses develop
• If chronic, scarring and loss of adnexa
• Bacteria often isolated include Staphylococcus spp., especially S. intermedius in dogs, Streptococcus spp., Corynebacterium pseudotuberculosis, Pseudomonas, Pasteurella, Proteus, E.coli

Subcutaneous abscesses

• Purulent exudate within dermis and subcutis
• Commonly occurs in cats due to contamination of penetrating wounds
• Surrounding wall of collagen and fibroblasts may develop
• Common bacteria (often normal mouth flora)
  • Pasteurella multocida, Fusobacterium spp., beta - haemolytic streptococci, Bacteroides

Bacterial granulomatous dermatitis

• Usually due to saprophytes
• Grossly:
  • Diffuse or nodular lesions
  • May ulcerate and form drainage fistulas
• Microscopically:
  • Macrophages +/- multinucleated giant cells
  • Caseous necrosis and neutrophils
• Mycobacterial granulomatous or pyogranulomatous lesions
  • Usually caused by Mycobacterium lepraemurium (feline leprosy) or other Mycobacteria
  • Most commonly lesions appear on head, neck and legs
• Botryomycosis
  • Granulomatous dermatitis caused by nonfilamentous bacteria
  • Usually Staphylococcus aureus
  • Small, yellow granules are formed - sulfur granules
    • Central bacteria surrounded by homogeneous eosinophilic material
• Filamentous bacteria can also cause granulomas
  • E.g. Actinomyces and Nocardia, Actinobacillus

Bacterial pododermatitis

• Digital infections in ruminants
• Contagious footrot
  • Usually caused by Bacteroides nodosus together with Fusobacterium necrophorum
  • Moisture and trauma allow B. nodosus to enter -> aids bacterial penetration of epidermis -> F. necrophorum invades -> necrosis and inflammation
  • Grossly:
    • Early lesions - red, moist, swollen, eroded interdigital skin
    • Spreads to epidermal matrix of hoof -> separation of horn + malodorous exudate
    • Regeneration attempted as germinal epithelium is not destroyed
    • Chronic infections -> long , misshapen hoof
  • Benign footrot (scald)- only interdigital ski affected, slight separation of heel horn
    • Mostly the type occuring in cattle
• Necrobacillosis of the foot
  • Usually caused by Fusobacterium necrophorum with other bacteria
  • In sheep:
    • Ovine interdigital dermatitis
      • Acute necrotising dermatitis similar to benign footrot
    • Foot abscesses
      • Bulbular or lamellar
      • Mostly in wet conditions and in heavy sheep
  • In cattle:
    • Interdigital dermatitis and cellulitis
    • Caused by F. necrophorum and Bacteroides melaninogenicus
    • Predisposed by trauma
    • Grossly:
      • Fissures, necrotic swollen edges in interdigital spaces
      • Inflammation may spread to joint spaces

Systemic bacterial infections

• Salmonellosis
  • Capillary dilatation and congestion -> cyanosis of external ears and abdoman
  • Thrombosis -> necrosis of extremities
• Erysipelas in pigs
  • Caused by Erysipelothrix rhusiopathiae
  • Vasculitis, thrombosis, ischaemia -> cutaneous lesions - firm, raises, rhomboidal pink to dark purple areas
• Clostridium novyi
  • Severe cellulitis, toxaemia and death of young rams during breeding season (due to traumatised heads) - 'big head'
• Streptococcus equi
  • In horses
  • Immune complex vasculitis -> purpura