Bacterial skin infections - Pathology

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

Bacterial pododermatitis

• Digital infections in ruminants
• Contagious Footrot

• Necrobacillosis of the foot

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