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 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

= Superficial pustular dermatitis
Caused by coagulase-positive Staphylococcus spp.
Associated with:
- Abrasions
- Poor nutrition
- Viral infections
- Immunosuppression (often due to hyperadrenocorticism)
Grossly:
- Pustules -> crusts, mostly in hairless skin
Microscopically:
- Interfollicular neutrophilic subcorneal pustule

Dermatophilosis in a cow (Courtesy of Bristol BioMed Image Archive)

Caused by Dermatophilus congolensis
Affects cattle, horses, sheep mainly
More common in wet and warm weather
Transmotted from animal to animal
Lesions tend to form on dorsal back and extremities
Associated with skin trauma, prolonged wetting or parasites
- -> penetration of zoospores
Bacteria proliferate in outer sheath of hair follicles and superficial epidermis
Gram-positive, filamentous branching organisms, subdivided longitudinally and transversly
Causing:
- Acute inflammatory response -> neutrophil migration through dermis and epidermis -> formation of microabscesses
- Further penetration of bacteria is thus prevented
- Regenerated epidermis is invaded again by remaining organisms
- Repeated reinfection -> multilaminated pustular crusts
Grossly:
- Papules, pustules, crusts may coalesce and mat the coat
Microscopically:
- Hyperplastic superficial perivascular dermatitis
- Multilaminated crusts, alternating keratin and inflammatory cell layers

= Exudative epidermitis of pigs
Caused by Staphylococcus hyicus
Affects neonatal piglets - often fatal, older piglets - milder disease
Focal erosion of stratum corneum
Brown exudate
Dermatitis around eyes, ears, snout, chin and medial legs, may spread to ventral abdomen and thorax
Rapidly becomes generalised -> greasy exudate over red skin -> hardened, cracked exudate
In older piglets, milder localised disease develops around eyes, ears and face
Grossly:
- Thickened epidermis, scaling
Microscopically:
- Early - subcorneal pustular dermatitis extending to hair follicles -> superficial suppurative folliculitis
- Late - hyperplastic epidermis, thick keratin crusts with cocci, microabscesses

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

Occurs in dogs, horses, sheep and goats
Dogs:
- Localised or generalised lesions
- Affected areas include: muzzle, chin, bridge of nose, pressure points and interdigital areas
- German Shepherd deep pyoderma
  - Genetically predisposed
  - Dorsal lumosacral, ventral adbomen and thigh areas
Horses:
- Areas under tack ususlly involved
Goats:
- Extremities, ventral abdomen, udder, medial thigh, perineum tend to be affected
Sheep:
- Adults: face, limbs and teats
- Lambs: lips and perineum

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

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

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

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