Definition
Pyoderma is a bacterial infection of the skin that can be classified based on depth of lesion:
- Skin surface lesions include pyotraumatic dermatitis ('hot spots') and intertrigo (skin fold infection)
- Superficial bacterial pyoderma is a bacterial infection of the epidermis and follicular epithelium. Common examples include impetigo, mucocutaneous pyoderma and superficial bacterial folliculitis. Superficial pyoderma is common, usually secondary to other skin diseases such as atopic dermatitis, flea allergy dermatitis, demodicosis etc, although primary idiopathic pyoderma may also occur
- Deep pyoderma is a serious infection occurring in tissues deeper than the hair follicle, potentially including the dermis and subcutaneous tissues. Examples include deep bacterial folliculitis and furunculosis, cellulitis and subcutaneous abscesses. Deep pyoderma is less common and may follow trauma or superficial disease
Cause
- Bacteria are normal inhabitants of the skin surface
- Bacterial skin disease can result when the skin barriers are impaired or damaged, such as by trauma or underlying disease
- The infection may occur as a primary condition or, more commonly, secondary to other systemic or skin disease. The most common underlying skin diseases include seborrhoeic, follicular or allergic disease (for example atopic dermatitis)
- Self-trauma resulting from pruritic disease can predispose to infection, as can suppression of the immune system (for example from systemic disease or from medication)
- Some animals, particularly those with allergic skin disease, may develop a hypersensitivity reaction to bacterial antigens (for example Staphylococcus)
- Staphylococcus intermedius is the main pathogen involved in canine pyoderma, but other bacteria may be present particularly in secondary infections or deep pyoderma
Signalment and history
Pyoderma may occur in dogs of any age and type. Bacterial infections may be more common in warm, humid weather.
- Primary pyoderma generally occurs in dogs with otherwise healthy skin
- Impetigo is generally seen in puppies prior to puberty (or in immunosuppressed animals)
- Breed predisposition to secondary pyoderma and lesion distribution may vary depending on the underlying disease. Almost any skin disease can predispose to infection. A history of chronic or recurrent skin lesions may indicate secondary infection. History, breed, and distribution of lesions may be consistent with, for example: (i) allergic skin disease (for example atopic dermatitis, food allergy, flea allergy); (ii) parasitism (for example mange); (iii) metabolic or endocrine disease (for example hypothyroidism, hyperadrenocorticism); (iv) other microbial infection (for example ringworm, Malassezia)
Lesions may be focal or generalised. Localised body areas that may be affected include:
- Skin folds (intertrigo): (i) body folds - breeds with pronounced skin folds, for example English bulldog, Shar Pei; (ii) facial folds - brachycephalic breeds, for example Pekinese; (iii) lip folds - loose-lipped dogs, for example Spaniels. Halitosis may be reported; (iv) vulval folds - obese older females with infantile vulva; (v) tail folds - dogs with corkscrew tails, for example Pug, may be predisposed
- Feet (pododermatitis): may be seen more commonly in short-coated breeds
- Ears (otitis externa): breeds with narrowed or excessively hairy ear canals, or ears that hang down may be predisposed
Deep pyoderma usually has an underlying cause such as demodicosis, immunosuppression or trauma (for example acral lick dermatitis, self-trauma and bite wounds).
Clinical signs
Bacterial skin disease is generally pruritic.
Surface infections
- 'Hot spots' - acute onset, reddened moist lesions with alopecia and well demarcated edges
- Intertrigo (skin fold infections) - inflammatory, exudative lesions in skin folds
Superficial infections (superficial pyoderma)
- Impetigo: (i) non-follicular sub-corneal pustules in lightly haired areas of skin are characteristic; (ii) most common in ventral abdomen and axillary regions; (iii) generally non-pruritic (pruritus may indicate folliculitis); (iv) large pustules >0.5cm diameter (bullous impetigo) may indicate underlying endocrine disease
- Mucocutaneous pyoderma: (i) mostly lips and around mouth (+/- eyelids, nostrils, prepuce/vulva or anus); (ii) symmetrical swelling of the lips, especially the commissures; (iii) painful crusting, fissures , erosion, +/- exudate, +/- depigmentation
- Superficial folliculitis (bacterial infection of the hair follicle): (i) inflammatory papule then pustule around a hair shaft; (ii) epidermal collarettes and excoriation due to self-trauma; (iii) circular 'target' or 'bull's eye' lesions of erythema, scaling, crusting, hyperpigmentation and alopecia. Clipping may be required to observe lesions in heavily coated areas
- Progressive hair loss and spread of lesions may give coat a moth-eaten appearance
Deep infections
- Deep folliculitis and furunculosis (infection of hair follicles and deeper tissues). Often occurs as a progression from a superficial infection. Pain and pruritus are usual. Large pustules and nodules form and may rupture to form fistulae (draining tracts). Alopecia, hyperpigmentation and lichenification can occur. Scarring is common. Typical sites include groin, axillae, feet and pressure points. Affected animals may show signs of systemic illness and lymphadenopathy
- Generalised deep pyoderma - as above but as lesions deepen, affected area appears larger and redder in colour. Tissue necrosis, haemorrhagic bullae and ulcers may occur. Cellulitis and panniculitis indicate subcutaneous involvement
- Cellulitis (acute spread of infection below the skin surface) is a severe condition that may occur due to progression of superficial infections or may be introduced by trauma. The deep infection is poorly contained and spreads laterally through the subcutaneous tissue. Lesions include erythema, heat, oedema and pain. Depending on cause, the overlying skin may become fragile, darkly discoloured and sloughing may occur. Lymphadenopathy and pyrexia may occur. Infections due to anaerobic bacteria generally progress more rapidly and more severely. May be accompanied by panniculitis (inflammation of fatty tissues)
Diagnosis
Differentials
Consider other diseases that cause pustules such as auto-immune (for example pemphigus) or folliculitis such as demodicosis or ringworm.
Diagnostic approach includes:
- Observation of primary or secondary skin lesions
- Elimination of other potential skin diseases
- Confirmation of bacterial infection
- Investigation of why infection originally occurred, particularly in recurrent cases
Potential diagnostic tests include:
- Skin scrapings (particularly deep scrapings for Demodex mites)
- Trichogram (hair pluck) and/or fungal culture (for ringworm)
- Cytology (rule out Malassezia infection and confirmation of bacterial infection)
- Cytology should show bacteria and phagocytosing cells (neutrophils and macrophages)
- Trial treatment
- Culture and sensitivity considered particularly if bacterial rods are seen (suggests an atypical infection) or if infection fails to respond to appropriate antibacterial therapy
Treatment and management
Treatment is dependent on:
- Depth and location of lesion
- Causative organism
- Presence of underlying disease
General considerations
- Deeper infections generally require longer and more aggressive antibiotic therapy
- Infections involving the feet require immediate treatment to minimise scarring (predisposes to future disease)
- Management or treatment of underlying disease is required to reduce the risk of recurrence
- Additional infections (for example Malassezia or mites) should be resolved with an appropriate course of antimicrobial or parasiticide therapy
Breaking the itch-scratch cycle
- In some cases anti-inflammatory treatment may be required to break the itch-scratch cycle, for example short course of corticosteroids for 'hot spots'. Consider the immunosuppressive potential of such treatments
- Physical barriers may assist in reducing self-trauma (for example Elizabethan collars)
Considerations in selection of antimicrobial
- Spectrum of activity required: (i) most likely causative bacteria (based on location or history); (ii) cytology, i.e. gram positive or negative, cocci or rods; (iii) culture and sensitivity testing if required
- Compliance issues such as ease of administration
- Potential drug interactions or side effects: (i) use during pregnancy or lactation; (ii) underlying health of dog, or concomitant disease
- Route of administration (tissue penetration required). Topical therapies may be useful for surface lesions or to reduce surface microbial load
- Duration of treatment: minimum 3 weeks for superficial pyoderma; therapy should be continued 1-2 weeks beyond resolution of visible clinical signs
Surgical therapy
- May be required for removal of foreign bodies
- Surgical drainage and flushing, and removal of necrotic tissue, is also indicated in some deep infections
