Approach to pustues, crusts + scales (secondary infections)

Recall the definitions of pustule, crust and scale

Pustule: vesicle filled with pus

Crust: dries exudate containing blood, serum, scaled, pus

Scale: desquamated corneocytes visible on surface

What are the potential causes of pustules?

• Superficial bacterial pyoderma

• Pemphigus folliceus (immune mediated)

What are the potential causes of crusts?

• Ruptured pustules

• Dried exudate from erosions/ulcerations

What are the potential causes of scale?

• Primary cornification disorders

• Secondary to any other skin pathology

How should you approach pustules, crusts and scales?

What are the features of skin infections and what are the causes?

Always 2ary to another disease

• Allergic skin disease

• Ectoparasites

• Endocrinopathy

• etc.

What are skin infections usually associated with?

• Commensals

• Take advantage of either a problem with skin barrier or immune system

How do you diagnose skin infections?

• Cytology

  • Culture not diagnostic as present on skin anyways

What is the most common commensal of the skin?

Staphylococcus Pseudintermedius

(S. aureus, S. schleiferi and S. canis less common)

What is the most common fungi of the skin?

Malessezia pachydermatis

Give an example of primary skin pathogens

Dermatophytes

How are dermatophytes (pathogenic bacteria/not commensal) diagnosed?

Culture

What is bacterial pyoderma?

bacterial skin infection very common in dogs

Not very common in cats except in abscesses from fighting

How is bacterial pyoderma classified?

1. Surface - Superficial epidermis

2. Superficial - Epidermis and hair follicles

3. Deep - Epidermis, hair follicles, dermis +/- subcutaneous fat

(according to depth)

How does surface pyoderma present clinically?

• Hotsposts (moist pyotraumatic dermatitis)

• Intertrigo (skin fold pyoderma)

• Bacterial overgrowth

What bacteria will you see in surface pyoderma?

• Coccoid, rod-shaped or mixed bacteria depending on location

• Staphylococci most common

• May progress to superficial/ focal deep infections

What are the features of moist pyotraumatic dermatitis?

• Well demarcated flat eroded moist lesion with erythematous halo

• Usually on cheek, neck, rump

• Develops within hours due to self trauma

• FAD or pruritic/painful trigger

hotspot before + after clipping

What breeds are predisposed to moist pyotraumatic dermatitis?

Rottweiler, Golden Retriever & GSD

What are the features of intertrigo?

• Skin fold pyoderma

• Compromised barrier (friction, altered microclimate, loss of normal ventilation)

• Microbes proliferate, produce toxins, create inflammation

• May have concurrent skin disorder

• Can progress to superficial or deep infection

What are the features of bacterial overgrowth as a symptom of surface pyoderma?

• Common, usually due to underlying cAD

• Ventral trunk and interdigital spaces

• Can be very pruritic

• Erythema, hyperpigmentation, lichenification, excoriation & alopecia

• Ddx Malassezia dermatitis

What is an overgrowth?

Excessive presence of microorganism on skin surface

Not pyogenic - lack of neutrophilic inflam (in infection there is a presence of degenerative neutrophils)

What does superficial pyoderma affect? What is the primary lesion?

• Epidermis and superficial hair follicles

• Primary lesions: follicular papules & pustules

  • ‘Folliculitis

What does pyogenic mean?

Degenerative neutrophils & phagocytosis of bacteria

What are the symptoms of superficial of pyoderma?

• Can be pruritic or non-pruritic; can exacerbate pruritus e.g., cAD

• Very variable clinical picture

What aetiological agents usually causes superficial pyoderma?

Usually, staphylococci

• S. pseudintermedius dogs and cats

• less so, S. aureus or S. schlieferi

What is being shown here?

Superifical pyoderma

• Erythema, follicular papules & pustules, crusts, epidermal collarettes, erosions & hyperpigmented macules

What are epidermal collarettes?

Rims of scale

How does superficial pyoderma present differently in different breeds?

• Moth eaten look in short coated

  

• Long coated- disheveled look (crusts and scales)

  

What is superifical pyoderma a main DDx for?

Spontaneous multifocoal alopecia

What is impetigo?

• Type of superifical pyoderma

• Non follicular pustules —> not centred around hair follicles

What is the pathogenesis of deep pyoderma?

• Epidermis

• Folliculitis (entire hair follicle) -> Furunculosis (follicle rupture) -> Dermis +/- subcutaneous fat

• Folliculitis & furunculosis

What are the clinical signs of deep pyoderma?

• Heat, swelling, erythema, furuncles, nodules, bullae, plaques, sinus tracts, ulcers, exudation and crusts

• Lesions usually haemorrhagic to haemo-purulent (as BV in dermis)

• Pain, systemically ill, fever, lymphadenopathy

how does deep pyoderma present in cytology?

• Gram-negative or atypical bacteria

  • E.g., E. coli, Proteus spp., Pseudomonas spp.

How does deep pyoderma present grossly?

Heat, swelling, erythema, furuncles, nodules, bullae, plaques, sinus tracts, ulcers, exudation and crusts

Give some examples of deep pyoderma

• Chin folliculitis and furunculosis

  

• Acral lick granuloma

  

• Pressure point deep pyoderma

  

• Post grooming furunculosis

  

Summarise the clinical presentations of pyoderma

What are these lesions showing?

What are the features of malassezia dermatitis and how does it present clinically?

• Overgrowths 2ary to underlying disease

• Erythema, greasy exudate, scaling, crusting

• When chronic: Greasy alopecia, lichenification & hyperpigmentation

• Extremely pruritic

• Atopic dogs become sensitised to malaessezia allergens

• Creates resistance to anti inflammatories

What dog breeds are predisposed to Malaessezia dermatitis?

• Basset hounds

• WHWT

• Cockers

What cat breeds are predisposed to Malassezia dermatitis?

Devon Rex

What can malaessezia dermatitis be secondary to in cats?

• Chin acne

• Idiopathic facial dermatitis of Persians

• Feline Atopic Skin Syndrome

• Thymoma-induced exfoliative dermatitis

• Paraneoplastic alopecia

How does malassezia dermatitis present grossly in cats?

Brown sebaceous discharge, follicular casting, greasiness

How are skin infections diagnosed?

Cytology

Culture and susceptibility is not diagnostic but can help confirm diagnosis can guide therapy

Name some cytology techniques

• direct impression smear

• cotton tip/swab smear

• adhesive tape strip

• fine needle aspirate

What is being shown in this histology?

LHS = cocci overgrowth

RHS = malassezia overgrowth

both examples of surface infections

What is being shown in this cytology?

Superficial pyoderma (Left)

Deep pyoderma (Right)

When should you perform culture and suscpetibility for a skin infection?

• When there are risk factors for antimicrobial resistance (prev/multiple corses of AB, poor response to empirical therapy)

• Rod shaped or unusual organisms on cytology

• Deep infections

• Degenerate neutrophils but no bacteria

• Non-healing wounds

• Post op or nosocomial infection

• Life threatening infection

How does sampling for culture differ in superifical and deep infections?

Superficial: (crust/pustule)

• Rupture and sample intact lesion if present

• Sample erosion under a crust or at the edge of a collarette

Deep: (nodular, purulent bloody discharge)

• Biopsy (fresh tissue sample)

• Rupture intact lesion if possible and use swab

• Swab into deep sinus tract- last resort