Pruritic microbial skin disease Flashcards
What are the 2 most common microbial infections causing pruritus?
- Bacterial pyoderma (surface/superficial)
- Malassezia dermatitis
When do skin commensals cause disease?
- Compromise of cutaneous defences, e.g.
- Mechanical damage to skin
- Defects in skin barrier function
- Changes to innate/acquired immunity
- Increased microbial adherence (e.g. with canine atopic dermatitis (CAD)
- Changes to skin microclimate
- Changes to skin microbiome
What skin disease does dysbiosis often occur with?
Canine atopic dermatitis (CAD)
* Decreased bacterial/fungal diversity
* Increased proportion of Staph/Malassezia
What organism is the most common cause of pyoderma in cats and dogs?
S. pseudintermedius
Where are the bacteria located in surface pyodermas compared to superficial pyodermas?
Surface - bacteria proliferate on epidermal surface
Superficial - bacteria invade epidermis
What is the most common form of superficial pyoderma in dogs? What are other common examples?
Folliculitis
- impetigo
- exfoliative superficial pyoderma
Where does canine intertrigo mostly occur?
In moist warm environment of skin folds, e.g.
* Facial and tail folds
* Vulval folds
* Intertriginous (i.e. skin-skin frictional) areas, e.g. of obese animals
What is the most common form of surface pyoderma? What are other common ones?
Intertrigo
- acute moist dermatitis (hot spots)
- bacterial overgrowth syndrome
- mucocutaneous pyoderma
Why does acute moist dermatitis occur?
- Acute lesion caused by skin self-trauma
- Triggered by any irritant (flea bite, classically)
- Very rapid development of bacterial overgrowth – can -> to pyotraumatic folliculitis if not addressed rapidly (stop pruritus, cleanse and dry skin)
Describe bacterial overgrowth syndrome lesions
greasy, malodorous, erythematous, alopecia -> hyperpigmentation, lichenification
What clinical signs can you see with bacterial folliculitis?
Papules -> pustules - primary lesions but short-lived -> often present as secondary lesions
* Crusts
* Small epidermal collarettes – peripheral scale +/- central post-inflammatory pigmentation
Loss of hair from follicle
- multifocal patchy alopecia
- vague patchy thinning of hair
- thinning of undercoat
How does exfoliative superficial pyoderma present?
- Rapidly-expanding erythematous rings with peripheral peeling (epidermal collarettes) – may coalesce -> large collarettes
- Also central alopecia +/- hyperpigmentation
- Often highly pruritic
NB no preceding pustule/papule
What are the most common presentations of pyoderma in the cat?
- Feline acne
- Surface pyoderma superimposed on EGC lesions
- Folliculitis
- Miliary dermatitis (small crusted papules)
- Larger crusts
- Alopecia with minimal inflammation - d/d dermatophytosis, demodicosis (D cati)
What diagnostic test should you always do if pyoderma is on the d/d list?
Cytology
* always! - impression smear (direct or indirect) or stained acetate tape strip
* Perform on papules/pustules/erosions, skin under crust, under epidermal collarette rim
* NB Absence of surface bacteria does not rule out pyoderma – organisms may be within hair follicle
When should you undertake culture and sensitivity testing?
- Superficial pyoderma unresponsive to initial empirical therapy
- History of repeated antibiotic use
- Previous isolation of a meticillin resistant Staphylococcus (MRS)
- Rods seen on cytology
Where and how should you take your samples for a culture and sensitivity study?
Primary lesions – best, if lesions present
* Look for papules/pustule in groin axillae esp!
* Burst pustule with sterile needle, or gently remove top of papule with edge of microscope slide/back of scalpel blade
* direct swab of exudate
Secondary lesions
- Sample under crusts (if moist beneath) or under rim of collarette, using saline-moistened swab
If no primary or secondary lesions
* Small punch tissue biopsy
* Gently blot surface with alcohol swab to remove contamination and allow to dry – do not prep in normal manner
* Submit in sterile glass tube +/- spot of sterile saline
What is the common approach to treating pyoderma?
Address infection - immediate priority!
* Surface pyoderma – always topical (chlrhexidine, fusidic acid for cocci and silver sulphadiazine for rods)
* Superficial pyoderma
* Topical alone whenever possible (chlorhexidine)
* Add systemic first tier antimicrobial drugs if severe or extensive (amoxiclav, clindamycin, TMPS, cefalexin)
* NB accurate dosing, target the specific organism, use narrow-spectrum drugs where possible
Use of antipruritic agents?
* Surface pyoderma – drugs to control pruritus (including corticosteroids) indicated
* Superficial pyoderma
* traditionally advised against concurrent use of any drug that may hamper the host’s immune response, especially corticosteroids.
* However, short course (e.g. 3-5 days) now considered acceptable in cases where underlying inflammatory process likely to be driving infection (e.g. pyoderma secondary to atopic flare).
* Deep pyoderma (identify through palpation of thickened skin) – drugs that may hamper host’s immune system (e.g. corticosteroids, oclacitinib) contraindicated
Address underlying disease - essential for long term resolution of secondary infection
How does your treatment approach change between acute moist dermatitis and that with satellite lesions?
pyotraumatic dermatitis - a surface pyoderma
Treatment usually
* Clip lesions (under sedation/GA if painful)
* Treat with topical antimicrobial – e.g. chlorhexidine, fusidic acid*
* Control pruritus – corticosteroids* usually
*(in ‘Isaderm’)
BUT if satellite lesions present, infection may be deeper (pyotraumatic folliculitis/furunculosis) so avoid corticosteroids!
What are risk factors for development of clinical MRS infections?
- Previous antimicrobial therapy
- Repeated visits to vet surgery
- Invasive procedures
How can you treat MRS infections?
Surface/superficial infections
* Topical therapy alone – as per meticillin-sensitive infections
* Avoid systemic antibiotics if at all possible (selects for more resistance!)
Deep infections
* Systemic antibiotics, using lowest EMA Category drug shown to be effective (NB never amoxyclav, even if test indicates susceptibility) – seek specialist advice if no susceptibility to authorized drugs
* Plus topical therapies
Strict hygiene measures in home and surgery are required, and owners need to be advised of zoonotic risk – advise they check with doctor as precaution. See
* Owner advice sheets MRSA & MRSP – excellent resources!
* BSAVA recommendations
Establish and address underlying cause!
What are risk factors for Malassezia dermatitis?
Anatomical features (skin folds, pendulous lips, hairy feet) -> warm lipid-rich environment
Underlying disease (especially allergies, endocrinopathies, keratinisation disorders)
* Alter barrier function/lipids/humidity of skin
* Favour adhesion of organisms and ?predominance of more virulent strains
Breed – e.g. Bassets + Devon Rex cats with high normal mucosal populations
What clinical signs are associated to Malassezia dermatitis?
- Pruritus varies - mild to severe (NB severe pruritus may be seen with minimal lesions)
- Initially erythema with greasy exudate, scale, crust -> lichenification, alopecia, hyperpigmentation
- Focal/multifocal/generalised
- Common sites: ears, lips, muzzle, i/dig skin, flexor surfaces, ventral neck/body, axilla, medial limbs, perineum!
- +/- rancid malodour
A common trigger for flares of allergic skin disease
How would you diagnose Malassezia dermatitis?
Cytology
* Stained acetate tape strip
* Direct/indirect impression smear if moist/waxy
* Peanut/snowman/footprint/Russian doll appearance – may be mixed with bacterial overgrowth
* May be clustered/adherent to keratinocytes (esp cats), so need to examine wide area
* No fixed number for significance – interpret in light of clinical signs - final diagnosis depends on response (clinical + cytological) to treatment
Culture, skin biopsy
- May identify organisms but not used routinely currently
How is Malassezia dermatitis treated? How can it be prevented?
- Reduce number of organisms
- Topical treatment very effective - often aimed at Malassezia AND bacterial pyoderma - eg Miconazole/chlorhexidine shampoo, 2-4% chlorhexidine shampoo/foam, TrizEDTA/chlorhexidine wipes
- Systemic treatment (Itraconazole / ketoconazole) if topical fails
Prevention - establish and treat the primary cause
- Often need regular treatment if primary cause cannot be fully controlled
You have diagnosed a superficial pyoderma on the ventrum of a dog. What is the treatment of choice?
* Systemic amoxicillin-clavulanate
* Systemic clindamycin
* Topical 2-4% chlorhexidine shampoo
* Topical polymyxin B
Topical 2-4% chlorhexidine shampoo
