Allergic skin disease: pathophysiology and presentation

Question 1

Allergic skin disease is a common cause of pruritus

particularly in companion animals and it can be triggered by?

Answer 1

Ectoparasites

Environmental allergens

Foods

Contact allergens

Drugs

Therefore consider:

• Ectoparasite hypersensitivities
• Atopic dermatitis
• Cutaneous adverse food reactions
• Contact allergy
• Drug reactions

Question 2

What is the classic definition of atopy?

Answer 2

‘Inherited predisposition to develop a Type I hypersensitivity reaction to environmental allergens’

Question 3

What is pathogenesis of Canine Atopic Dermatitis (AD)?

Answer 3

• When body exposed to normal harmless foreign antigen (allergen), it should develop a tolerance to it
• But in atopic animals these tolerance mechanisms fail…
• Atopic dogs are genetically predisposed to
• Epidermal barrier function defect à allow entry of percutaneous allergens from environment
• Polarisation of lymphocytes towards Th2 subset

Question 4

What are the 3 phases of inflammation?

Answer 4

1. Acute phase response

• Th2-dominated cytokine profile
• 15-20mins after initiation
• Inflammatory mediators released from mast cells (eg histamine, serotonin, prostaglandins) –> local vasodilation, tissue oedema and influx of inflammatory cells; often pruritus seen

2. Late phase response

• Eosinophil-dominated
• Seen after 6-12 hours

3. Chronic phase

• Subsequent infiltration of lymphocytes
• Th1-dominated or unpolarised cytokine profile

Question 5

What are the clinical aspects of canine allergic dermatitis?

Answer 5

• Chronic relapsing dermatitis
• Often seasonal initially, progressing to perennial
• Onset typically young adult, 6mo-3 years
  
  • Can be as early as 4-5 months
  • Unusual after 5-6 years of age
• Glucocorticoid-responsive (at anti-inflammatory doses)
• Predisposed breeds NB geographical variation
  
  • Terriers, Retrievers, English setters, Boxers, GSDs etc
• Due to
  
  • Domestic mite allergens – most important (eg housedust mites, storage mites)
  • Epidermals (eg human epidermal allergens)
  • Pollen allergens and moulds less important in UK than USA
• Pruritus
  
  • Face, feet, inguinum, axillae, flexural surfaces of limbs, pinnae
  • NB face rubbing and foot licking often misinterpreted by owners
• Otitis externa
  
  • May be sole presenting sign in up to 20%
• Erythema, papules, wheals, alopecia, lichenification, hyperpigmentation
• Secondary pyoderma/Malassezia dermatitis, often recurrent episodes

Question 6

What are Favrot’s diagnostic criteria for canine AD?

Answer 6

• Age at onset <3 years
• Living mostly indoors
• Glucocorticoid-responsive pruritus
• Chronic or recurrent yeast infections
• Affected front feet
• Affected ear pinnae
• Non-affected ear margins
• Non-affected dorsal lumbosacral area

If 5 positive criteria…

• Sensitivity 85% and Specificity 79.1% for atopic dermatitis
• ie false positive diagnosis in every 5th dog…!

If 6 positive criteria…

• Sensitivity 58%, Specificity 89%

Question 7

What is the allergic threshold principle?

Answer 7

• Atopic dermatitis may co-exist with other allergic diseases
• Secondary infections common
• Pruritus from concurrent conditions may “add up”
• Individuals have pruritic threshold

Below –> no clinical signs

Above –> clinical signs evident

• Important to address all compounding concurrent or secondary pruritic diseases

Question 8

Discuss feline atopic dermatitis?

Answer 8

• Still not well defined disease in cats, though now have evidence for existence of IgE
• Chronic, relapsing disease, may be seasonal or perennial
• Most present at young age (6m -3y). Genetic predilection recognised by some reports, esp purebreds?
• Sometimes referred to as ‘non-flea non-food induced hypersensitivity dermatitis’ (NFNFIHD)

Question 9

What are Favrot’s criteria for the diagnosis of non-flea hypersensitivity dermatitis (NFHD)?

Answer 9

• Presence of at least 2 body sites affected
• Presence of at least 2 of the following clinical patterns:
  
  • Symmetrical alopecia
  • Miliary dermatitis
  • Eosinophilic dermatitis
  • Head and neck erosions/ulcerations
  • Presence of symmetrical alopecia
  • Presence of any lesion on lips
  • Presence of erosions/ulcerations on chin or neck
  • Absence of lesions on rump
  • Absence of non-symmetrical lesions on rump/tail
  • Absence of nodule or tumours
• If 5 positive criteria: sensitivity 75%, specificity 76% for diagnosis of NFHD
• NB feet rarely involved, cf dog

Question 10

Cats with a hypersensitivity dermatitis usually present with one or more of the following cutaneous reaction patterns (ie a commonly recognised clinical presentation with multiple underlying causes). Outline them?

Answer 10

1. Head and neck excoriations
2. Symmetrical self-induced alopecia
3. Miliary dermatitis (multiple small papulocrustous lesions)
4. Eosinophilic granuloma complex (eosinophilic granulomas/ulcers/plaques)

Question 11

What is equine atopy?

Answer 11

• Equine IgE isolated and characterised
• Allergic respiratory and cutaneous disease recognised for
  
  • Culicoides /insect bite hypersensitivity
  • Atopic dermatitis

Question 12

What are the clinical signs of equine allergic dermatitis?

Answer 12

• Pruritus and/or Urticaria
• Tail flicking, stamping, rubbing (head-shaking)
• Initially no dermatological lesions

–> Self-inflicted damage

• excoriations, alopecia, lichenification, thickening, hyperpigmentation, secondary infection

Many manifestations

• Urticarial wheals, rugae, angio-oedema
• +/- pruritus
• Tufted papules: may coalesce –> crust –> focal alopecia
• Histopathology: eosinophilic mural folliculitis
• Nodules
• Histopathology: eosinophilic granulomata
• Rarely concurrent respiratory disease
• COPD, allergic rhinitis/conjunctivitis

Question 13

What are the allergens implicated in Equine AD?

Answer 13

• Dust mites, forage mites >90%
• Dust extracts (grain mill, house), epidermals, feathers
• Mould spores
• Tree, grass weed pollens
• Concurrent insect-bite hypersensitivity

Question 14

What are Cutaneous Adverse Food Reactions (CAFR)?

Answer 14

Inappropriate reactions to elements of diet

Non-immunological mechanisms

• Food intolerance

Immunological mechanisms

• Dietary hypersensitivity
• Underlying immunopathology poorly defined; hypersensitivity mechanisms implicated:
• Type I – IgE antibodies demonstrable
• Type III
• Type IV

Recognised in both dog and cat

• Uncommon in dog
• May be as common as atopic dermatitis in cats !!

Question 15

What is the Cutaneous Adverse Food Reaction (CAFR) presentation in the dog?

Answer 15

• Any age of onset - Including young and older animals (cf atopic dermatitis..)
• Otherwise indistinguishable clinically from atopic dermatitis to environmental allergens
• Pruritus
• Erythema, wheals, ulceration, crusting
• Recurrent pyoderma
• Non-seasonal, perennial
• Generalised or localised (including unilateral otitis externa)

Question 16

What is the cutaneous adverse food reaction presentation for the cat?

Answer 16

Can present as:

• Pruritus – especially head and neck?
• Miliary dermatitis
• Symmetrical alopecia/overgrooming
• Disease of the Eosinophilic Granuloma Complex

Question 17

CAFR presentation in horses?

Answer 17

• RARE!
• Foods often suspected by owners
• “protein bumps”
• But not supported by elimination diets and provocative challenge
• Possible effect of plane of nutrition on threshold of pruritus/urticaria due to other causes (hypothesis only!)
• Only one convincing case in the literature
• presented with Telogen Defluxion, not pruritus

Question 18

Just have a look at this!

Answer 18