Allergic Skin Disease Flashcards
What is an allergy?
Condition characterised by a hypersensitivity response to allergens
* Common cause of pruritus in small animals
Common triggers of allergy
Environmental allergens
Foods
Ectoparasites
Contact allergens
Micro-organisms (malassezia)
Drugs
Environmental allergen conditions
Environmental atopic dermatitis - dog
Feline atopic skin syndrome (FASS)
Equine atopic dermatitis
Food allergen conditions
Food-induced atopic dermatitis (FIAD) – dog
Feline food allergy (FFA) - cat
Equine food allergy
Ectoparasite allergen conditions
Flea allergic dermatitis
Insect bite hypersensitivity
Mite hypersensitivity
What is a hypersensitivity reaction?
Exaggerated immune system response to an antigen
Type I
Immediate
* Cross-linking of IgE molecules on mast cell surface by allergen
* Degranulation of mast cells
* Tissue inflammation
Type II
Antibody mediated cytotoxicity
* Destruction of target cell by effects of antibody and complement pathway
Type III
Imune-complex
* Antibody-antigen complexes
* Form locally in tissue when antibody in excess
* Circulate systemically if antigen in excess
Type IV
Delayed
* Cell mediated
* Sensitised mononuclear inflammatory cells release proinflammatory cytokines
What type of hypersensitivity is most common in allergic disease?
Type I - IgE
Causes mast cell degranulation
What is required fro allergic reaction?
Sensitisation of individual by repeated exposure to allergen
Subsequent exposure
* Immunologically excessive response
* Clinical disease
Environmental atopy dermatitis
Type I hypersensitivity involved in most cases
Acute lesions
* Lymphocytes classically follow Th2 pathway
* Produce IgE
Chronic lesions
* More complex pattern with Th1, Th2 and other T-cell responses
Causes of environmental atopic dermatitis
Aberrations in skin barrier
Dysregulation of microbiome
Mechanism of Sensitisation Phase
- Allergen crosses skin barrier
- Picked up by Langerhans cell
- Presented to T-helper cell
- Immunological response directed down Th2 route
- B cell produce IgE which is allergen specific
- Binds to mast cell ready for next exposure
Mechanism of Provocation Phase
- Allergen crosses skin barrier
- Picked up by Langerhans cell
- Triggers T-helper cell down Th2 route
- Tiggers production of IgE and cytokines
- Cytokine IL31 causes pruritus
- Target for allergy therapy
- Allergen can also be met by IgE on mast cell
- Causes degranulation and release of inflammatory mediators
cytokine IL31
Causes pruritus
Target for allergy therapy
Clinical features of environmental atopic dermatitis
Very common
6mo-3yrs
Highly heritable
Chronic relapsing pruritus dermatitis - seasonal
Usually responsive to glucocorticoids
Clinical signs of environmental atopic dermatitis
○ Initially erythema/papules
Self-induced alopecia/excoriations
Chronic changes
○ Affects esp ears/face, feet, carpi/tarsi, axillae/inguinum, ventrum, perineum
Food-induced atopic dermatitis
Immunological reaction to dietary allergens
Pathogenesis is relatively unknown
Can occur at any age
Response to glucocorticoids is variable
Non-seasonal
Feline Atopic Syndromes
Feline atopic skin disease (FASS)
Feline food allergy (FFA)
Feline atopic skin disease (FASS)
Equivalent of environmental atopic dermatitis in dog
* Inflammatory/pruritic skin syndrome
* Likely associated with IgE to environmental allergens
* Usually young adult – 6mo-5y onset (occasionally older)
* Seasonal/ non-seasonal
Feline food allergy (FFA)
Equivalent of food-induced atopic dermatitis in dog
* Can occur at any age (3mo+) but 27% cats <1yo
* Non-seasonal
* +/- GI/conjunctivitis/respiratory signs
Cutaneous Reaction Patterns of cats
- Face, head, neck prurutius (FHN)
- Self-induced alopecia (SIA)
Symmetrical - Miliary dermatitis (MD)
- Eosinophilic granuloma couples (EGC)
Equine atopic dermatitis
Horses suffer from a range of environmental allergens
* Type I hypersensitivity
* Less defined than in dogs/cats
Seasonal/non-seaosnal
Age of onset 1.5-6yrs
Often co-exist with insect-bite hypersensitivity
Equine atopic dermatitis clinical signs
Pruritus
Urticaria - Oedematous wheals
Sometimes nodules (eosinophilic granulomas)
Lesion distribution poorly-defined
Generalised/localised lesions
Ectoparasite allergies
Flea allergic dermatitis
Insect bite hypersensitivity
Mite hypersensitivity
Flea allergic dermatitis
Common in dogs/cats
Type I & IV hypersensitivity to flea salivary antigens
Dogs
* Affects dorsal/caudal region of body
* Pruritus, self-trauma, chronic changes
Cats
* Any of the 4 cutaneous patterns
Insect bite hypersensitivity
- Culicoides in horses - ‘Sweet itch’
- Culicoides in sheep - affects non-wooled areas
- Mosquitoes in cats - affects face and pads
‘Sweet itch’
Culicoides spp. hypersensitivity in horses
* Effects mane and tail
* Type I & IV hypersensitivity to midge salivary proteins
Mite Hypersensitivity
- In affected individuals -> increase pruritus from infestation
- Potential reason for persistence of pruritus after parasite killed
- E.g. Psoroptes (sheep), Sarcoptes (dog)
Contact dermatitis
Uncommon
Type IV reaction
Sensitisation over a prolonged period of time
○ Plants, topical drugs/shampoos
○ Chemicals, cleaning products, rubber, plastic, leather, metal etc
Lesions only occur in contact areas
Drug reactions
Any type of hypersensitivity
Variable pruritus
Common in antibiotics
* Potentiated sulphonamides
