Allergic skin disease Flashcards
Allergy definition
= a condition caused by a hypersensitivity response to allergens
- common cause of pruritus in dogs and cats
Triggers of allergic skin dz
- Environmental allergens*
- Environmental atopic dermatitis - dog*
– Feline atopic skin syndrome (FASS) – cat* - Foods*
– Food-induced atopic dermatitis (‘food allergy’)– dog*
– Feline food allergy (FFA) – cat* - Ectoparasites*
– Flea allergic dermatitis*
– Insect bite hypersensitivity
– Mite hypersensitivity – especially Sarcoptes - Contact allergens
- Micro-organisms, e.g. Malassezia hypersensitivity
- Drugs
Factors of allergic skin dz
- Environmental influences
- Immunological dysregulation
- Genetic background
Hypersensitivity involved
- Type I hypersensitivity most commonly involved
- But >1 type of hypersensitivity proposed for some conditions
- Picture is complex and incompletely understood
- But all require stages of:
– Sensitisation of individual by repeated exposure to allergen
– Subsequent exposure -> immunologically excessive/inappropriate response
-> clinical disease
Canine atopic dermatitis (CAD)
- A common chronic, relapsing, pruritic and inflammatory skin syndrome with characteristic clinical features
- Traditionally associated with IgE antibodies to environmental allergens BUT
- <30% cases have no evidence of IgE (’intrinsic atopics’) (based on serology/intradermal testing (IDT))
- <30% cases may have dietary trigger (not diagnosed by serology or IDT)
Environmental CAD pathogenesis
- Defective cutaneous barrier function
- Microbial dysbiosis/overgrowth/infection
- Hypersensitivity reaction
Common allergens associated with environmental CAD
- dust mites*
- pollen
- moulds
Hypersensivity/immunology associated with environmental CAD
- Type I hypersensitivity response is involved in most cases, but not all
- Adaptive immune response changes with chronicity
– Acute lesions: lymphocytes classically follow Th2 pathways -> IgE (Type I hypersensitivity)
– Chronic lesions: more complex pattern - include Th1, Th2 and other T-cell responses - Cells/mediators of innate immune system (including keratinocytes) play a role in initiating/maintaining inflammation
- NB skin lesions sometimes accompanied by other manifestations of atopy (atopic conjunctivitis /rhinitis)
- Allergen specific IgE binds to tissue mast cells (+basophils + in circulation + Langerhans cells). IL31 is a major player in IgE allergic response
- Keratinocytes also cause substances that cause pruritus to be released in these cases
Type I HS overview (for environmental CAD)
- Initial sensitisation -> no clinical signs
- On re-exposure to the allergen:
– degranulation of mast cells -> release of inflammatory mediators
-> keratinocytes/activated T-cells -> cytokines - All -> recruit inflammatory cells (esp neutrophils, then eosinophils) to dermis
- Inflammatory cells activated and proliferate via Janus kinase (JAK) pathways on cell surface
-> itch/inflammation -> scratch
-> epidermal hyperplasia -> worsening epidermal barrier function
With chronicity of Type I HS reaction (for environmental CAD)
From scratching, toxins from skin microbes, continued exposure to allergens:
-> activate keratinocytes and other immune cells (eg macrophages) -> polarisation to Th1 phenotype
-> Increase monocyte/macrophage recruitment and activation
-> thickening of epidermis/ stratum corneum etc…
-> worsening epidermal barrier function
Favrot proposed diagnostic criteria for CAD
- Age at onset <3 years
- Living mostly indoors
- Glucocorticoid-responsive pruritus
- Non-lesional pruritus
- Affected front feet and/or pinnae
- Unaffected ear margins
- Unaffected dorsal/lumbar area
If 5 positive criteria:
- Sensitivity 85%
- Specificity 79%
May help, but cannot be used in isolation
Other factors also involved
Environmental CAD: Signalment
Breed predisposition, varies with location.
Commonly, e.g.
- Golden/Labrador retriever
- WHWT/other terriers
- English/French bulldog, Pug, Boxer, Lhasa Apso, GSD, Shar Pei
Environmental CAD: History
- Nearly aways pruritic
– (v occasionally present as pyoderma that is 100% responsive to therapy) - Scratch, lick, rub, scoot
- Onset usually 6m-3y
- May start seasonally -> year round
Most will respond to anti-inflammatory dose of corticosteroids
Environmental CAD: Clinical signs
Affected areas commonly
- Face, ears (concave pinnae, ear canals)
- Axillae, ventral abdomen, inguinum, perineum
- Carpi/tarsi, feet
- Essentially most of the dog apart from the dorm and flank are commonly affected
Uncomplicated case
- Erythema
Self-induced alopecia, excoriations
- Primary papular eruption
With chronicity
- Lichenification, hyperpigmentation
NB Lesions of secondary infection often superimpose
CAD occasionally manifests as
- acral lick dermatitis (produce lesions by licking the same spot repeatedly)
- pyotraumatic dermatitis
Food-triggered AD: common reactions
- dogs: beef, dairy, chicken, wheat
- cats: beef, fish, chicken
Food-triggered AD (dog): age of onset
- Can develop at any age
30-50% start at <1yo - More likely than environmental CAD if onset <6mo?
- Sensitisation not associated with diet change
- affects <30% dogs with CAD
Food-triggered AD (dog): clinical signs
- Skin signs clinically indistinguishable from environmental CAD
- +/- Concurrent clinical signs, e.g.
– GI signs
– Urticaria/angioedema
– Malassezia dermatitis - Less responsive to steroid anti-inflammatories than environmental CAD?
Feline atopic syndrome (FAS): 4 feline cutaneous reaction patterns
- Face, head, neck pruritus (FHN)
- Self-induced alopecia (SIA)
- Miliary dermatitis (MD)
- Eosinophilic granuloma complex (EGC)
(eosinophilic granulomas/ulcers/plaques)
- Can show 1 or more simultaneously
Adverse food reactions:
Food allergy: - Hypersensitivity reaction
(likely Types I, III +/or IV)
- immunological reaction
Food intolerance:
- Metabolic reaction
- Pharmacologic reaction
- Idiosyncratic reaction
- Non-immunological reaction
Intoxication:
- Bacterial/ fungal/ other toxins
- Non-immunological reaction
Most common causes for feline cutaneous reaction patterns
- FASS
- FFA
- Flea allergic dermatitis (FAD)
Causes of FAS
- Feline atopic skin syndrome (FASS)
- Feline food allergy (FFA)
- Feline asthma
FASS (& FAD) clinical signs
- 4 feline cutaneous reaction patterns
FFA cutaneous clinical signs
- 4 feline cutaneous reaction patterns
- urticaria
- non-pruritic nodules
- plasma cell pododermatitis
FASS: Signalment, history, clinical signs
- Inflammatory/pruritic skin syndrome, likely associated with IgE to environmental allergens
- Usually young adult – 6mo-5y onset (occasionally older)
- Seasonal/ non-seasonal
FFA: Signalment, history, clinical signs
- Can occur at any age (3mo+) but 27% cats <1yo
- Non-seasonal
- +/- GI/conjunctivitis/respiratory signs
FAS diagnosis in most cases
- Eliminate ectoparasites and bacterial/fungal infections
2.-> Exclusion diet trial -> FFA - -> FASS
Contact hypersensitivity
- Very uncommon
- Type IV reaction: affects isolated individual
- Sensitisation usually over prolonged period, eg to:
– Plants, topical drugs/shampoos
– Chemicals, cleaning products, rubber, plastic, leather, metal etc - Lesions in areas of contact only
- Particularly affects sparsely haired regions
2 types of contact dermatitis
- Contact irritant dermatitis (due to irritant nature of substance (not hypersensitivity) – often affects >1 animal)
- Contact hypersensitivity