Canine atopic dermatitis

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Atopic dermatitis

Atopic dermatitis

Definition

Atopy is a genetic predisposition to develop hypersensitivity to environmental allergens. This can manifest itself in a number of different ways, for example atopic dermatitis, otitis, conjunctivitis or asthma. Atopic dermatitis is a genetically predisposed inflammatory and pruritic skin disease, most commonly resulting from hypersensitivity to environmental allergens.

Cause

Atopic dermatitis is one of the most common skin diseases of dogs, affecting 10 to 15% of the canine population.

Typical allergens causing atopic dermatitis include house dust mites, pollens, mould spores and occasionally foods. Many dogs have multiple allergies and may even suffer from a combination of atopic dermatitis and other hypersensitivities, most notably flea allergy.

Each dog may have an allergic threshold. When the level of allergen exposure is below this threshold, there are no clinical signs. Once this threshold has been exceeded, pruritus develops. Any additional factors that cause skin inflammation, such as the presence of fleas or a microbial infection, may contribute to the development of clinical signs. These are known as ‘flare factors’.

Signalment and history

Any breed of dog may be affected but some are more prone than others. The breed incidence varies geographically as genetic factors appear to contribute to the susceptibility of certain breeds to develop atopic dermatitis.

Some of the breeds most commonly reported to be affected by atopic dermatitis include:

  • Terriers, especially West Highland White Terriers and Staffordshire Bull Terriers
  • Boxers
  • Labrador Retrievers, Golden Retrievers
  • German Shepherds

Signs of atopic dermatitis usually first appear between 6 months and 3 years of age. Dogs may present initially with acute episodes of pruritus and erythema, but signs rapidly become chronic.

Initially the disease may be seasonal, but usually this progresses to non-seasonal disease, often with seasonal exacerbations. The clinical signs often increase and decrease in severity, depending on the level of exposure to allergens and the presence of secondary infections or concurrent allergic diseases.

Clinical signs

Atopic dermatitis causes pruritus. Initially there may be no visible lesions or just erythema. Many of the signs that develop are due to secondary damage caused by self-trauma. Other lesions such as pustules, crusted papules and epidermal collarettes may develop due to secondary infections.

The degree of pruritus is typically moderate (for example rated 5–7 out of 10). Secondary infections often increase the degree of pruritus and may make the pruritus poorly responsive to treatment for atopic dermatitis.

Typically lesions start at friction areas and skin folds:

  • Face – around the mouth and eyes, inside the pinnae
  • Feet
  • Axillae
  • Groin and perineum
  • Flexural surfaces (front of elbows, front of hocks)

Some dogs present with only recurrent otitis externa.

In chronic cases the skin develops alopecia, excoriation, lichenification and hyperpigmentation.

Diagnosis

There is no specific diagnostic test for atopic dermatitis. It is a clinical diagnosis based on compatible clinical signs and exclusion of other conditions that could cause similar findings.

Differentials

Any other cause of pruritus including:

  • Parasites (for example fleas, mange)
  • Bacterial or yeast infection of the skin

A logical stepwise approach is required and owners should be made aware that it may take weeks to reach a confirmed diagnosis.

Diagnostic approach

  1. Compatible history and clinical signs (for example age of onset, lesion distribution).
  2. Rule out/treat parasites
    • Comprehensive flea control
    • Skin scrapes, coat brushings +/- trichograms to look for fleas and mites
    • Serology and/or trial therapy may also be necessary to rule out sarcoptic mange
  3. Rule out/treat infections
    • Cytology (tape strips/impression smears/swabs) to identify yeast (Malassezia) and bacterial infections
    • If pruritus persists once parasites and infections have been excluded, then atopic dermatitis is likely
  4. Consider attempting to identify the offending allergens
    • Elimination food trial in dogs with non-seasonal signs (10-20% of patients may have food allergy)
    • Intradermal skin testing or serology if allergen-specific immunotherapy is planned

Treatment and management

There are many treatment options for atopic dermatitis and frequently a combination of therapies is required to manage the disease, particularly to control flare factors.

Factors to consider when choosing treatment include:

  • Severity of pruritus
  • Seasonality
  • Compliance issues (for example bathing large/aggressive dogs, cost of medication)
  • Potential side effects

Treatment objectives

  • Reduce allergen challenge if possible (for example with low dust mite bedding, regular shampooing, dietary modification)
  • Control secondary flare factors by rigorous flea control, regularly carrying out cytology to check for bacterial/yeast infections and treating if present
  • Improve the skin barrier function if it is altered (for example by dietary essential fatty acids, topical therapy)
  • Control the allergic reaction to reduce inflammation and pruritus for example with glucocorticoids, antihistamines, atopica (cyclosporine)
  • Prevent future flares: consider long-term anti-inflammatory medication, for example atopica, and/or immunotherapy (requires identification of allergens)

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