Atopic Dermatitis, a Common Skin Disease of West Highland White Terriers

Editor's note; This is part one of Dr. Olivry's aricle. The conclusion will be printed in our fall issue

Atopic Dermatitis, a Common Skin Disease of West Highland White Terriers
Dr. Thierry Olivry DrVet, PHD, DipACVD, DipECVD

Summer 2004, NEWS

DEFINITION

Atopic dermatitis (AD) is an itchy chronic skin disease that can affect humans and animals such as dogs. A task force recently defined canine AD as "a genetically-predisposed inflammatory and pruritic (itchy) allergic skin disease with characteristic clinical features that is associated most commonly with IgE antibodies to environmental allergens."

EPIDEMIOLOGY

The prevalence of atopic diseases in humans (asthma, allergic rhinitis/conjunctivitis and AD is estimated to exceed 30% in developed countries, with AD affecting greater than 10% of children.

Unfortunately, to date, there are no reliable epidemiological data of the true incidence and prevalence of AD in the general canine population. Some insights into the prevalence of canine AD can be gained from the following published data:

In private general practices in the US, 8.7% of dogs presented to veterinarians are diagnosed with atopic/allergic dermatitis.
In private general practices in the US, 21.6% of dogs presented to veterinarians for skin and ear disease have atopic/allergic dermatitis.
In university dermatology referral practice, between 3.3% and 12.7% of dogs have AD
In private dermatology referral practice, as many as 30% of dogs could have AD.

Whatever the true prevalence of AD is in the general canine population, there is a variable frequency of the disease among dog breeds. Such predisposition varies depending upon the geographical location, and it has changed over time for some breeds. In the USA and Europe, one of the breeds most commonly reported for AD development is the West Highland White Terrier (WHWT). Indeed, in a recent survey, AD was estimated to affect 15.5% (363/2,343) of WHWT dogs surveyed by the American Westie Foundation. This disease was the one with highest prevalence in this breed, and it was the number one source of concerns among WHWT breeders.

The heritability of AD in dogs is suggested from limited mating experiments and a longitudinal study of WHWT puppies. In this report, 154 puppies from 33 litters were followed for three years. At that end point, 75 of 114 WHWT dogs (66%) had skin diseases, 28 (25%) were diagnosed with AD, and more than half of 33 litters encompassed at least one atopic dog. Unfortunately, a clear pattern of heritability could not be established in this study. Altogether, these reports suggest that AD is a heritable allergic skin disease commonly seen in dogs, and especially in WHWT.

CLINICAL SIGNS

In the dog, the clinical signs of AD are remarkably similar to those seen in the human disease. The typical age of onset of canine AD is reported to be between six months (before puberty) and three years of age (young adult). Occasionally, AD is diagnosed in puppies as well. A sex predisposition is not apparent. Clinical signs may be seasonal or perennial (all year long) with or without seasonal exacerbation, depending upon the nature of flare factors.

A dog with AD will suffer from pruritus (itching) of the face, ears, paws, extremities, axillae (armpits) and groin. Any one, any combination, or all of these areas can be affected. Generalized pruritus is reported in approximately one half of the dogs.

There is disagreement in the literature as to whether primary skin lesions are seen in canine AD. The consensus appears that, in some atopic dogs, there are no visible primary lesions, even in pruritic areas. When present, primary lesions of AD likely consist of erythematous (red) macules (spots) and papules (small bumps).

Skin lesions usually seen in dogs with AD are of secondary origin. These lesions reflect chronic pruritus and self-trauma, and chronic skin inflammation, with or without concurrent secondary infections or microbial overgrowth. Secondary lesions reported in textbooks include red-brown "salivary" staining, excoriations (scratch marks), self-induced alopecia (hair loss), dry lusterless hair, hyperpigmentation (dark skin), lichenification (thick skin) and scaling (dandruff). These lesions are observed principally at sites of pruritus (see above), such as the face (muzzle, periocular skin), ear pinnae, dorsal and ventral aspect of the paws, flexural aspects of joints on the extremities, axillae, abdomen, groin and medial thighs. Otitis externa is seen commonly.

A good mnemonic aid for the localization of canine AD skin lesions is the following formula: AD in dogs = 5F (flexure + friction + folds + face + feet) The common occurrence of secondary bacterial and/or yeast skin infections can add papules (bumps), pustules (pimples) and crusts, scaling and seborrhea to the cutaneous lesions. Dogs with AD can be affected with other atopic diseases, such as conjunctivitis, rhinitis (hay fever) and more rarely asthma. Thus, clinical signs of these diseases can be present (lacrymation, conjunctival congestion (red eyes), sneezing, serous nasal discharge (nose dripping). When concurrent food allergies are present, vomiting and diarrhea can be observed. DIAGNOSIS In the mid-1980's clinical criteria for canine AD were transposed from those proposed for the human disease. Unfortunately, the validity (sensitivity, specificity, accuracy) of these criteria was not tested nor proven. More recently, Prélaud and colleagues examined clinical signs exhibited by dogs with and without AD. Statistical analyses suggested that the presence of three of the criteria below would yield 80% sensitivity and specificity for canine AD. Even though these criteria could be useful in orienting a clinician toward the diagnosis of AD, there is ample possibility for misdiagnosing AD in one of five patients (i.e. 20%). Prélaud's Major Criteria for Canine AD (Prélaud, 1998): A canine patient must exhibit at least three out of five criteria: " Corticosteroid-sensitive pruritus " Erythema of pinnae " Bilateral cranial erythematous pododermatitis " Cheilitis " Appearance of first signs between the ages of six months and three years After confirming that a dog's clinical signs are consistent with those of AD, resembling pruritic skin diseases must be ruled-out. Such diseases are usually of parasitic, allergic and infectious origins. One of the most important considerations is the elimination of the diagnosis of scabies (sarcoptic mange). Indeed, this disease also is associated with severe pruritus, and the distribution and nature of skin lesions mirror that of AD. Other ectoparasitic diseases, such as chiggers (trombilulidiasis) also resemble AD. Traditionally, allergic skin diseases such as flea allergy dermatitis (FAD) and cutaneous adverse food reactions (CAFR) must be eliminated before the diagnosis of AD can be made. This work-up, however, does not consider the possibility that multiple allergies could coexist in a single patient. For example, an evidence-based analysis of published data supports the hypothesis that atopy predisposes dogs to the development of hypersensitivity to flea salivary antigens. Eventually, such flea-hypersensitive dogs can manifest signs of FAD upon challenge. Lesions of FAD thus can be superimposed to the symptomatology of AD. A strong flea control program therefore must be in place when evaluating a dog with AD. The relationship between canine atopy, AD and CAFR remains the subject of debate. Whereas there is little doubt that dogs can exhibit CAFR without being atopic (for example food-induced urticaria), there is good evidence that both AD and CAFR can coexist in the same patient. Data from naturally occurring colonies of dogs with atopy (for example, maltese-beagle crossbred dogs at North Carolina State University) suggest that atopy can predispose dogs to IgE-mediated food hypersensitivity. Challenge with food allergens results in digestive signs and pruritus, and oftentimes skin lesions of AD. The recognition of resembling or concurrent CAFR, and their elimination with appropriate dietary measures, thus are indispensable in dogs with AD. Dogs with AD commonly exhibit skin infections, either with bacteria or yeast. Some of these infections are believed to contribute to the exacerbation of cutaneous inflammation and clinical signs of AD. These signs could occur in addition to those of the infectious process sensu stricto. The identification of skin infections, and their treatment, thus is warranted for patients with AD. The Place of "Allergy Testing" in the Diagnosis of Atopic Dermatitis Historically, veterinarians have relied on intradermal and IgE serum tests to demonstrate the presence of hypersensitivity to environmental allergens and confirm the diagnosis of AD in dogs. Unfortunately, these tests DO NOT confirm allergies, but merely reveal hypersensitivities to the allergens. Additionally, because of the documentation of common occurrence of such hypersensitivities, albeit not clinically relevant, in normal non-allergic dogs, the ACVD task force on canine AD recommended that this practice be altered. It is believed that usage of intradermal or serological tests should be restricted to the following situations: ä if there are historical grounds to suspect that specific environmental allergens are exacerbating the clinical signs, and avoidance measures would be considered upon confirmation of relevant hypersensitivities ä if owners are ready to afford the time, expense and efforts of allergen-specific immunotherapy regimens, once relevant allergens have been documented. Dr. Olivry will address treatment and conclusions in the next issue.