Atopic Dermatitis


Table 46-1 PHARMACOLOGIC AGENTS USED FOR THE SYMPTOMATIC CONTROL OF CLINICAL SIGNS OF ATOPIC DERMATITIS IN DOGS

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Chapter 46 Atopic Dermatitis



Andrew Hillier


Atopic dermatitis (AD) is a genetically predisposed inflammatory and pruritic allergic skin disease with characteristic clinical features. It is associated most commonly with immunoglobulin E (IgE) antibodies to environmental allergens: dust mites (house dust and storage mites), pollens (trees, grasses, and weeds), mold spores, danders, insects (moth, cockroach, etc.), and other miscellaneous allergens.


In dogs, this results in skin disease in most cases, with occasional concurrent involvement of other organ systems. Cats generally develop dermatitis as well, but they may also develop asthma-like respiratory tract disease in association with IgE antibodies to environmental allergens. However, the link between feline asthma and AD is uncertain.



ETIOLOGY


AD is a classic example of an IgE-mediated, type I hypersensitivity reaction. Patients may have hypersensitivity to one, a few, or many allergens. AD is a complex skin disease, and a state of “cutaneous hyperreactivity” often exists. The following factors may contribute to clinical disease:


• Environmental allergens

• Staphylococcal pyoderma

• Malassezia (yeast) dermatitis

• Concurrent flea allergy or food allergy

• Irritants such as chemicals, plants, and medications

• Temperature and humidity

• Local skin factors such as dry skin, excoriations, etc.

Recognition of factors contributing to disease in each individual patient is critical in the development of successful treatment strategies. As there is a summation of effects (added effects of multiple stimuli), elimination of some of the most significant stimuli may reduce the skin inflammation below the patient’s pruritic threshold (the point at which pruritic stimuli induce skin disease and pruritus) without a need to address all factors that may be contributing to the pruritus.



Incidence in Dogs


AD is a common skin disease that affects 10% to 15% of the dog population. Data from 52 general practices in the United States documented that AD was diagnosed in 8.7% of all canine patients and in 21.6% of all dogs presented with skin or ear disease. In regions where fleas are not a significant problem, AD is likely the most common allergic disease of dogs and cats.



Age and Sex Predilection



• Age at onset: Clinical signs are first seen between the ages of 6 months and 3 years in the majority of dogs. AD will rarely develop for the first time once animals are over 7 years of age.

• Sex predilection: None known.


Breed Predilection


AD can affect dogs of any breed as well as mixed-breed dogs.


• High-risk breeds: Beauceron, Boston terrier, boxer, Cairn terrier, Chinese Shar-Pei, cocker spaniel, Dalmatian, English bulldog, English setter, fox terrier, golden retriever, Irish setter, Labrador retriever, Labrit, Lhasa apso, miniature schnauzer, pug, Scottish terrier, Sealyham terrier, West Highland white terrier, wirehaired fox terrier, and Yorkshire terrier.

• Low-risk breeds: Dachshund, Doberman pinscher, German shepherd, German short-haired pointer, and poodle.


Seasonal Incidence


Typically, AD is seasonal in nature initially, but the majority of dogs progress over a period of years to develop non-seasonal (year-round) disease. However, many dogs with non-seasonal AD have seasonal exacerbations due to pollen allergies or concurrent flea allergy dermatitis.



Incidence in Cats


There is no reported sex or breed predilection, although Abyssinian cats were suggested in one review to have a higher incidence of AD. The age of onset of clinical signs is similar to dogs.



CLINICAL SIGNS


AD should be considered a differential diagnosis in any dog with the following:


• Pruritus

• Recurrent staphylococcal or Malassezia dermatitis

• Otitis externa


Pruritus


Pruritus is the initial and most outstanding clinical sign of AD in dogs and cats. Pruritus is mild to moderate in most cases but may become severe in some chronically affected dogs. It is responsive to glucocorticoid therapy, at least initially.


• The pruritus in AD typically involves one or more of the following body locations:
• Face—Especially periocular, muzzle and chin

• Ears—Most commonly the ear canal but sometimes also the ear pinna and skin at the base of the ear

• Ventrum—Including the neck, axilla, abdomen, and groin

• Distal limbs—Including the carpal and tarsal area, digits, and interdigitally

• Other locations sometimes affected include the perineum and the flexural surface of the elbows and hocks.

• Pruritus may become generalized with chronicity or when a secondary infection is present.


Staphylococcal and Malassezia Dermatitis


AD in dogs is commonly complicated by recurrent staphylococcal pyoderma (see Chapter 38) and Malassezia yeast dermatitis (see Chapter 41).


• Both infections contribute to the skin disease and pruritus. Once the infections are treated and resolved, pruritus is usually diminished, sometimes significantly. However, residual pruritus typically remains—a strong clue to a primary underlying allergic dermatitis. Rarely, treatment of the secondary infection leads to resolution of the pruritus as well—these patients only have pruritus when the infection flares and is active.

• Staphylococcal infections tend to involve the trunk, while Malassezia infections are more frequently found in the ventral neck, interdigital area, and nail folds. Either infection may become generalized.


Otitis Externa


Otitis externa is common in dogs with AD—one study reported that 86% of cases had otitis. One or both ears may be affected.


• Early cases have mild erythema of the ear pinna and ear canals. As the disease progresses, hyperplasia of the epithelium and ceruminous exudation usually develop.

• Most frequently, allergic otitis becomes complicated by secondary bacterial and yeast infections (see Chapter 59).


Skin Lesions


There is currently debate about the skin lesions that typify AD (assuming that secondary infections are not present).


• Some dogs with AD have no primary lesions, while others may have a macular to papular dermatitis (i.e., a rash).

• Chronic self-trauma as a result of the allergic inflammation eventually leads to secondary lesions such as alopecia, lichenification, hyperpigmentation, and seborrhea. The most important aspect of these lesions is whether their location on the body is typical for AD. Similar secondary lesions are seen in other pruritic but non-allergic dermatoses; however, the lesions are not in the locations on the body that are associated with AD.


Clinical Signs in Cats


Pruritus is the hallmark of the disease in cats as well. However, the other characteristics of AD in cats do not mimic the disease in dogs very closely.


• Pruritus due to AD in cats may manifest as symmetric alopecia, miliary dermatitis, or one of the eosinophilic granuloma complex lesions. These cutaneous reaction patterns may also be observed in cats with non-atopic pruritic skin disease as well (see Chapters 52 and 53 for differential diagnoses).

• Secondary bacterial or yeast infections and otitis are less common than in dogs but may be present in some atopic cats and must be considered part of the diagnostic workup.


DIAGNOSIS


The diagnosis of AD in dogs and cats is based on the following:


• Typical historical features

• Presence of characteristic clinical signs

• Exclusion of other differential diagnoses


Differential Diagnoses


The following are the most significant differential diagnoses that need to be considered.



Seasonal Disease



• Flea allergy dermatitis

• Insect bite hypersensitivity


Non-seasonal Disease



• Food allergy

• Scabies

• Contact dermatitis


Diagnostic Approach


Use the following three-step approach in patients with suspected AD.



Step 1



• Diagnose and treat secondary infections.

• Diagnose and treat concurrent flea allergy.

• Diagnose and treat scabies.


Step 2



• Confirm that secondary infections, flea allergy, and scabies have been ruled out.

• Determine that residual pruritus is still present.

• If pruritus is seasonal and flea allergy has been ruled out, the patient has AD.

• If pruritus is non-seasonal, perform a food trial to rule out food allergy (see Chapter 47).


Step 3



• If pruritus resolves with a food trial, the likely diagnosis is food allergy, and it is confirmed when pruritus recurs with a challenge trial using the former diet.

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Aug 27, 2016 | Posted by in SMALL ANIMAL | Comments Off on Atopic Dermatitis
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