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Allergic Skin Diseases

Patrick Hensel, Dr. Med.Vet., DACVD

Tierdermatologie, Basel, Switzerland

This chapter includes common allergic skin diseases of History and Clinical Signs
dogs and cats with the exception of atopic dermatitis and Urticaria and angioedema typically have an acute onset
cutaneous adverse food reactions, which are covered in (minutes to a few hours). Urticaria is characterized by
Chapters 159 and 161, respectively.
erythematous wheals which are small (1–5 cm in diame-
ter) and present in a multifocal to coalescing pattern
Urticaria and Angioedema (Figure 160.1). They are transient and normally fade
within 24 hours but new lesions form as long as the ani-
Etiology/Pathophysiology mal remains exposed to the triggering factor. Angioedema
presents as a locally extensive and ill‐defined swelling of
Immunologic and nonimmunologic mechanisms are the face and ventral neck due to edema in the deep
suspected to be involved in the pathogenesis of urticaria dermis and subcutaneous tissue (Figure 160.2). In severe
and angioedema, causing degranulation of mast cells or cases, serum may leak through the skin. Very rarely and
basophils which results in vascular dilation and recruit- only in severe cases, fever, lethargy, and anorexia may be
ment of inflammatory cells. The inflammation remains present. Pruritus may or may not accompany urticaria
limited to the dermis in patients with urticaria, but it and angioedema.
extends to the subcutis with angioedema. While the
immunologic process is mediated by a predominantly
type I or III hypersensitivity reaction (e.g., allergies), Diagnosis
nonimmunologic causes (e.g., heat, cold, stress, chemi- The diagnosis is based on the patient’s history, clinical
cals) can also trigger mast cell degranulation (Table 160.1). presentation, behavior of the skin lesions, diascopy
A hypersensitivity reaction to an offending substance (blanching) and, if necessary, histopathology. Identifying
may not result in clinical signs at the first exposure, but any triggering factor should be a priority. For this reason,
will more likely occur with repeated exposures. Repeated a thorough patient history, including vaccination and any
exposures may also potentially result in more severe drug exposure, should be recorded. Pyoderma and vascu-
reactions. litis are differential diagnoses consistent with urticaria,
but the lesions of those diseases will not blanch on
Epidemiology diascopy. Mast cell tumor, cutaneous lymphoma, and
amyloidosis should also be considered as differential diag-
Urticaria and angioedema are rare in cats and uncom- noses for urticarial lesions. Differential diagnoses consist-
mon in dogs and have been associated with various ent with angioedema are cutaneous lymphoma, juvenile
triggering factors as described above. cellulitis, mast cell tumor, and infectious cellulitis.

Signalment
Therapy
Urticaria and angioedema can occur at any age and affect The triggering cause should be identified and eliminated
male and female animals equally. A specific breed to evade symptomatic therapy and prevent recurrences;
predisposition has not been established.

Clinical Small Animal Internal Medicine Volume II, First Edition. Edited by David S. Bruyette.
© 2020 John Wiley & Sons, Inc. Published 2020 by John Wiley & Sons, Inc.
Companion website: www.wiley.com/go/bruyette/clinical