Small Animal Dermatology, 3rd edition

chapter 39 Photodermatoses.....................................

DEFINITION/OVERVIEW

 Electromagnetic radiation, primarily ultraviolet light (UVL), is absorbed by and

directly damages keratinocytes, and alters the epithelial environment.

 Nonpigmented and/or glabrous areas of both dogs and cats are most affected.

 Photodermatoses include solar dermatitis, actinic keratoses (AK), actinic comedones

and furunculosis, hemangioma (HA), hemangiosarcoma (HSA), and squamous cell

carcinoma (SCC).

ETIOLOGY/PATHOPHYSIOLOGY

 Both UVA and UVB cause photodermatitis by:
Direct phototoxicity (sunburn)
Alteration of cell markers (seen with discoid lupus erythematosus and pemphi-

gus erythematosus)

Damage by photoactive compounds (photosensitivity)

Cellular hyperproliferation and mutagenesis (actinic keratosis and solar-induced

neoplasia).

 Natural barriers to UVL (e.g., melanin) are overcome by chronic and prolonged expo-

sure to sunlight.

 UVL causes DNA damage directly and indirectly by free radicals; specific UVL-

induced mutations have been documented in the tumor suppressor genep53, leading

to expansion of mutated keratinocytes.

 Patients often develop a spectrum of UVL-caused disorders concurrently, includ-

ing nonneoplastic (actinic comedones and furunculosis), preneoplastic (actinic ker-

atoses), and neoplastic (hemangioma, hemangiosarcoma, and squamous cell carci-

noma).

SIGNALMENT/HISTORY

 Most affected dogs and cats are known sunbathers; dogs may preferentially expose

one side when sunbathing, resulting in asymmetric lesions.

Blackwell’s Five-Minute Veterinary Consult Clinical Companion: Small Animal Dermatology, Third Edition.
Karen Helton Rhodes and Alexander H. Werner.
©2018 John Wiley & Sons, Inc. Published 2018 by John Wiley & Sons, Inc.

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