CHAPTER 29 KERATINIZATION (CORNIFICATION) DISORDERS 431
Increased production (epidermopoiesis), increased or decreased desquamation,
and/or decrease in the cohesion of keratinocytes results in abnormal shedding of epi-
dermal cells individually (fine scale) or in sheets (coarse scale).Normal epidermal turnover in the dog is approximately 21 days; this may decrease
to as low as 7 days in primary seborrhea, resulting in a more rapid accumulation of
scale.A flawed epidermal lipid barrier may allow transepidermal water loss (TEWL), result-
ing in decreased epidermal water content (loss of hydration) causing xerosis; normal
skin hydration is 20–35%.An accelerated TEWL may be due to an inherent genetic defect in the epidermal
lipid barrier (primary) or as a result of inflammatory or metabolic skin disease
(secondary).In an attempt to heal the flawed barrier and restore normal hydrophobicity by making
more lipid, the epidermis becomes hyperplastic and hyperkeratotic (thickened – seen
clinically as excess scaling).Secondary infection (bacterial or yeast) produces inflammation and pruritus with
excoriation and further epidermal damage.
Abnormalities of sebaceous or apocrine gland function alter intercellular lipids and
disrupt the epidermal barrier function.
Primary disorders (ichthyotic and nonichthyotic): keratinization defects, in which
the genetic control of epidermal cell proliferation and maturation and/or epider-
mal barrier formation is abnormal; known defects are related to mutations in genes
that encode the structural proteins that form the corneocyte (cells of the stratum
corneum) and/or enzymes responsible for lipid transport or formation.Secondary disorders: effects of disease alter the normal maturation and proliferation
of epidermal cells; most keratinization disorders (over 80%) result from an underlying
etiology.Assigning a specific category (primary versus secondary) for each of the discussed
disorders is difficult and may be premature since the exact defect has yet to be iden-
tified.The nomenclature “seborrhea” is misused in the veterinary literature and may be
more of an adaptive response to describe a defect in the keratinization (cornification)
process; seborrhea sicca (dry scale) and seborrhea oleosa (greasy scale); seborrhea
literally means “flow of sebum.”SIGNALMENT/HISTORY
Primary: apparent by 2 years of age; characteristic in affected breeds; no sex predilec-
tion.
Secondary: any age; any breed of dog or cat; any disorder affecting the skin can result
in symptoms described as “seborrheic.”
Numerous syndromes have been identified; more frequent disorders listed below.