Canine Seborrhea Study Guide

Overview and Clinical Importance

Seborrhea (also called seborrheic dermatitis) is a common keratinization disorder in dogs characterized by defective keratinization of the epidermis, hair follicles, and claws, resulting in abnormal scale formation, excessive greasiness, and often secondary infections. Understanding seborrhea is essential for the NAVLE because it is frequently encountered in clinical practice and requires systematic diagnostic workup to identify underlying causes.

The keratinization cycle normally takes approximately 21 days in healthy dogs. In seborrheic dogs, this cycle is dramatically accelerated to approximately 7-8 days, leading to accumulation of scale and abnormal sebum production. This accelerated epidermal turnover is well-documented in research on affected Cocker Spaniels.

Pathophysiology of Keratinization

Keratinization is the process by which the protective outer layer of skin (stratum corneum) is constantly renewed by new skin cells migrating from the basal layer. Keratin is a structural protein that gives skin and hair its form and creates a protective outer coating.

Normal Keratinization Process

• Basal keratinocytes divide in the stratum basale
• Cells migrate upward through stratum spinosum and granulosum
• Terminal differentiation produces corneocytes filled with keratin
• Normal cycle takes approximately 21 days
• Sebaceous glands produce sebum for skin lubrication

Pathological Changes in Seborrhea

In seborrheic dogs, the epidermal turnover time is accelerated to approximately 7-8 days (versus normal 21 days). Research by Kwochka demonstrated that affected Cocker Spaniels have increased DNA synthesis in basal cells, with the defect being epidermal in origin (not related to serum or dermal factors).

Classification of Seborrhea

Clinical Classification

Etiological Classification

Breed Predispositions

Primary Seborrhea Breeds

Causes of Secondary Seborrhea

Identifying the underlying cause is essential for successful treatment of secondary seborrhea. The differential diagnosis list is extensive and should be approached systematically based on patient signalment and history.

Causes by Category

Clinical Signs and Distribution

Primary Lesions

• Scaling: Dry, white/gray (sicca) or greasy, yellowish-brown (oleosa)
• Follicular casts: Accumulation of keratin around hair shafts
• Comedones: Dilated, plugged hair follicles
• Waxy/greasy coat: Oily feel with adherent debris

Secondary Lesions

• Hyperpigmentation: Indicates chronic skin irritation
• Lichenification: Thickened, "elephant skin" - indicates chronic pruritus
• Erythema: Redness from inflammation or infection
• Alopecia: Hair loss from follicular damage or self-trauma
• Papules/pustules: Indicates secondary bacterial pyoderma
• Epidermal collarettes: Circular scaling - indicates superficial pyoderma

Distribution

Seborrhea typically affects areas rich in sebaceous glands: dorsal trunk, flanks, neck, ventral body, skin folds, ear margins. Intertriginous areas (axillae, groin, interdigital spaces) are often more severely affected.

Ceruminous otitis externa is commonly associated with seborrhea, especially in Cocker Spaniels and Basset Hounds.

Diagnostic Approach

A systematic approach is essential to identify underlying causes of secondary seborrhea. The diagnosis of primary seborrhea should only be made after ruling out all other causes.

Step 1: History and Signalment

• Age at onset: Less than 5 years suggests allergies; greater than 5 years suggests endocrinopathy/neoplasia
• Breed: Predisposed breeds for primary seborrhea
• Pruritus level: None suggests endocrine; moderate-severe suggests allergies/parasites
• Seasonality: Seasonal suggests environmental allergies
• Systemic signs: PU/PD, weight changes, lethargy (endocrine disease)

Step 2: Physical and Dermatological Examination

Document distribution, type of scale (dry vs. oily), presence of secondary lesions, odor, and concurrent ear disease.

Step 3: Diagnostic Testing

Secondary Malassezia Dermatitis

Malassezia pachydermatis is a commensal yeast that is normally present in low numbers on canine skin. In seborrheic dogs, changes in the skin microenvironment allow Malassezia to overgrow, causing secondary dermatitis that significantly contributes to pruritus and odor.

Clinical Features

• Intense pruritus
• Characteristic malodor (yeasty, musty, "corn chip" smell)
• Erythema and greasy seborrhea
• Lichenification and hyperpigmentation (elephant skin)
• Distribution: Interdigital spaces, ventral neck, axillae, groin, lip folds, ear canals
• Paronychia: Brown discoloration of nail beds

Diagnosis

Diagnosis is based on cytology. Malassezia appears as characteristic peanut-shaped or footprint-shaped budding yeast organisms, 3-8 μm in diameter. The presence of ≥1 yeast/HPF on skin or ≥5 yeast/HPF in ears is generally considered abnormal.

Breed Predisposition

West Highland White Terrier, Basset Hound, American Cocker Spaniel, Shih Tzu, Poodle, Boxer, Cavalier King Charles Spaniel, German Shepherd Dog, Dachshund

Treatment

Treatment of seborrhea involves three concurrent approaches: (1) treating any secondary infections, (2) addressing the underlying cause, and (3) managing the seborrhea symptomatically with topical therapy.

Topical Therapy

Topical therapy is the cornerstone of seborrhea management. The choice of shampoo depends on whether the seborrhea is primarily dry (sicca), oily (oleosa), or complicated by infection.

Bathing Protocol

• Initial phase: Bathe 2-3 times weekly until improvement (2-3 weeks)
• Contact time: Allow shampoo to sit for 5-15 minutes
• Maintenance: Reduce to 1-2 times weekly as condition improves
• Moisturizer: Apply emollient spray/rinse after bathing for seborrhea sicca

Systemic Therapy

Treatment of Underlying Causes

• Hypothyroidism: Levothyroxine (0.02 mg/kg PO q12h) - lifelong
• Cushing's disease: Trilostane (Vetoryl) or mitotane
• Allergies: Flea control, food trial, immunotherapy, oclacitinib, lokivetmab
• Parasites: Appropriate antiparasitic therapy

Prognosis

• Secondary seborrhea: Good to excellent if underlying cause identified and controlled
• Primary seborrhea: Guarded; incurable but manageable with lifelong topical therapy
• Client education: Essential to set realistic expectations