Canine Diseases of Pads Study Guide

Overview and Clinical Importance

Diseases of the pads represent a challenging and clinically significant category of dermatologic conditions in dogs. The pads are specialized structures composed of thick, heavily keratinized epithelium overlying adipose tissue, designed to withstand mechanical stress. When diseased, pad pathology can cause significant pain, lameness, and disability. For the NAVLE, understanding the differential diagnoses, diagnostic approaches, and treatment strategies for pad diseases is essential.

Pad diseases can be categorized into several major groups: hyperkeratotic disorders, immune-mediated conditions, infectious diseases, nutritional deficiencies, and neoplastic processes. Recognizing the clinical patterns and breed predispositions is critical for accurate diagnosis.

Hyperkeratotic Pad Diseases

Hyperkeratosis refers to excessive production of keratin, the structural protein that forms the outer layer of the epidermis. In pad diseases, hyperkeratosis manifests as thickened, crusty, or horn-like protrusions from the pad surface, often described as 'hairy paw pads.'

Familial (Hereditary) Footpad Hyperkeratosis

Pathophysiology

Hereditary footpad hyperkeratosis is caused by genetic mutations affecting keratinocyte differentiation and cohesion. Several gene variants have been identified including DSG1 (desmoglein 1), FAM83G, and KRT16. These mutations result in abnormal keratin production and accumulation on the pad surface.

Breed Predispositions

Highly Predisposed Breeds:

• Dogue de Bordeaux
• Irish Terrier
• Kerry Blue Terrier
• Rottweiler (specific DSG1 mutation)
• Golden Retriever
• Labrador Retriever

Clinical Presentation

Age of Onset: Typically presents between 2-6 months of age, although not always congenital. In Rottweilers, signs typically appear by 6 months of age.

Clinical Signs: All four footpads are affected with severe, diffuse thickening of the stratum corneum. The pads develop a hard, horn-like texture and may develop deep fissures and cracks. When severe, secondary bacterial pyoderma develops in the fissures, causing pain and lameness. Unlike other hyperkeratotic conditions, no other skin lesions are present elsewhere on the body.

Diagnosis

Diagnosis is typically clinical based on breed, age of onset, and characteristic appearance. Biopsy shows marked orthokeratotic and parakeratotic hyperkeratosis with acanthosis but no acantholysis. Genetic testing is available for specific breeds, particularly Rottweilers (DSG1 mutation).

Treatment and Management

No cure exists. Treatment is palliative and lifelong:

• Daily soaking in 50 percent propylene glycol for 5-10 minutes to soften keratin
• Regular trimming of excess keratin after soaking (every 1-2 months)
• Application of keratolytic agents: salicylic acid, urea-containing creams
• Antimicrobial therapy for secondary bacterial infections
• Protective booties when walking on rough or extreme temperature surfaces

Idiopathic Nasodigital Hyperkeratosis (Age-Related)

Idiopathic nasodigital hyperkeratosis is the most common form of pad hyperkeratosis, typically affecting middle-aged to senior dogs (8-12 years of age) without an identifiable underlying cause.

Clinical Features

Distribution: May affect pads alone, nose alone, or both (nasodigital). Usually less severe than familial forms.

Appearance: Thickened, dry, sometimes cracked pads with a rough texture. May develop 'hairy' or feathered appearance. Usually painless unless fissuring occurs.

Differential Diagnoses to Rule Out

Before diagnosing as idiopathic, exclude:

• Pemphigus foliaceus (check face, ears for crusting)
• Zinc-responsive dermatosis (check diet, breed, other affected sites)
• Canine distemper (check vaccination status, systemic signs)
• Hepatocutaneous syndrome (check liver enzymes, albumin)
• Leishmaniasis (if travel history to endemic areas)

Treatment

Management is symptomatic with good outcomes:

• Regular application of moisturizing paw balms (lanolin-based, vitamin E)
• Warm water with Epsom salt soaks 2-3 times weekly
• Gentle trimming of excess keratin as needed
• Omega-3 fatty acid supplementation may provide benefit

Canine Distemper Virus - Hard Pad Disease

Canine distemper virus (CDV) is a paramyxovirus that causes severe systemic disease. Hard pad disease is the classical cutaneous manifestation, characterized by severe nasodigital hyperkeratosis.

Pathophysiology

CDV selectively infects keratinocytes in the stratum spinosum and granulosum, as well as epithelial cells of eccrine sweat glands. Viral replication causes orthokeratotic hyperkeratosis, irregular acanthosis, and thickened rete ridges. The selective infection of these keratinocyte layers is the key event in hard pad disease development.

Clinical Presentation

Systemic Signs (Precede or Accompany Pad Disease):

• Biphasic fever, lethargy, anorexia
• Mucopurulent ocular and nasal discharge
• Respiratory signs: coughing, dyspnea, pneumonia
• GI signs: vomiting, diarrhea
• Neurologic signs: seizures, myoclonus, ataxia, paresis

Pad Changes: Severe hyperkeratosis of footpads and nasal planum. Pads become extremely hard and thick to palpation. Often associated with neurologic disease. Enamel hypoplasia in incompletely erupted teeth is also classic.

Diagnosis

History: Unvaccinated or incompletely vaccinated puppies, exposure to infected dogs

Clinical Signs: Multisystemic disease with respiratory, GI, and neurologic involvement plus hard pads

Laboratory: Lymphopenia in acute phase, RT-PCR on conjunctival swabs, urine, or blood. Immunohistochemistry on skin biopsy can identify viral antigen in keratinocytes.

Histopathology: Orthokeratotic hyperkeratosis, irregular acanthosis, thickened rete ridges. Viral inclusions may be present but are not always seen.

Treatment and Prognosis

Treatment is supportive: IV fluids, nutritional support, antibiotics for secondary bacterial infections, anticonvulsants for seizures. Prognosis is guarded to poor, especially with neurologic involvement. Mortality rate is 50 percent or higher.

Zinc-Responsive Dermatosis

Zinc-responsive dermatosis is a nutritional skin disease resulting from zinc deficiency or malabsorption. Zinc is essential for epithelial cell differentiation, keratinocyte maturation, and immune function.

Two Clinical Syndromes

Syndrome I (Malabsorption):

• Breeds: Siberian Huskies, Alaskan Malamutes, Samoyeds
• Age: Young adults (1-3 years)
• Pathophysiology: Genetic defect causing decreased intestinal zinc absorption despite adequate dietary zinc

Syndrome II (Dietary Deficiency):

• Breeds: Any breed, especially rapidly growing large-breed puppies
• Age: Puppies during rapid growth phase
• Pathophysiology: Diet low in zinc or high in phytates/calcium that bind zinc

Clinical Presentation

Distribution of Lesions: Footpad hyperkeratosis, periocular, perioral, pinnal, chin, mucocutaneous junctions (prepuce, vulva, anus), pressure points (elbows, hocks)

Lesion Progression: Erythema to alopecia to crusting and scaling/hyperkeratosis to suppuration. Pruritus is present in approximately 50 percent of cases. Dull, dry hair coat with excessive sebaceous secretions.

Diagnosis

Clinical Diagnosis: Based on breed, age, distribution of lesions, and dietary history

Serum Zinc Levels: Often normal or low-normal, not always diagnostic

Biopsy: Parakeratotic hyperkeratosis, follicular hyperkeratosis, epidermal pallor

Therapeutic Trial: Response to zinc supplementation is diagnostic

Treatment

Immune-Mediated Pad Diseases

Pemphigus Foliaceus

Pemphigus foliaceus (PF) is the most common autoimmune skin disease in dogs. It is characterized by production of autoantibodies against desmocollin-1, a component of keratinocyte desmosomes, leading to acantholysis (loss of cell-to-cell adhesion) and intraepidermal pustule formation.

Pathophysiology

IgG autoantibodies target desmocollin-1 (and to lesser extent desmoglein-1) in the superficial epidermis. Autoantibody binding causes disruption of intercellular adhesion, resulting in acantholysis. Acantholytic keratinocytes and neutrophils accumulate in subcorneal pustules, which rapidly rupture leaving crusts, erosions, and hyperkeratosis.

Breed Predispositions

Most Commonly Affected: Akita, Chow Chow, Bearded Collie, Doberman Pinscher, Cocker Spaniel, Dachshund, Newfoundland, Schipperke, Chinese Shar Pei

Age: Middle-aged dogs (mean age 4.2 years), but can occur at any age

Clinical Presentation

Distribution: Classically starts on the face (dorsal muzzle, nasal planum, periorbital), pinnae, and then spreads to trunk, limbs, and pads. Pads may be the only site affected in some cases. Lesions are bilaterally symmetrical.

Primary Lesions: Large superficial pustules (rare to observe intact, very transient)

Secondary Lesions: Crusts (honey-colored), erosions, scales, alopecia, erythema. On pads: hyperkeratosis, fissuring, and painful crusting. Lameness may be present.

Pruritus: Variable, may or may not be present

Systemic Signs: Fever, lymphadenopathy, lethargy (in severe cases)

Diagnosis

Cytology (From Intact Pustule): Acantholytic keratinocytes (round cells with large nuclei), neutrophils, occasional eosinophils. This is a rapid screening test.

Histopathology (Gold Standard): Subcorneal to intragranular pustules containing acantholytic keratinocytes and neutrophils. Must biopsy early lesions (fresh pustules or crusts). Submit multiple samples to increase diagnostic yield. Use a dermatopathologist.

Rule Out Differentials: Deep bacterial pyoderma and dermatophytosis can cause acantholysis. Perform bacterial and fungal cultures if uncertain.

Treatment Protocol

Goal: Induce remission with high-dose immunosuppression, then taper to lowest effective maintenance dose.

Adjunctive Therapies

• Antimicrobial therapy for secondary bacterial infections (cephalexin, clindamycin)
• Antiseptic shampoos (chlorhexidine) 2-3 times weekly
• Topical corticosteroids for localized lesions
• Vitamin E (400-800 IU PO q12-24h) - anecdotal benefit
• Fatty acid supplementation

Prognosis

Fair to good with lifelong treatment. Approximately 53 percent achieve long-term remission. Relapse is common if medications are discontinued. About 13 percent of cases require euthanasia due to poor response, severe side effects, or owner finances.

Other Important Pad Diseases

Hepatocutaneous Syndrome (Superficial Necrolytic Dermatitis)

Hepatocutaneous syndrome (also called necrolytic migratory erythema, metabolic epidermal necrosis, or superficial necrolytic dermatitis) is a rare cutaneous manifestation of severe hepatic disease or, less commonly, glucagonoma. It is characterized by severe hyperkeratosis, crusting, ulceration, and erythema affecting the pads, mucocutaneous junctions, and pressure points.

Pathophysiology

The exact pathogenesis is unclear, but involves severe hypoaminoacidemia due to liver dysfunction or excessive glucagon secretion. This causes keratinocyte degeneration and necrosis in the superficial epidermis. A characteristic 'red, white, and blue' pattern is seen histologically.

Clinical Features

Systemic Signs: Lethargy, depression, weight loss, PU/PD, diabetes mellitus

Skin Lesions: Severe crusting, ulceration, and erythema of pads, mucocutaneous junctions (perioral, perianal, prepuce), pressure points. Pads often ulcerated and extremely painful.

Diagnosis

Bloodwork: Elevated liver enzymes (ALT, ALP), hypoalbuminemia, hyperglycemia, anemia, increased bile acids

Imaging: Hepatic abnormalities on ultrasound - nodular regeneration, cirrhosis, or pancreatic mass (glucagonoma)

Skin Biopsy: Classic 'red, white, and blue' pattern - superficial keratinocyte necrosis (red), edema (white), and parakeratotic hyperkeratosis (blue)

Treatment and Prognosis

Prognosis is grave. Treatment focuses on the underlying hepatic disease. Amino acid supplementation (egg yolks, high-quality protein diet), supportive care. Median survival is only a few weeks to months from diagnosis. If glucagonoma can be surgically resected, prognosis improves significantly.

Canine Papillomavirus - Footpad Warts

A distinct papillomavirus type causes multiple pigmented warts (verrucae) on the footpads of dogs. This is different from the oral papillomavirus that causes gingival warts in young dogs.

Clinical Presentation

Appearance: Multiple pigmented, cauliflower-like growths on footpads, particularly affecting the larger metacarpal and metatarsal pads. Can cause significant lameness if lesions are extensive.

Treatment

Many cases self-resolve spontaneously. Treatment options for persistent or painful lesions include surgical excision, topical imiquimod (Aldara 5 percent cream), or intralesional interferon injections.

Pododermatitis (Interdigital Furunculosis)

While technically not a pad disease, interdigital pododermatitis and furunculosis frequently affects the webbing between pads and can extend to involve pad margins. This is a complex, multifactorial condition.

Pathophysiology

Follicular trauma leads to folliculitis, furunculosis, and foreign body reactions to keratin and hair shafts in the dermis. Secondary bacterial infection (often Staphylococcus pseudintermedius) is nearly universal. Underlying allergic dermatitis, demodicosis, or conformational abnormalities predispose.

Breed Predispositions

Breeds with wider paw conformations: English Bulldog, Labrador Retriever, German Shepherd Dog, Bull Terrier, Boxer, Great Dane

Clinical Signs

Painful, nodular, erythematous lesions in interdigital spaces. May have draining tracts with hemorrhagic or purulent discharge. Licking, lameness, reluctance to walk. Chronic cases develop fibrosis and scarring.

Diagnosis and Treatment

Diagnosis based on clinical appearance and history. Cytology shows neutrophils, bacteria, and may show Demodex if present. Deep skin scrapings to rule out demodicosis. Bacterial culture for resistant infections. Treatment includes addressing underlying allergies, long-term systemic antibiotics (6-12 weeks), topical antiseptic therapy, laser ablation or surgical debridement for chronic cases.

Summary - Highest Yield Facts for NAVLE

1. Familial Footpad Hyperkeratosis: Autosomal recessive, Dogue de Bordeaux and Irish Terrier, onset less than 6 months, all four pads, severe horn-like hyperkeratosis, NO other sites affected, treat with propylene glycol soaks and trimming.

2. Zinc-Responsive Dermatosis: Syndrome I = HSM breeds (Huskies, Samoyeds, Malamutes), 1-3 years, genetic malabsorption, lifelong supplementation. Syndrome II = dietary deficiency in puppies, correctable with diet change. Key feature: MULTIFOCAL (pads PLUS face PLUS mucocutaneous junctions).

3. Pemphigus Foliaceus: Most common autoimmune skin disease in dogs, Akitas and Chow Chows overrepresented, starts on FACE then spreads to pads, bilateral symmetry, acantholytic keratinocytes on cytology, treat with immunosuppression (prednisone plus azathioprine or cyclosporine), lifelong management.

4. Canine Distemper - Hard Pad Disease: Unvaccinated puppies, severe nasodigital hyperkeratosis with multisystemic signs (respiratory, GI, neurologic), biphasic fever, paramyxovirus, RT-PCR diagnostic, poor prognosis with neurologic signs, prevention through vaccination is key.

5. Hepatocutaneous Syndrome: Rare, grave prognosis, older dogs, severe pad ULCERATION (not just hyperkeratosis), systemic illness, elevated liver enzymes and low albumin, 'red, white, and blue' histologic pattern, treat underlying liver disease, median survival weeks to months.