Rats/Mice Ulcerative Pododermatitis Study Guide

Overview and Clinical Importance

Ulcerative pododermatitis (commonly called bumblefoot or sore hocks) is a chronic, granulomatous, ulcerative dermatitis affecting the plantar (and occasionally palmar) surfaces of the feet in rats and mice. This condition is one of the most common integumentary disorders encountered in pet rodents and laboratory animals. Secondary bacterial infection, primarily with Staphylococcus aureus, frequently complicates the condition and can lead to osteomyelitis, septicemia, and death if left untreated.

Understanding ulcerative pododermatitis is essential for the NAVLE because it represents a classic example of a husbandry-related disease with multisystemic implications. The condition demonstrates the critical relationship between environment, host factors, and opportunistic infection.

Etiology and Pathophysiology

Primary Pathophysiology

The pathogenesis of ulcerative pododermatitis begins with ischemic necrosis of soft tissues compressed between bone and hard surfaces. The sequence typically follows this pattern:

• Initial Trauma: Microabrasions or pressure necrosis from rough or wire flooring
• Keratinized Lesion Formation: Small, reddish, raised areas of keratinized growth develop and form crusts/scabs
• Ulceration: Continued pressure causes blood-filled areas to rupture, drain, and reform cyclically
• Secondary Infection: Bacterial invasion (primarily S. aureus) leads to abscess formation
• Deep Tissue Involvement: Infection can spread to tendons, joints, and bone causing osteomyelitis
• Systemic Spread: Bacteremia and sepsis may occur in severe cases

Bacterial Pathogens

Risk Factors and Predisposing Conditions

Clinical Signs and Physical Examination

Clinical Presentation

Ulcerative pododermatitis typically affects the plantar metatarsal region (hind feet) and occasionally the volar metacarpal surfaces (front feet). Clinical signs vary based on severity and include:

• Early Stage: Erythema, mild swelling, hair loss on plantar surface
• Moderate Stage: Raised, reddened bumbles (nodules), hyperkeratosis, scab/crust formation
• Advanced Stage: Open ulceration, abscess formation, purulent discharge, pain
• Severe/Systemic: Osteomyelitis, septicemia, reluctance to walk, weight loss, fever

Severity Grading System

Diagnosis

Diagnostic Approach

Diagnosis is primarily based on clinical signs and history. A thorough diagnostic workup should include:

• Complete History: Cage type, bedding material, sanitation practices, diet, body condition
• Physical Examination: Assess all four feet; evaluate body condition score; assess pain response
• Culture and Sensitivity: Essential for appropriate antibiotic selection, especially in non-responsive cases
• Cytology: Gram stain of exudate to identify bacterial morphology
• Radiography: Evaluate for osteomyelitis in advanced cases; look for bone lysis or periosteal reaction

Exam Focus: Culture and sensitivity testing is crucial because Staphylococcus aureus may demonstrate resistance to commonly used antibiotics. This is especially important in laboratory animal settings where antibiotic resistance is increasingly common.

Differential Diagnoses

Treatment

Successful treatment requires a multimodal approach addressing both the infection and underlying husbandry factors. Treatment duration often spans weeks to months.

Pharmacological Treatment

Surgical Intervention

Surgical intervention is indicated for Grade IV-V lesions with abscess formation, when medical management fails, or when osteomyelitis is present. Options include:

• Debridement: Removal of necrotic tissue and abscess drainage
• Wound packing: Consider antibiotic-impregnated polymethylmethacrylate (AIPMMA) beads
• Amputation: May be necessary for refractory osteomyelitis or severe tissue damage

Husbandry Modifications and Prevention

Environmental modification is essential for treatment success and prevention. Without addressing husbandry factors, recurrence is virtually guaranteed.

• Flooring: Replace wire floors with solid flooring; cover wire with vinyl, fleece, or plastic canvas
• Bedding: Use soft, absorbent bedding (paper-based products, fleece); avoid pine/cedar shavings and corn cob
• Sanitation: Daily spot cleaning; complete bedding change at least twice weekly
• Weight management: Reduce high-fat diet; encourage exercise; prevent obesity
• Nail trimming: Regular nail trims to prevent gait abnormalities and self-trauma
• Monitoring: Regular foot examinations, especially in at-risk animals

Prognosis

Prognosis depends heavily on the grade of disease at presentation and owner compliance with treatment and husbandry changes:

• Grade I-II: Good to excellent prognosis with appropriate husbandry changes; may resolve with environmental modification alone
• Grade III: Fair prognosis; requires weeks to months of treatment; recurrence common without husbandry changes
• Grade IV-V: Guarded to poor prognosis; bone involvement significantly worsens outcome; amputation or euthanasia may be necessary

Species-Specific Considerations

Rats (Rattus norvegicus)

• Most commonly affected rodent species; often associated with obesity and wire caging
• Concurrent respiratory disease (Mycoplasma pulmonis) may compromise immune function
• Pruritus from sialodacryoadenitis virus may increase scratching and foot trauma

Mice (Mus musculus)

• C57BL/6 and related strains have increased susceptibility to ulcerative dermatitis
• Exercise wheels may contribute to hind foot lesions
• May be associated with barbering behavior and self-trauma