Feline Dermatophytosis Study Guide

Overview and Clinical Importance

Dermatophytosis (commonly known as ringworm) is a superficial fungal infection of keratinized structures including skin, hair, and nails in cats. Despite its common name, this condition is not caused by a worm but by fungi called dermatophytes. This is a high-yield NAVLE topic due to its clinical prevalence, zoonotic potential, and importance in shelter medicine.

Microsporum canis is the causative agent in greater than 90% of feline dermatophytosis cases. The disease is self-limiting in immunocompetent cats (typically resolving within 4 months) but treatment is always recommended to shorten clinical course, reduce environmental contamination, and prevent zoonotic transmission.

Etiology and Pathogenesis

Causative Organisms

Dermatophytes are keratinophilic fungi that utilize keratin as a nutrient source. They are classified by their natural reservoir:

Pathogenesis

The infection cycle follows these key steps:

• Exposure: Arthrospores contact skin (direct animal contact or fomite transmission)
• Adhesion: Spores adhere to keratinocytes within 2 hours of contact
• Germination: Requires microtrauma/abraded skin for successful invasion
• Invasion: Hyphae penetrate stratum corneum and hair follicles
• Shedding: Infected hairs with arthrospores shed into environment (can occur within 7 days)

Risk Factors and Predispositions

Clinical Presentation

Classic Clinical Signs

Dermatophytosis is a follicular disease, meaning lesions follow hair follicle distribution. Clinical presentation is highly variable - there is no single 'classic' appearance.

Atypical Presentations

• Asymptomatic carriers: Culture-positive cats with no clinical lesions - especially long-haired breeds
• Dermatophytic pseudomycetoma: Nodular/subcutaneous lesions almost exclusively in Persian cats
• Exudative paronychia: Nail bed infection with discharge
• Kerion: Highly inflammatory, nodular reaction (less common in cats than dogs)

Exam Focus: The 'classic ring' lesion seen in humans is UNCOMMON in cats. Feline lesions are often irregular, asymmetrical patches of alopecia with scaling. Always include dermatophytosis in differentials for any cat with patchy alopecia!

Diagnostic Approach

No single test is the gold standard for diagnosing dermatophytosis. A combination of diagnostic modalities provides the most accurate diagnosis.

Diagnostic Tests Comparison

Wood's Lamp Examination

The Wood's lamp emits long-wave ultraviolet light (320-400 nm, peak 365 nm). M. canis produces a tryptophan metabolite called pteridine that fluoresces a characteristic bright apple-green color.

Technique

• Allow lamp to warm up for 3-5 minutes before use
• Examine in a completely dark room
• Hold lamp 2-4 cm from skin surface
• Start at head, examine systematically including face, ears, feet, interdigital spaces
• Gently lift crusts to visualize hairs beneath
• Look for fluorescence in individual hair shafts, not scales or debris

Interpretation

• Positive: Bright apple-green fluorescence of hair shafts
• False positives: Lint, soap, topical medications, doxycycline, lime sulfur residue (yellow/orange/blue)
• False negatives: 30-50% of M. canis strains; Trichophyton and M. gypseum NEVER fluoresce

Direct Microscopic Examination (Trichogram)

Pluck hairs from lesion edges (preferably Wood's lamp positive hairs) and examine microscopically.

Technique

• Mount hairs in mineral oil or 10-20% KOH (KOH clears keratin debris; incubate overnight)
• Examine at 10x and 40x magnification
• Positive findings: Arthrospores (ectothrix pattern) surrounding or within hair shaft; hyphae

Fungal Culture (DTM)

Dermatophyte Test Medium (DTM) is the standard culture medium containing antibiotics (chloramphenicol, cycloheximide) to inhibit bacteria and saprophytic fungi, plus phenol red pH indicator.

Sample Collection

• Lesional cats: Pluck hairs from lesion periphery; include scales
• Screening/carriers: MacKenzie toothbrush technique - brush entire coat vigorously with new sterile toothbrush, then press bristles onto DTM

Incubation and Reading

• Incubate at 25-30°C in dark, humid environment
• Monitor DAILY for up to 21 days
• Do not seal lid tightly (allow air exchange)

Interpretation

Positive culture = White/buff/pale yellow FLUFFY colony growth SIMULTANEOUS with red color change (alkaline pH from protein metabolism)

Treatment

Treatment requires a three-pronged approach: (1) systemic antifungal therapy, (2) topical therapy, and (3) environmental decontamination.

Systemic Antifungal Therapy

Monitoring During Systemic Therapy

• Monitor liver enzymes (ALT) at baseline and periodically - especially with itraconazole
• If using griseofulvin: CBC every 2 weeks; test for FIV/FeLV before starting
• Watch for anorexia, vomiting, hepatotoxicity signs

Topical Therapy

Topical therapy is used to disinfect the haircoat and reduce environmental contamination. Whole-body treatment is preferred over spot treatment.

Clipping

Clipping is NOT required for single-cat households. Recommended for multi-cat environments, shelters, or long-haired breeds to reduce environmental contamination and improve topical penetration.

Environmental Decontamination

Critical concept: M. canis arthrospores can survive in the environment for up to 18 months.

• Effective disinfectants: Dilute bleach (1:10), accelerated hydrogen peroxide, enilconazole
• Vacuum frequently; dispose of vacuum bags
• Wash bedding, toys in hot water with bleach
• Discard items that cannot be effectively cleaned (scratching posts, soft toys)

Treatment Duration and Cure Criteria

Continue treatment until mycological cure is documented - resolution of clinical signs alone is insufficient.

• Typical treatment duration: 6-12 weeks
• Cure criteria: Resolution of clinical lesions + negative Wood's lamp (except tips) + TWO consecutive negative fungal cultures 2-4 weeks apart

Prognosis

Excellent in immunocompetent cats with appropriate treatment and environmental management. Recurrence usually indicates inadequate treatment duration, environmental recontamination, or unidentified carrier animals.

Zoonotic Considerations

M. canis is highly zoonotic. Approximately 50% of humans exposed to infected cats develop infection.

• Human presentation: Classic circular, erythematous, scaly 'ring' lesion (tinea corporis)
• Children, elderly, and immunocompromised individuals at highest risk
• Advise clients to practice good hygiene, limit direct contact during treatment, seek human medical attention if lesions develop

Memory Aids

"M. CANIS" Mnemonic for Diagnosis

• M - Most common cause (greater than 90% of feline cases)
• C - Culture on DTM is gold standard
• A - Apple-green fluorescence under Wood's lamp
• N - Not normal flora (isolation = exposure)
• I - Itraconazole first-line treatment
• S - Self-limiting but STILL treat!

"PULSE" for Itraconazole Protocol

• P - Persists in keratin (drug accumulates)
• U - Use 5 mg/kg daily
• L - Licensed product only (no compounding)
• S - Seven days ON, seven days OFF
• E - Expect 3 cycles (weeks 1, 3, 5)