Canine Dermatophytosis Study Guide

Overview and Clinical Importance

Dermatophytosis (commonly known as ringworm) is a superficial fungal infection affecting the skin, hair, and occasionally claws of dogs. Despite its name, this condition is not caused by a worm but by keratinophilic fungi belonging to the genera Microsporum, Trichophyton, and Epidermophyton. Understanding dermatophytosis is essential for the NAVLE examination due to its zoonotic potential, clinical significance in shelter medicine, and the importance of accurate diagnosis.

Dermatophytosis represents less than 4% of all canine skin disorders, yet it is frequently over-diagnosed clinically due to its classic appearance that can mimic staphylococcal pyoderma. A common clinical adage states: "If it looks like ringworm in a dog, it's probably staphylococcal pyoderma."

Etiology

Primary Causative Agents

Three main dermatophyte species cause the vast majority of canine infections. Understanding their relative prevalence, sources, and characteristics is high-yield for the NAVLE.

Pathogenesis

Infection Mechanism

The infective form of dermatophytes is the arthrospore (arthroconidium), which is formed by fragmentation of fungal hyphae.

Steps of Infection

• Transmission: Direct contact with infected animal or fomite (contaminated bedding, grooming tools, clippers)
• Adherence: Arthrospores bind tightly to keratin within 6 hours of exposure
• Germination: Requires microtrauma to skin surface plus moisture; spores germinate and penetrate hair follicle
• Invasion: Keratinophilic fungi produce keratinases and other proteolytic enzymes to digest keratin
• Resolution: In immunocompetent hosts, cell-mediated immunity clears infection in 6-12 weeks (self-limiting)

Epidemiology and Risk Factors

Breed Predispositions

Certain breeds demonstrate increased susceptibility to dermatophytosis.

Additional Risk Factors

• Age: Young puppies (immunological immaturity, deficiency of fungistatic sebum)
• Environment: Shelters, kennels, hoarding situations (overcrowding, stress)
• Climate: Warm, humid environments; seasonal in temperate climates
• Immunosuppression: Concurrent illness, chemotherapy, prolonged corticosteroid use
• Skin barrier disruption: Pre-existing skin disease, ectoparasites, microtrauma from clipping

Clinical Signs and Presentation

Classic Presentation

Clinical signs are highly variable and often non-specific. The classic "ringworm" lesion with central clearing is less common in dogs than in humans.

Common Clinical Features

• Alopecia: Circular to irregular patches; may be focal, multifocal, or generalized
• Scaling and crusting: Often with erythematous margins
• Erythema: Variable; may be subtle in dark-coated dogs
• Papules and pustules: Follicular in distribution
• Hyperpigmentation: In chronic infections
• Pruritus: Highly variable - may be absent, mild, or severe

Predilection Sites

Lesions typically first appear on the face (around eyes, ears, muzzle), distal limbs, and tail.

Kerion (Nodular Dermatophytosis)

A kerion is a localized, nodular inflammatory lesion representing vigorous host immune response. These present as erythematous, alopecic, dome-shaped, exudative nodules - most commonly on the muzzle or distal limbs of working/hunting dogs. Associated with M. gypseum or T. mentagrophytes.

Differential Diagnosis

Clinical signs overlap with many conditions. The NAVLE frequently tests differentiation from other causes of alopecia and folliculitis.

Diagnosis

No single test is the gold standard for dermatophytosis diagnosis. Multiple complementary tests are typically used to confirm infection.

Wood's Lamp Examination

The Wood's lamp emits ultraviolet light at 320-400 nm. M. canis produces a metabolite (pteridine) that causes infected hairs to fluoresce apple-green.

Technique

• Allow lamp to warm up for 3-5 minutes before use
• Perform examination in complete darkness (allow eyes to dark-adapt)
• Hold lamp 2-4 cm from patient's skin
• Start at head and move slowly; examine inside ears, between toes
• Look for fluorescence of individual HAIR SHAFTS (not crusts or scale)

Direct Microscopic Examination (Trichogram)

Direct examination of plucked hairs can provide rapid confirmation by identifying arthrospores surrounding or within the hair shaft.

• Pluck hairs from margin of active lesion (broken, abnormal-appearing hairs)
• Mount 10-20 hairs with mineral oil or 10-20% KOH
• Lower microscope condenser to increase contrast
• Ectothrix infection: Arthrospores form sheath AROUND hair shaft (M. canis)

Fungal Culture (Dermatophyte Test Medium - DTM)

Fungal culture remains the reference standard for precise species identification and confirmation of viable infection.

• Toothbrush technique: Best for screening, asymptomatic carriers, or widespread disease
• Hair pluck: Pluck hairs from margin of lesion; select broken, abnormal hairs
• Incubate at room temperature (24-30°C); examine DAILY
• Hold cultures for at least 14-21 days before calling negative

Microscopic Identification of Macroconidia

Exam Focus: Remember M. canis macroconidia: "Thick-walled, Spindle-shaped, 6+ cells, Terminal knob." M. gypseum has thin walls and fewer than 6 cells. T. mentagrophytes are cigar-shaped.

Treatment

Treatment involves a multimodal approach: systemic antifungal therapy, topical therapy, and environmental decontamination.

Systemic Antifungal Therapy

Topical Therapy

Treatment Duration and Monitoring

• Duration: Minimum 6-8 weeks; continue until MYCOLOGICAL CURE
• Clinical cure: Resolution of all skin lesions
• Mycological cure: Negative fungal culture - TRUE endpoint of therapy

Environmental Decontamination

Arthrospores can survive in the environment for up to 18 months.

• Confine infected animal to easily cleaned area
• Vacuum/damp mop daily to remove shed hairs
• Disinfect with 1:10 dilute bleach or accelerated hydrogen peroxide

Zoonotic Considerations

Dermatophytosis is a ZOONOSIS. Transmission occurs through direct contact.

• M. canis is the most zoonotic of the canine dermatophytes
• Young children and immunocompromised individuals at highest risk
• Human lesions present as circular, red, scaly patches
• Disease is TREATABLE and CURABLE in both pets and people

Prognosis

• Superficial dermatophytosis: Excellent prognosis; self-limiting in immunocompetent animals
• Kerion reactions: Good prognosis with appropriate treatment
• Subcutaneous nodular disease: Guarded; may require surgery plus prolonged systemic therapy

"70-20-10" Rule for Canine Dermatophytes

M. canis = 70% (Cats, fluoresces, most zoonotic) | M. gypseum = 20% (Ground/soil, kerions) | T. mentagrophytes = 10% (Rodents)

"CULTURE = CURE" Mnemonic

Confirm with culture | Use systemic therapy | Lime sulfur topically | Treat environment | Until culture negative | Remember zoonotic risk | Expect 6-12 weeks