Dermatophytosis

Etiology

• Pathogens:

  • Microsporum canis: Primarily affects cats and dogs, less commonly larger animals.

  • Trichophyton mentagrophytes, T. verrucosum, and T. erinacei: Mainly affects hedgehogs.

  • Microsporum gypseum: A soil-borne fungus causing inflammatory lesions.

• Transmission:

  • Direct contact with infected animals is the main route of transmission.

  • Fungal spores may also be transmitted indirectly via contaminated objects (e.g., clippers, tack).

  • Dermatophytosis requires microtrauma (e.g., scratches, cuts) to infect the skin, as exposure alone does not always lead to disease.

  • It is considered zoonotic, meaning it can be transmitted from animals to humans.

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Clinical Signs

• Common Symptoms:

  • Hair loss (alopecia), scaling, crusting, and erythema (redness).

  • Papules and hyperpigmentation.

  • Pruritus (itching) may be present, but it varies by case.

  • Nodular Lesions (kerion) in dogs and pseudomycetomas in Persian cats.

  • Cats can develop exudative paronychia (inflammation of the nail folds).

  • Pustular dermatophytosis may mimic other skin conditions.

• Affected Areas:

  • Lesions often appear on areas of the body where hair is thinner, such as the face, ears, and feet.

  • In severe cases, the lesions can become larger and more inflamed.

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Diagnosis

• Direct Examination:

  • Hair and skin scrapings, or biopsies, can confirm fungal invasion of the hair follicles and skin.

  • Wood’s Lamp: A black light (320-400 nm) can be used to detect infected hairs (Microsporum canis), which fluoresce apple green.

  • Fungal Culture: Helps identify the presence of spores. It is essential to collect samples directly from infected lesions rather than the entire coat.

  • Dermoscope: A handheld tool used to find abnormal hairs for direct examination.

  • PCR Testing: Confirms the presence of fungal DNA but cannot differentiate between viable and nonviable spores.

• Differentiating Conditions:

  • Bacterial Pyoderma: Common in dogs and often misdiagnosed as dermatophytosis.

  • Skin Biopsy: Not usually required but may be indicated for nodular lesions or atypical presentations.

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Treatment and Management

• Self-Curing:

  • In otherwise healthy animals, dermatophytosis is self-limiting and often resolves without treatment. However, treatment can accelerate recovery and reduce transmission risk.

• Systemic Antifungals:

  • Itraconazole (5 mg/kg, PO, once daily, on a week on/week off schedule) is commonly used for cats and small dogs.

  • Ketoconazole (5 mg/kg, once daily) or Terbinafine (30–40 mg/kg, once daily) can be used for other dogs.

  • Griseofulvin is no longer recommended due to superior alternatives.

• Topical Therapy:

  • Lime Sulfur Rinse (1:16) or Enilconazole (1:100) should be applied twice a week. These treatments have residual antifungal activity.

  • Chlorhexidine & Miconazole Shampoos: Effective when lime sulfur or enilconazole is not available, though they have no residual activity.

  • Adjunct Topical Therapy:

    • 1–2% Miconazole Cream can be used on the face.

    • Otic Products: Ear medications containing clotrimazole, miconazole, or ketoconazole/chlorhexidine combinations can treat fungal infections in the ears.

• Environmental Cleaning:

  • Mechanical Cleaning: Remove organic material and hair from surfaces using detergent.

  • Disinfection: Use a disinfectant that is effective against Trichophyton species to kill spores.

  • Soft Surfaces: Wash bedding and towels with regular detergent; bleach is not required. Carpets can be cleaned with steam or carpet shampooing.

  • Frequency: Clean the environment thoroughly once or twice a week, and use wipes between cleanings to remove organic material.

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Prognosis

• Self-Limiting in Healthy Animals: Most animals will recover without treatment, but infected animals can spread the fungus to others.

• Treatment Accelerates Recovery: Systemic antifungals and topical therapy help reduce contagion and shorten the disease course.

• Cure Confirmation:

  • Clinical Cure: Resolution of lesions with no new lesions appearing.

  • Mycologic Cure: Confirmed through a negative fungal culture or PCR test after treatment.

  • Wood’s Lamp: Can be used to check for glowing infected hairs in M. canis infections, which may indicate residual infection.