Dermatophytosis in Pets

Dermatophytosis, commonly known as ringworm, is a superficial fungal infection affecting the skin, hair, and, less frequently, claws of dogs and cats. The primary causative agents are dermatophyte fungi, notably Microsporum canis, Microsporum gypseum, and Trichophyton mentagrophytes. Merck Veterinary Manual

Etiology and Transmission:

• Causative Agents:

  • Microsporum canis: Predominantly affects cats and dogs; zoonotic potential.

  • Microsporum gypseum: A soil organism causing inflammatory lesions.

  • Trichophyton mentagrophytes: Affects various species, including dogs and cats.

• Transmission:

  • Direct Contact: With infected animals.

  • Indirect Contact: Via contaminated environments, grooming tools, or bedding.

  • Environmental Persistence: Dermatophyte spores can survive in the environment for extended periods, facilitating transmission.

Pathophysiology:

Dermatophytes invade keratinized tissues, such as the stratum corneum of the skin and hair shafts. Infection requires sufficient spore load, microtrauma to the skin, and moisture. The fungi produce enzymes that degrade keratin, allowing colonization and proliferation. Host factors, including age, immune status, and physiological stress, influence susceptibility and severity. Merck Veterinary Manual

Clinical Presentation:

• Lesions:

  • Dogs: Circular areas of alopecia with scaling, crusting, erythema, and possible papules. Nodular lesions (kerions) may develop.

  • Cats: Similar lesions, often on the face, ears, tail, and feet. Kittens are particularly susceptible. Some cats may develop miliary dermatitis or exudative paronychia.

• Pruritus: Variable; some animals may exhibit significant itching, while others remain asymptomatic.

• Zoonotic Potential: Infected animals can transmit dermatophytosis to humans, emphasizing the importance of early diagnosis and management.

Diagnosis:

• Wood's Lamp Examination: Approximately 50% of M. canis infections fluoresce under ultraviolet light, appearing as apple-green fluorescence on hair shafts. However, this method has limitations and should not be solely relied upon.

• Direct Microscopic Examination: Evaluation of plucked hairs and skin scrapings treated with potassium hydroxide (KOH) to identify fungal elements.

• Fungal Culture: Considered the gold standard; samples inoculated onto appropriate media to identify fungal growth. Results may take up to three weeks.

• PCR Testing: Detects fungal DNA; however, it cannot distinguish between viable and nonviable organisms.

Treatment:

• Topical Therapy:

  • Antifungal Shampoos/Dips: Containing miconazole, ketoconazole, or lime sulfur applied regularly to reduce surface contamination.

  • Clotrimazole or Miconazole Creams: Applied to localized lesions.

• Systemic Therapy:

  • Griseofulvin: Administered orally; effective but with potential side effects, including gastrointestinal upset and bone marrow suppression. Contraindicated in pregnant animals due to teratogenicity.

  • Itraconazole: Effective against dermatophytes; dosing regimens vary. Monitor for potential hepatotoxicity.

  • Terbinafine: Another systemic antifungal option; dosing and duration depend on the severity of infection.

• Environmental Decontamination: Regular cleaning and disinfection of the animal's environment to eliminate fungal spores and prevent reinfection.

Prognosis:

Dermatophytosis is generally self-limiting in healthy animals, with spontaneous resolution occurring over weeks to months. However, treatment is recommended to expedite recovery, reduce environmental contamination, and minimize zoonotic risk. Animals with underlying health issues or those under physiological stress may experience more persistent infections requiring prolonged therapy.

Prevention:

• Hygiene: Regular grooming and bathing to maintain skin health.

• Environmental Management: Routine cleaning and disinfection of living areas to reduce spore load.

• Quarantine: Isolate infected animals to prevent transmission to other pets and humans.

References:

• Dermatophytosis in Dogs and Cats. Merck Veterinary Manual.