Zoonotic Diseases in Dogs and Cats: Ringworm

Zoonotic diseases are those that can be transmitted to people from dogs or cats. It is important toZoonotic Diseases in Dogs and Cats: Ringworm treat these diseases when they are noticed to prevent illness in pet owners.

Ringworm is a fungal infection, its misleading name being carried over from the old and incorrect assumption that the characteristic circular skin lesion was caused by a worm. Several types of fungi falling under the category “dermatophytes” (pronounced der-mat-o-fites) cause the disease. Though humans have their own unique dermatophytes to worry about (such as athlete’s foot), dogs and cats can share their itch-causing Microsporum canis (contact/fomites), Microsporum gypseum (soil), and Trichophyton mentagrophytes as well. Importantly, bacterial skin infections are far more common in pets and people and are often indistinguishable from ringworm without testing.

Who Can Get Ringworm from their Pet?

Ringworm is a highly transmissible disease, meaning that even healthy pets and humans can develop signs following exposure to infected spores. The very young and the very old are predisposed to any infectious organism, including dermatophytes. Additionally, damaged or unhealthy skin has reduced immunity to opportunistic infections. Malnutrition, crowding (such as in catteries or shelters), stress, pregnancy, lactation, immunodeficiency syndromes, cancer, and immunosuppressive therapy all predispose pets and people to ringworm infection.

How Ringworm Works in Dogs and Cats

Dermatophytes invade the follicles through actively growing hairs and dead skin, creating brittle hairs and skin lesions that may or may not be circular. Affected areas can become red, inflamed, and itchy with scale and hair loss. Signs typically develop 7-14 days after exposure to hairs, clothing, soil, skin, or other objects containing infective spores, which can survive in the environment for months or years. Ringworm may be diagnosed by your veterinarian by looking at affected hairs under a microscope, but a fungal culture is the definitive test. Importantly, dogs and cats can carry and transmit infective spores without showing signs of disease.

How is Ringworm in Dogs and Cats Treated?

The inflammation associated with infection typically clears the infection within 2-3 months without treatment. However, systemic and/or topical antifungal medications can hasten resolution and reduce transmission to people and other pets. It may be necessary to shave a pet to allow topical medications to penetrate more fully, and treatment may be necessary for greater than 6 weeks. Treatment should continue at least 1-2 weeks beyond a negative fungal culture. This disease should be discussed and monitored closely with your veterinarian throughout diagnosis and treatment.

How to Reduce the Risk of Transmission Between Pets and People

Affected pets or suspected carriers should be isolated from humans and other pets wheneverRingworm and Zoonotic Diseases in Dogs and Cats possible. Environmental decontamination is crucial to limit transmission, including the removal as much hair as possible via vacuuming or other methods. Bleach (1:10 dilution) is the only environmental decontamination agent available to the general public. Other chemicals available to the public, even steam, have not been shown to inactivate dermatophyte spores in a real-world setting.

For more information about ringworm, visit the Cornell University College of veterinary Medicine website.

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Dr Roger Johnson and Dr Derek Calhoon

Roger K. Johnson, DVM, Diplomate ACVIM (internal medicine) is a board-certified veterinary internal medicine specialist. His professional interests include cardiology as well as using advanced diagnostics to help his patients. His particular favorites include echocardiography, abdominal ultrasonography, and endoscopy. Derek Calhoon, DVM is a veterinary general practitioner.

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