Skin conditions in dogs can be baffling and require some detective work to diagnose. Ringworm is an especially important infection to get right because it's contagious to humans and other pets in the home.
What Is Ringworm?
Ringworm is the popular name for dermatophytosis, a frequently-identified fungal skin infection seen in dogs. Its name originates from the red, ring-shaped irritation that accompanies infection in humans; it's not actually infection by a worm.
Fungal dermatophyte species identified in canine ringworm infections include Microsporum canis (most commonly identified), Microsporum gypseum, and Trichophyton mentagrophytes. These robust and hardy fungi can live for a long time in the environment (from months to years) in the form of infective spores (seed-like structures) and take their nutritional supply from the keratin protein in the hair, nails, and skin layers. Different strains can vary in virulence, or disease-causing ability.
How Is Canine Ringworm Spread?
Ringworm is transmitted by contact with skin lesions, hair, and scales of an infected pet or person. The fungus can also be isolated from carriers as well as from the soil and surrounding environment. Carriers do not show signs of infection, but can infect surrounding animals. Hairs, which are covered with infectious spores, are continuously shed from the infected dog. The veterinarian should examine the dog’s environment and living style in cases of infection, including close contact with any humans or pets with dermatitis in the household. Puppies and sick and immune-compromised dogs (as well as those from shelters) are at the greatest risk for infection.
What Does Ringworm in Dogs Look Like?
Ringworm lesions can appear quite variable in dogs; they do not demonstrate the classic human ring-shaped lesion. Dogs typically display bald spots with associated scaly, crusty, irritated, and sometimes itchy lesions that mimic other clinical veterinary conditions such as mites, allergy, or bacterial infection. Your veterinarian has the diagnostic tools to differentiate these variable conditions from one another.
How Is Ringworm Diagnosed?
Ringworm is diagnosed by fungal culture of hair and scale or crust from lesions, skin biopsy, or KOH-DMSO preparation tests. Hair from some cases will shine a fluorescent green color if examined under a Wood’s lamp. The Mackenzie toothbrush technique test (rubbing a sterile toothbrush over infected areas) is an effective way to obtain samples for culture, particularly in asymptomatic carrier animals.
How Is Ringworm Treated?
Topical and/or oral antifungal medications are used in ringworm treatment. Some of these medications can have serious side effects, and they should only be administered under the direction of your veterinarian. Antibiotics are used for secondary skin infections. The flea preventive Program® (lufeneron) may have some utility, but its efficacy is still unclear. Treatment should be continued until hair and skin cultures are negative. Environmental decontamination, though difficult, is essential in effective therapy, and can include vacuuming, steam cleaning, and bleach de-contamination.
Can Humans Catch Ringworm from an Infected Dog?
Ringworm is a zoonosis, or a shared disease between human beings and animals. Immune suppressed humans (very young or very old individuals, those with chronic diseases such as HIV or cancer, and people undergoing chemotherapy or post-transplant therapy) are most susceptible to ringworm infection and clinical disease.
Please consult your veterinarian if you suspect that your dog has ringworm in order to confirm the diagnosis and start treatment.
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