Red Circular Rash on a Dog


If you find a red, circular rash on your dog, odds are he has ringworm. It's not a worm, but a fungus, and you can catch it from him. So can any felines in your household. Formally known as dermatophytosis, ringworm infects the top layer of your dog's skin, or nails and hair. The fungus consumes keratin in these tissues. If you suspect your dog has ringworm, take him to the veterinarian for a definite diagnosis and treatment.


  • While any dog can develop ringworm, the condition is more common in older dogs or those with impaired immune systems. Some breeds, such as the Yorkshire terrier, appear particularly prone to ringworm.

Canine Ringworm Fungus

Although symptoms of ringworm are the same, three different ringworm fungi primarily infect canines, and all are transmissible to people. These are Microsporum gypseum, Microsporum canis and Trichophyton mentagrophytes.

Ringworm Signs

The red, circular rash is generally accompanied by hair loss in the affected area. Ringworm may or may not cause itching. If your dog scratches the lesions, a secondary bacterial infection could occur.

Diagnosis and Treatment

Your vet diagnoses ringworm not by observing the circular lesions, but by a Wood's lamp, an ultraviolet light, above the affected area in a darkened room. If the culprit is M. canis, the lesion usually will glow. However, the other ringworm fungi don't fluoresce, so no glowing doesn't mean ringworm isn't present. Your vet will take a skin or hair sample and have it cultured for a definitive diagnosis.

Treatment generally includes combination therapy, but it may take up to two weeks before you see results. Therapy includes:

  • Bathing with antifungal shampoo.
  • Topical antifungal ointments.
  • Griseofulvin, an oral antifungal drug, or other oral antifungal medications.

Severely affected dogs may require coat shaving. If the dog also has a bacterial infection, your vet may prescribe antibiotics.


  • If your dog has ringworm, throw out or disinfect all of his brushes, bowls, beds, toys and other items. You can use diluted bleach for disinfection, with a ratio of 1 part bleach to 10 parts water.

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