Dermatophytosis: Ringworm

Dermatophytosis : Ringworm
By Dr. Ed Mapes
Stonebridge Animal Hospital

The skin disorder called ringworm is not actually caused by a worm, rather it is a fungal infection within the skin that often spreads outward from a central area and has a red ring of inflamed skin around it. The infection is caused by one of two fungal organisms, called dermatophytes. These are Microsporum canis and Trichophyton mentographytes.


These infections are quite contagious between animals and humans, so it’s wise to have the skin checked whenever a suspicious lesion appears. Infection can easily spread between pets and people within the household, so I attempt to make the correct diagnosis as soon as possible.

Lesions are characteristically hairless with a flaky, grey appearance. They are often seen on the head and limbs of pets, but can be anywhere over the body. Infections can become generalized, having spread to all areas of the body. You may see scaling that looks like dandruff on the hairs, and lesions are often in circular patches surrounded by a reddish ring of inflammation. The lesions can cause some itch sensation, but patients do not scratch at them as much as most other skin disorders.

The often characteristic appearance is a clue in the diagnosis. Examination of hair shafts under the microscope can sometimes reveal the presence fungal spores that may be attached. I use the Dermatophyte Test Medium as the definitive diagnostic test. We place a few hair shafts with the scaly debris with the matrix of growth medium, and observe for 5 to 7 days. A test is deemed to be positive when a fluffy white and green growth appears in conjunction with the medium turning colors from orange to red.

The DTM above shows no growth and is a negative test. Note the fluffy white growth present on the sample to the right, along with a red medium. This test is considered positive.

There are several strategies used in the treatment of ringworm infections. Localized and isolated infections such as one spot on the head can be treated with ointments applied directly to the lesions. Treatment must be continued for at least one month, and the patient should be observed carefully for the development of other lesions during treatment; these could signal the need for other methods to combat a spreading infection.

Dips can be effective on their own or in conjunction with other measures. The dips are applied (using rubber gloves) to freshly-washed hair and allowed to dry on the pet.

Oral medication is the most effective means of treatment. Several medications are available (prescribed by a veterinarian). The oral form must also be given for a least one month to eradicate the fungal infections of ringworm.

Cleaning the house and furniture

As ringworm lives on both skin and hair, it can be easily transmitted by loose hair on carpet or furniture. At the same time as treating your animal for ringworm, it is recommended that you do a thorough clean of your home environment to remove any contaminated hairs.

  • Vacuum any carpet or furniture that your pet has contact with (including underneath beds and couches)
  • Wash down surfaces with a good cleaning agent
  • Restrict your pet to areas of the house that are easy to clean, such as rooms with tiles or floorboards.
  • It can take up to six weeks for treatment to be effective. During this time, your pet may still be contagious, so it’s important that members of the family (particularly children, the elderly or anyone with a compromised immune system) have minimal contact with the animal.

Prevention of ringworm

The fungi that cause ringworm love to live in warm, damp environments like soil. They then attach to hair and skin cells shed by humans and animals. While there is not much you can do to prevent these conditions, there are a number of other actions that you can take to prevent you and your family from catching the infection:

  • Regularly clean pet blankets and other bedding from your cat or dog’s quarters
  • Regularly dispose of any hairs from your pet’s grooming brush
  • Remove skin cells and hair from your home by regularly vacuuming the house
  • Disinfect other common areas of the house where your pets tend to live

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