Dog: Dermatophytosis

Other common/scientific names: ringworm, Microsporum sp, Trichophyton sp, fungal skin infection

Dermatophytosis is a zoonotic skin and hair infection caused by a fungus.

There are three types of dermatophytes (fungi) which cause disease in dogs:

• Microsporum canis causes 70% of disease, commonly carried by cats
• Microsporum gypseum causes 20% of disease, found in the soil
• Trichophyton mentagrophytes causes 10% of disease, carried by rodents

Dermatophytosis is spread by direct contact with an infected animal and objects contaminated by the fungus such as bedding, grooming supplies, furniture and clothing. Fungal spores can remain infective in the environment for up to eighteen months. Contact with a dermatophyte does not always result in infection. Whether an infection develops depends on the type of fungus, the dog’s age, immunity, nutritional status and condition of exposed skin. Young or debilitated dogs are more prone to infection. Dogs may also be asymptomatic carriers, meaning they can still spread the fungus without showing clinical signs.

Dermatophytes usually grow only in keratinized tissue (skin and hair). The fungus begins in the growing hair and penetrates the hair shaft weakening it which leads to hair loss. As the infection progresses, clusters of fungi develop on the hair shafts which become the main source for spread of the disease.

Patchy hair loss

Clinical signs include circular areas of hair loss. The skin can be reddened and scaly. Some cases of dermatophytosis will have a secondary bacterial infection showing raised papules and pustules. Unlike in humans, ringworm is rarely pruritic (itchy) in dogs.

Abb. GFTH3I5B: Dermatophytosis.

This is a photograph of a typical circular lesion with a secondary bacterial infection.

Abb. GFTH5FTY: Dermatophytosis.

This is a photograph of a shaved dog with several circular areas of ringworm.

• A Wood’s lamp is a special type of ultraviolet light that can be used to diagnose ringworm. The lamp is held over a suspected area and some infections will fluoresce an apple green color. While this test is easy to perform, only 50-80% of Microsporum canis cases fluoresce and the other fungal species in dogs do not glow.
• Examination of the hairs and skin scrapings under the microscope can be used to detect the fungus. However, this test is not always 100% reliable since the fungus may not always be visible.
• A fungal culture is needed to confirm diagnosis. This test can be performed at your veterinarian’s clinic using a special medium called dermatophyte test medium (DTM). Hair stubble and skin scale are placed on the DTM and checked daily. The fungal culture not only confirms the presence of a dermatophyte but it also allows for identification of fungus. The disadvantage to this test is the fact that it can take up to 10 days for results.
• A skin biopsy of the lesion can be obtained to also confirm the diagnosis. However, this procedure is more involved and generally involves sedation or anesthesia.

Abb. GFTHAFHT: Obtaining hair samples.

This is a photograph depicting removal of hair follicles from the edge of a lesion to test for dermatophytosis.

Abb. GFTHC63Z: Fungal Culture or Dermatophyte Test Medium (DTM).

This is a photograph showing a positive test for ringworm. Hair and skin samples were placed on the DTM. The color change from yellow to red indicates a fungal infection. This sample is then examined under the microscope to identify the specific type of fungus.

Abb. GFTHEDQT: Ringworm in a child.

This is a photograph of a child with ringworm.

Dermatophytosis in dogs is usually self-limitin g (heals on its own) but resolution can be hastened by treatment. Another reason to provide treatment is to prevent the spread of infection to other animals and humans.

Treatment of dermatophytosis must involve the animal and its environment. Animals can be treated with systemic and/or topical antifungal medication. Systemic medication consists of oral tablets for many weeks. These medications can be quite costly and have the potential for side effects. Topical treatments consist of shampoos and dips and should be repeated twice weekly. The animal must be shaved to allow adequate skin penetration when using topical treatments. Treatment should only be discontinued after a repeat culture is negative. All animals in a household may need to be treated.

Fungus like to grow in dark spaces on hair and debris. Because the spores last for months in the environment, rooms need to be vacuumed often and the vacuum bags changed each time. Surfaces should be treated with diluted bleach to kill the ringworm spores. Affected animals should be confined to one room.

While dermatophytosis is never life-threatening, treatment to rid the dog and environment is lengthy, time consuming and can be expensive.

In the US, there currently is no vaccine for ringworm in dogs.

If a person in your household has signs of dermatophytosis, all pets should be examined by a veterinarian.

Dermatophytosis is zoonotic meaning the infection can be transmitted to humans from dogs. Young children are more susceptible to ringworm. Gloves should be worn when treating a dog with ringworm and contact should be minimized.