Ringworm in companion dogs can mimic a number of other skin disorders and probably is over-diagnosed. Sometimes, a presumptive diagnosis is made based on what the dog’s skin lesions look like, without actually identifying the cause of the problem. Fortunately, several tests are available to help veterinarians arrive at an accurate diagnosis of ringworm infections. The initial evaluation typically includes taking a thorough history from the dog’s owner and conducting a complete physical examination. The veterinarian will pay particular attention to any dry, scaly areas of the dog’s skin and any patchy areas of hair loss. Routine blood work is not usually done for cases of suspected ringworm infection.
There are several advanced diagnostic techniques that can help a veterinarian confirm a diagnosis of ringworm. One of those is called a trichogram, which involves plucking hairs from the edge of one or more of the patchy lesions and immersing them in a mixture of oil or potassium hydroxide and a special ink stain. The hairs are put onto a glass slide and examined under a microscope for evidence of fungal spores. Trichograms are not very sensitive and can be tricky to perform accurately. Another technique involves examining the areas of hair loss under a Wood’s lamp, which emits a certain wavelength of ultraviolet light that causes many of the strains of Microsporum canis to fluoresce an apple-green color. Unfortunately, a number of other substances also fluoresce under a Wood’s lamp, which can lead to false positive test results. Ultraviolet light is useful primarily as a screening tool but is not dependable for definitive diagnosis of ringworm.
The gold standard for diagnosing ringworm is fungal culture. Hairs from affected areas are plucked or collected on a toothbrush and put onto special media that supports fungal growth. The samples are sent to a referral laboratory, where they are cultured (grown) at room temperature. Within 7 to 14 days, the special test media will turn red if ringworm fungi are growing. The fungal colonies look fluffy, powdery and white; they will not be green, grey, black or multi-colored. Unfortunately, even fungal culture is not one-hundred-percent foolproof. Suspect colonies should be examined under a microscope to accurately identify the species of fungi that is causing the infection. The veterinarian may recommend taking skin biopsies and submitting them to a pathology laboratory for detailed evaluation, although usually this more invasive procedure is not necessary.
Veterinary and laboratory personnel should be very careful when taking samples and handling animals suspected of having ringworm, to reduce the chance of contagion. Owners also should be cautious and should avoid touching circular or oval areas of patchy hair loss on their dogs. It is a good idea to wear rubber or latex gloves when handling any animal that may have ringworm.