Following a thorough history and physical exam, testing will be performed to rule out other skin diseases that have similar signs, such as a bacterial skin infection and skin mite infestation. A special light, called a Wood's lamp, can be used as a crude screening test for ringworm. Unfortunately, only 50 percent of a specific type of ringworm called Microsporum canis will fluoresce within the animal's fur with the characteristic apple green color. Therefore, a negative result from a Wood's lamp does not rule out the possibility of ringworm.
A more reliable way to diagnose ringworm is to conduct a fungal culture on hairs taken from around the skin lesions by plucking them with a clean instrument or brushing them with a new toothbrush. In order to identify the source of the infection, the fungal growth is evaluated under a microscope to determine the type of fungus present. This assessment of the material subsequent to its growth in a medium will rule out false positives that would otherwise be caused by environmental contaminants.
The veterinarian may evaluate plucked hairs under a microscope to look for evidence of fungal units associated with the hair shaft. However, this test is more time-consuming and only carries a 40 to 70 percent success rate in detecting a ringworm infection.
In animals with severe skin abnormalities, skin biopsies may be obtained. Although a skin biopsy can indicate a true fungal infection of the skin as opposed to a temporary presence, this procedure offers a less reliable diagnosis than a fungal culture. Often, this test is performed when the skin lesions are impossible to culture for ringworm.
