How to Check for Ringworm in Dogs With a Black Light

by Catherine Troiano
Physical examination, Wood's lamp testing and fungal cultures serve to diagnose ringworm.

Physical examination, Wood's lamp testing and fungal cultures serve to diagnose ringworm.

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If you have spotted a patch of hair loss or a scaly, crusty lesion on your canine companion, he may have ringworm, or dermatophytosis, a highly contagious fungal infection of his skin. Seventy percent of ringworm cases in dogs and nearly all cases of ringworm in cats are caused by microsporum canis. The first diagnostic tool that your veterinarian will be reaching for is a handheld black light device known as a Wood’s lamp. Once the examination room lights are turned off, the Wood's lamp is held above your furry friend's body, and when its light shines on his hair, you and your veterinarian will look for areas that appear apple green in color.

Using a Wood’s Lamp

A Wood’s lamp is an ultraviolet light that is filtered through a cobalt or nickel filter so that its light wavelengths can make the microsporum canis fluoresce and appear apple green in color. The test is economical, noninvasive, easily performed and perfectly tolerated by all patients, including your nervous canine friend. When using a Wood’s lamp, it needs to be warmed up for 5 to 10 minutes before use. All lights in the room are then turned off. The suspect area of Rocky's skin and coat and coat will be observed with the Wood’s lamp in a dark room. The lamp should be positioned over the area for at least five minutes because some strains of microsporum canis take longer to fluoresce than others.

Understanding the Results

The results of the Wood’s lamp test are not always accurate, and the test should only serve as a first step in diagnosing ringworm. Only 30 to 80 percent of microsporum canis strains fluoresce. The other two less common causes of ringworm in dogs are M gypseum and trichophyton mentagrophytes, neither of which fluoresce. Therefore, to rule out potentially false negative results of a Wood’s lamp test, hairs should be plucked from the suspected area and cultured in a laboratory to rule out the presence of ringworm. If any hairs on Rocky's body fluoresce under the Wood’s lamp, these hairs will be plucked and cultured to confirm the ringworm diagnosis. Fungal cultures, or dermatophyte test mediums, do not provide immediate results, but they are the most accurate method for achieving a definitive ringworm diagnosis. A confirmed diagnosis is the key to treating your canine companion and restoring his handsome coat.

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