I recommend using a good quality electric Wood’s lamp because of inconsistent results with battery-operated models. There are 2 types of hand-held lamps available—one has only a lamp (Figure A) and the other has a light surrounding the magnifying lens (Figure B). Both are acceptable; however, care must be taken to clean the magnifying lens in the latter type, which is also larger and sometimes more difficult to use.
Wood’s lamps produce a wide range of fluorescent colors depending upon the substrate being examined. Many things fluoresce under a Wood’s lamp including scales, sebum, seborrhea, crusts, and topical agents. True M canis fluorescence is bright apple green and infected hairs glow from the bulb to the tip. Skin oils often produce a blue-green to orange fluorescence. Some fabric fibers, especially synthetic carpets, produce bright apple-green fluorescence, the color most consistently associated with M canis infections. A Wood’s lamp is only a screening tool for M canis infections, however, because not all strains will fluoresce.
Points to Remember
- Wood’s lamp examinations are effective only as a screening tool for M canis infections. Negative fluorescence does not rule out dermatophytosis.
- Wood’s lamp examinations are cost- and time-effective when examining known culture-positive animals (M canis) or highly suspect animals (eg, cats with skin lesions).
- Wood’s lamps can be used in any species susceptible to M canis infections, but because dermatophytosis is more common in cats and more pleomorphic in presentation, it is considered a core diagnostic test in cats with skin disease.
- Fungal culture is always recommended to confirm the infection.
- Allow the lamp to warm up for 3 to 5 minutes to be sure it is being used at full power. Perform the test in complete darkness to allow for the examiner’s retinas to be totally dark-adapted. The gradual “development” of fluorescence is the result of one’s eyes becoming adapted to the light.
- Most mistakes made with the Wood’s lamp are due to failure to allow the lamp to warm up completely, not having the room dark enough, not taking enough time to perform a complete examination, and not allowing time for the examiner’s eyes to adapt to darkness.