An update on dermatophytosis (Proceedings)

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An update on dermatophytosis (Proceedings)

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Aug 01, 2009

Dermatophytosis is another of our common diagnoses, especially in general practice. Yet, it sends chills down many of our spines as we know that it may be difficult to treat depending on the situation.

The first step in the process is to recognize the clinical signs. Unfortunately, dermatophytosis can present with a multitude of clinical signs. Clinical signs may include pruritus or no pruritus, alopecia and scaling or crusting, erosive or ulcerative lesions, paronychia, folliculitis or furunculosis, feline "miliary dermatitis", feline "acne" and occasionally non-healing wounds or ulcerated/draining nodules. Some of our patients are seemingly lesion free, but are asymptomatic carriers.

Therefore, as usual, history becomes very important. For instance, the history of a recent arrival of a kitten from a cattery, shelter or other place that houses multiple animals may signal a search for dermatophytes. Recent addition of a stray animal to the fold may promote testing for dermatophytes. Finally, as in a number of our dermatologic diseases, dermatophytosis is contagious and zoonotic. Therefore, it should be asked, "Are there any other pets or humans in the household with signs suspicious of dermatophytosis?" If so, appropriate diagnostics should be done quickly. For many reasons, the zoonotic diseases are very important to recognize and treat promptly.

Diagnosing this process should be commenced based on clinical and historical suspicions. The usual means of diagnosis would include Wood's lamp, skin scrapings/trichograms, dermatophyte culture and/or histopathology. In my experience and based on several articles, the Wood's lamp is overused as a single diagnostic tool and should be limited to selecting any fluorescent hairs for a dermatophyte culture. Many false negatives and false positives can occur with this test. Additionally, the only dermatophyte of veterinary importance that fluoresces is M. canis. Even then, only some M. canis will fluoresce.

Skin scrapings/trichograms can be a great diagnostic tool as they're simple and inexpensive. When viewing the skin scrapings, you should first start with a scanning objective and look for "sick" hairs, then move to a higher power (high dry). Treatment can be based on finding ectothrix spores or filamentous hyphae at higher power if you're confident with this method. However, a fungal culture should still be performed.

The most reliable and best method of diagnosing dermatophytosis is an actual dermatophyte culture. This can be done in our hospitals or sent to an appropriate laboratory. In our hospital, we use Sab-duets (Bacti-Lab/Hardy Diagnostics). When doing a dermatophyte culture, it is most reliable to collect fluorescent hairs and hairs via toothbrush technique depending upon presenting clinical signs. The collected hairs are then placed on either Saboraud's dextrose agar and/or dermatophyte test media. Most people find the dermatophyte test media easiest to interpret, but it is not perfect. When interpreting the growth on dermatophyte test media, you should expect to see white growth at the same time or shortly after the red color change in the media. Since occasional saprophytes will turn the media red immediately, one should always examine samples from the colony growth microscopically. An advantage to the Sab-duets is that the color change can be seen on the dermatophyte test media and the microscopic sample can be examined from the Saboraud's dextrose agar as the macroconidia are easier to identify from the plain Saboraud's dextrose agar. Dermatophytes generally grow within 7-14 days, but cultures should be retained and examined daily for 3-4 weeks to be sure (especially if the case is already under treatment).

To examine microscopically, clear cellophane tape should gently be pressed to the colony, then placed onto a drop of lactophenol cotton blue on a microscope slide. Then add another drop of lactophenol cotton blue and then a cover slip. This should enable you to visualize the macroconidia. This may take 4-7 days or occasionally much longer.

Occasionally, a suspicious mass, non-healing wound or draining lesion is present where a fungal organism is suspected. Then several samples of representative tissue can be submitted for histopathology. Be sure to alert the histopathologist that you are concerned about dermatophytosis. Additionally, a tissue sample should be submitted in these cases for fungal culture to identify the genus and species as this can not be elucidated with just histopathology.