An update on dermatophytosis (Proceedings)

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.

Treatment options are wide and varied. It is important (especially in older animals) to screen for underlying issues prior to treatment. This may impact which treatments are used and also affect the prognosis of the therapy. There are many approaches to treatment, but in general you will need to treat the pet(s) both topically and systemically and consider the environmental contamination as well.

A big question that I always receive from practitioners is "Do I clip the hair?" There are many opinions regarding this. This needs to be evaluated on a case by case basis. When clipping is done, it should be done with a #10 blade, not a #40 blade. In general, it may not be necessary in every single cat household or in every short haired feline, but may facilitate the "dipping" process. In catteries, it is beneficial. In cases of dermatophytosis in household with small children, elderly people or immunosuppressed individuals, or in long-haired cats, it is quite important to clip the hair coat. In all cases, it will decrease the potential environmental contamination by the hairs. However, I usually strongly recommend treating the pets topically 1-2 times before clipping as I have consulted on several cases where the clinic became contaminated after clipping infected cats!!

Topicals may include local treatment. Many clients are very adverse to more aggressive therapy when they only see 1 lesion on their pet. However, it is best to treat the whole animal as there may be affected areas that have not been detected The best way to apply topicals is by whole body rinses 1-2 times per week. Many topicals are available. One of the most efficacious therapies is lime sulfur rinse. This is malodorous, can stain, but is quite safe. This is generally the preferred treatment in the U.S. Enilconazole has been used in other countries for the dog and horse (not approved for the cat), but is not sold for this application in the U.S. For cats in particular, an E-collar should be applied until the cats are dry to prevent licking the wet solution. Another option is shampoos (miconazole or ketoconazole and chlorhexidine) which may aid in the treatment when used with oral medications, but generally are not as effective as the rinses. Topicals, in any case, are best used in conjunction with systemic antifungals.

Many systemic medications are available. The most commonly used in veterinary medicine for dermatophytosis include: griseofulvin, ketoconazole, itraconazole, fluconazole and terbinafine.

Griseofulvin is becoming increasingly difficult to obtain. This is very teratogenic and should not be used in pregnant animals. Other side effects may include: unpredictable bone marrow suppression, hepatotoxicity or dermatitis. All cats should be tested for FIV or feline leukemia virus prior to usage as these conditions may predispose to neutropenia or panleukopenia.

Ketoconazole, although relatively inexpensive, should not be used in cats due to a high rate of hepatotoxicity and the availability of less toxic medications for the cats. It is still used in dogs for dermatophytosis, but can cause hepatotoxicity especially at higher dosages.

Itraconazole, although somewhat expensive, is probably the most favored drug currently for the treatment of dermatophytosis. Avoid the temptation to use the bulk compounded formulations even though they are less expensive as they are not equivalent due to low solubility and poor stability. Toxicity of itraconazole is primarily manifested as anorexia and hepatotoxicity (usually manifested by increased ALT). Dosages vary from 5-10 mg/kg/day. Two different pulse regimens have been used: 5 mg/kg once daily on an every other week basis or 5 mg/kg for 2 consecutive days per week. Both pulse therapies decrease the likelihood of hepatoxicity and are most cost effective.

Fluconazole has had mixed reviews. Some suggest that it has decreased activity against Microsporum canis.

Terbinafine has had limited usage in veterinary medicine, but appears to be well tolerated. The most common side effects to date in pets have been gastrointestinal, but liver issues have been reported in humans. The dosage ranges from 30-40 mg/kg/day PO.

All the azoles are capable of causing nausea, anorexia, vomiting. No definitive studies have been done regarding usage in pregnancy for the azoles or for terbinafine.

Treatment should not be discontinued until 2-3 negative consecutive fungal cultures are obtained at weekly or bi-weekly intervals.

In conclusion, dermatophytosis can present in multiple ways. There are many methods of treating this, but the best involves topical and systemic therapy and great dedication by the person or persons doing the therapy. Environmental control needs to be addressed in cases of dermatophytosis on an individual basis as well.