Experimental diagnosis of canine and feline dermatophytoses.

Additional examinations in feline dermatology should be performed in various circumstances. First, they are obviously necessary when dermatophytosis is suspected. It should be remembered that it presents great clinical pleomorphism and can thus manifest as alopecia, scaling, but also as pruritic dermatosis or even simple hyperpigmented areas. Examinations are also carried out in debilitated cats or kittens with any dermatosis. They are also useful in cases of lesions compatible with dermatophytosis in a person in contact with the animal.

 

Wood’s lamp has partial interest

The first additional examination that can be performed is using a Wood’s lamp. This is an ultraviolet lamp that emits radiation between 320 and 400 nm, causing fluorescence. This varies between yellow and green, or even blue to green, and may be present on different parts of the hair, depending on the evolutionary stage of the dermatosis.
Half of Microsporum canis strains, which is the main species responsible for feline dermatophytosis, cause fluorescence. Thus, the absence of fluorescence does not rule out dermatophytosis, far from it. This absence can occur in the presence of non-fluorescent Microsporum canis strains, during simple cutaneous carriage without infection, dermatophytosis due to another species, prior application of antifungal topicals and, of course, when there is no dermatophytosis. There are also causes of false positives. Thus, sebum and various topicals give a greenish coloration that can sometimes be confused with fluorescence. It is recommended to use electric lamps rather than battery-operated ones. Indeed, the latter can lead to false negatives related to the low light intensity they emit after a certain time. To prevent these false negatives, it is also important to let the lamp warm up, since the wavelength and light intensity depend on its temperature.

Direct examination is quick

It is also possible to perform a direct examination of ringworm hairs, on which spores and hyphae are sought. This is an inexpensive and quick examination to perform.
Hairs can be placed in different agents to improve the visualization of fungal agents. Lactophenol is generally used, but potassium hydroxide or calcofluor are also interesting. Then, the whole is covered with a coverslip and observed under the microscope at 4, 10 and 40x objectives. A little tip is to look for fluorescence with a Wood’s lamp to determine which area to observe preferentially under the microscope. Under microscopic examination, hairs that no longer have any “classic” structure, that are broken, wider, of filamentous appearance, are sought. At a higher magnification, spores and hyphae at the periphery must be observed. This additional examination is interesting, because, unlike the Wood’s lamp, it allows obtaining a definitive diagnosis and the immediate implementation of treatments, without requiring a mycological culture.

Mycological culture is the reference

Mycological culture is the reference method for diagnosing dermatophytosis. It can be performed directly in the clinic or the samples can be sent to a laboratory where people specialized in veterinary mycology work, because the identification of dermatophytosis is delicate.
In the clinic, a DTM (Dermatophyte Test Medium) can be used. The sample can consist of hairs and scales gently placed on the agar. Above all, they should not be buried. DTM media used in veterinary structures have a color indicator that turns red during fungal growth or in the presence of a contaminant, by changing the pH of the medium. If it is a dermatophyte, the color change is simultaneous with fungal growth, while it is late when a contaminant is present. Indeed, the color change only indicates the growth of an element on the agar. However, care should be taken, as some dermatophytes, such as Microsporum persicolor, turn the medium late, like a contaminant.

If a specialized laboratory is chosen, these same samples are sent to it. Other sampling techniques involve using a previously sterilized carpet or a toothbrush. They are vigorously rubbed on the animal and then gently applied to the Sabouraud medium. Once the culture medium is inoculated, it is kept at a temperature between 24 and 27 °C, in the dark. The culture medium must be observed daily, and kept for three weeks.
Whatever the technique used and the place where the culture will be carried out, the definitive diagnosis can only be obtained by performing a Roth flag. This method consists of applying a small piece of scotch tape to the culture before placing it on a slide, after having previously deposited a drop of methylene blue on it. A second drop is placed on top of the scotch tape, before applying a coverslip and proceeding with microscopic examination. Hyphae, spores, vrilles and macroconidia are then sought, which will allow both to establish a definitive diagnosis and to identify the responsible dermatophyte.

Biopsy is rarely required

Skin biopsies are generally unnecessary in the diagnosis of dermatophytoses. However, they may be necessary, particularly in cases of granulomatous infection, masses, or deep pyoderma. Fungal elements are highlighted after PAS staining. If the biopsies do not contain any, it is interesting to add scabs to the formalin, taking care to specify this to the histologist.

The diagnosis of dermatophytosis therefore employs simple techniques, generally inexpensive. However, the interpretation of these additional examinations can be delicate. One should not hesitate to implement another additional examination, or to have it confirmed by a specialized laboratory and, finally, to reconsider the initial diagnosis.