FELINE DERMATOPHYTOSIS: A PRACTICAL APPROACH
César Yotti Alvarez
Ctro Dermatológico Veterinario Skinpet I-II
C/Nazaret 2 (Móstoles)
C/ Andrés Torrejón 18 (Madrid)
Etiopathogeny
The most common dermatophytes infecting cats are Microsporum and Trichophyton, with M. Canis being
the most common by far. Other less common pathogens are M.persicolor or M.gypseum.
M. Canis is not a common resident of feline skin. Therefore, isolating it always means there is infection or
contamination of the sample from the environment or fomites such as forceps for sample collection.
Dermatophytes are transmitted by contact with infected hairs or scales, from sick animals, the
environment or fomites.
The hair is invaded by ectotrix and endothrix spores and hyphae will eventually colonize the ostium of the
hair follicle, proliferating on the surface of the hair and producing keratolytic enzymes that dissolve the
hair cuticle.
Fungus components may occasionally cause an immunological reaction that causes itch due to the
release of various compounds such as:
- Cell wall carbohydrates; mannans or chitin.
-
Cell wall proteins such as glycoproteins
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Keratinases.
Prevalence
The true prevalence of the disease in feline species is little known. Traditionally, the prevalence of feline
ringworm was considered high but, in fact, there are currently no scientific data to support this assertion.
In a recent review of the most common feline skin diseases, ringworm was not listed among the ten most
common diseases.
Clinical signs
Clinical signs of ringworm are extremely variable and heterogeneous, depending mainly on two factors:
The pathogenesis of infection
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Patient Health Status
The most common clinical pictures, in order of frequency, are:
- Focal or multifocal flaky alopecia.
-
Papulocrusted dermatitis or miliary dermatitis.
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Recurrent feline acne
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Tail gland seborrhoea
-
Exfoliative erythroderma
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Onychomycosis
-
kerion
-
Pseudomycetoma, subcutaneous ulcerated nodule
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Annular dark-haired areas and longer hair in Devon Rex.
Diagnostic workup
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Wood's lamp is an ultraviolet lamp with a wavelength of 253.7 nm that is filtered through a cobalt
or nickel filter. 70-90% of hairs invaded by M.canis can emit a yellowish green fluorescence due to
tryptophan metabolites produced by the fungus. Always use lamps plugged into the power grid
and equipped with a good-quality magnification system. Hair observation must be made very
close to the skin surface to avoid artifacts that distort the interpretation. Its reliability has been
classically considered moderate, although recent research revealed a high reliability in the hands
of an experienced examiner. The use of Wood's lamp has also proved useful in monitoring
response to treatment of feline patients with ringworm as the fluorescence moves toward the distal
region of the hair as it grows free from infection in the affected follicle.
-
Direct examination of the hair is a valuable technique but you need to have experience in the
interpretation of trichograms. Ideally, hairs selected based on the fluorescence observed under
Wood's lamp are then collected on a slide and suspended in mineral oil to for immediate direct
observation. KOH can be employed in some cases to clear the sample and facilitate observation,
although it requires 10-20 minutes of incubation prior to the observation of the sample. This
technique allows us suspecting the disease if hairs with loss of structure, enlarged or swollen, with
a complete loss of demarcation of the cuticle, cortex and medulla are found.
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Dermatophyte test medium. (DTM)
DTM was introduced as a routine diagnostic method for ringworm in 1970. Its principle is based on the
addition of cycloheximide to limit the growth of saprophytic fungi, the antibiotic chloramphenicol to inhibit
the growth of contaminating bacteria and a pH indicator, phenol red. Dermatophyte fungi increase the
medium's pH during its development, so that the original yellow-orange color of the culture medium
changes to red during the development of the colony. Although it is a very practical method, it has many
limitations such as the false negative results due to massive growth of saprophytic fungi or false positive
results by alkalinization of the medium by bacterial elements or non-pathogenic fungi.
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PCR
It is not recommended as a reference diagnostic test due to its inability to differentiate between sick
animals and animals whose fur is contaminated by spores from the environment.
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Biopsy / Special stains
It is specially useful in the case of nodular lesions such as pseudomycetoma in Persian cats or pustular
lesions that mimic autoimmune pemphigus-like diseases, particularly with infections caused by
Trichophytum. Its main limitation is that it does not allow the identification of the responsible fungus, and
culture or PCR from tissue samples are required for that.
Treatment
The treatment of ringworm necessarily involves considering several elements in a combined mode:
1- Isolation.
Any patient with ringworm should be confined inside home to prevent reinfection from the environment or
fomites. In multi-cat homes, the diseased cat must be physically isolated from healthy individuals. It is
very important to avoid sharing grooming or cleaning tools with other cats living in the same environment.
2- Cleaning
The spores are mostly found on the hair and are easily removed using mechanical or vacuum cleaning or
washing with soapy water. It is generally unnecessary to use the traditionally recommended more
powerful and aggressive products such as bleach.
The fabrics in contact with the sick cat should be washed twice a week, with long washing cycles and
being careful to not "overloading" the capacity of the washing machine.
For a successful treatment of ringworm, in general, it is essential to treat on two fronts: topical and
systemic.
3- Topical Treatment
Shaving the coat prior to topical treatment, using scissors or a #10 blade is highly recommended in most
cases, especially in Persian or any long haired cats, to decrease the spore load of the patient. However, it
is advisable to perform a gentle shaving, as there are reports of spreading the disease after a too
aggressive shaving, producing micro-abrasions easily colonized by fungal spores.
It is advisable to perform baths or rinses two or three times per week, for at least three consecutive
weeks.
- Chlorhexidine-miconazole shampoo
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Climbazole foam
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Enilconazole
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Lime sulphur
4- Systemic treatment
The currently recommended drugs for the treatment of feline dermatophytosis are terbinafine and
itraconazole, at least for 21 consecutive days or, in the case of Itraconazole, 6 alternate weeks at doses
of 5 mg / kg every 24 hours.
- Itraconazole 10 mg / kg every 24 hours
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Terbinafine 20-40 mg every 24 hours
References
-
Moriello K; dermatophytosis: Diagnosis and Effective Treatment “ This Much I know to be
true” 8 WCVD Burdeos 2016
-
Frymus T, Gruffydd-Jones T, Pennisi MG, Addie D, Belák S, Boucraut-Baralon C, et al.
Dermatophytosis in cats. ABCD guidelines on prevention and management. J Feline Med Surg
2013; 15: 598-604
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28. Newbury S, Moriello KA, Kwochka KW, Verbrugge M, Thomas C. Use of itraconazole
and either lime sulphur or Malaseb Concentrate Rinse® to treat shelter cats naturally infected
with Microsporum canis: an open field trial. Vet Dermatol 2011; 22: 75-79.