322 DISEASES/DISORDERS
DIFFERENTIAL DIAGNOSIS
Dermatophytosis may mimic many scaling and alopecic dermatoses in the dog (e.g.,
bacterial folliculitis, demodicosis, hyperkeratotic disorders, endocrine dermatoses)
and cat (e.g., allergy, lymphocytic mural folliculitis, cutaneous lymphoma) and must
be ruled in/out during an initial diagnostic plan.Acantholytic dermatophytosis mimics clinical lesions consistent with pemphigus
complex.
DIAGNOSTICS
Wood’s Lamp Examination
Variable screening tool; poor confidence in this test may be due to inadequate equip-
ment and diagnostic techniques; some pathogenic dermatophytes do not fluoresce.
Fluorescence is due to a pigment on the hair and is not associated with arthrospores or
the infection itself. As the infection resolves, the pigment will be lost in the proximal
portion of the hair shaft while the tips may demonstrate residual fluorescence.Spontaneously occurring felineM. canis: 72% of cats fluoresce.
False fluorescence is common; medications, keratin associated with epidermal scales,
and sebum may all produce false-positive fluorescence yet the color is different from
that seen in dermatophytosis.Topical therapy will not remove fluorescence.
Plug-in lamps are more effective than battery-operated lamps.
A true positive reaction associated withM. canisconsists of apple-green fluorescence
of the hair shafts (Figure 21.14).
Microscopic Examination of Hair
Examination of plucked hairs placed in mineral oil can help provide a rapid diag-
nosis. Positive and negative predictive values are as high as 93% with an educated
investigator. KOH preparation is not necessary.Infected hairs often appear broken or misshapen due to ectothrix invasion of the
cuticle (Figure 21.15).
Use hairs that fluoresce under Wood’s lamp illumination to increase the likelihood of
identifying the fungal hyphae associated with the hair shaft.
Saprophytic fungal spores may be visualized microscopically (dermatophytes do not
form macroconidia in tissue but may demonstrate a yeast form in cytology) (Figure
21.16).Fungal Culture with Identification
Hairs that exhibit a positive apple-green fluorescence under Wood’s lamp examination
are ideal candidates for culture.