CHAPTER 21 DERMATOPHYTOSIS 323
Pluck hairs from the periphery of an alopecic area.
Use a new toothbrush to collect hairs from an asymptomatic animal (Figure 21.17).
Dermatophyte test media: Sabouraud’s dextrose agar with phenol red indicator: der-
matophyte growth changes media color from yellow to red as it becomes alkaline;
dermatophytes produce color change simultaneously with the early growing phase of
the culture; saprophytes may cause color change after significant colony growth, so it
is important to examine the media daily to match growth with potential color change
(Figure 21.18).Microscopic examination of the culture growth for microconidia and macroconidia is
necessary to confirm a pathogenic dermatophyte; identify genus and species to help
identify the source of infection;Trichophytonspp. are often more difficult to identify
(Figure 21.19).Dermatophyte colonies are white to buff in color; contaminants are often blue, green,
black, or dark brown (Figure 21.20).
Positive culture indicates presence of a dermatophyte; organisms may be transient
(i.e., geophilic dermatophytes on the feet).
Skin Biopsy
Not usually required for diagnosis.
Can be helpful in confirming true invasion and infection, or to diagnose suspicious
cases with negative fungal culture.
Most helpful in cases of granulomatous dermatophytosis (kerion, pseudomycetoma).
Histopathology results: folliculitis, perifolliculitis, or furunculosis are common;
hyperkeratosis, intraepidermal pustules, and pyogranulomatous reaction pattern may
occur.Fungal hyphae may be observed in H&E-stained sections; special stains allow easier
visualization of the organism (Figure 21.21).
THERAPEUTICS
Drugs of Choice
Itraconazole: dogs: 3–5 mg/kg PO q24h, refractory cases 10 mg/kg PO BID; cats:
3–5 mg/kg PO q24h alternating dose (week on/week off) may be used after the first
week of daily dose; available in 100 mg capsules and as 10 mg/mL liquid contain-
ing cyclodextrin; liquid preferred over compounded formulations due to absorption
variability; best for small dogs and cats.Griseofulvin: no longer a drug of choice: microsized formulation: 25–60 mg/kg PO
BID to q24h for 4–10 weeks; ultramicrosized formulation: 2.5–15 mg/kg PO BID to
q24h; absorption increased by dividing the dose twice a day and giving with a fatty
meal; higher doses are associated with an increased likelihood of toxicity and should
be used with extreme caution; gastrointestinal upset is the most common side effect –
alleviate by reducing the dose or dividing the dose for more frequent administration;