Methodologies

• Culture is best at room temperature and a positive result may be seen in as little as 5 days.
• Leave the culture for up to 14 days before treating as negative. Increased humidity is helpful for fungal growth and a pan of water in the incubator will provide this.
• Identifying particular dermatophytes is not often possible with DTM as macroconidia are not produced. Macroconidia are more commonly produced by colonies older than seven days on BCP-milk solids-yeast extract agar.
• Setting up a subculture on Sabouraud’s dextrose agar will culture colonies that sporulate and mature macroconidia can then be identified using a fungal guide (experience required).
• The most common dermatophytosis of cattle is Tricophyton verrucosum. This fungus has a requirement for thiamine and some varieties also for inositol. It is also very slow growing and may need to be incubated at 37° and for up to 1 month.
• Colonies on Sabouraud’s agar are white to cream-colored sometimes ochre, they are initially waxy, generally small and raised showing folds, and they rarely show macroconidia without vitamin enrichment.
• Once colonies are seen, take a small square of acetate tape e.g. Scotch pressure sensitive tape® (3M), holding it in forceps touch it lightly to the surface of the culture then place it on a glass slide with a drop of lactophenol cotton blue and examine under x100 magnification. Chains of chlamydospores are seen described as like fir trees with club-shaped tips.

Availability

• Fungal culture can easily be carried out in the practice situation. DTM is commercially available eg Dermaphyt^R (Kruuse).
• Alternatively, all commercial laboratories will offer the test.

Validity

Sensitivity

• As alluded to before, choosing sample material carefully remains the single most important determinant of success.
• Trichophyton verrucosum can be fastidious in its cultural requirements and may not always grow.

Specificity

• Visual analysis of gross cultural morphology and microscopic analysis of hyphal form and macroconidial morphology is necessary for species identification and requires experience

Technique intrinsic limitations

• Fastidious cultural requirements.
• Overgrowth by non-pathogenic fungi or bacteria on Sabouraud's dextrose agar plates.

Technician extrinsic limitations

• Veterinarian or laboratory technician experience required.