Overview

• Dermatophyte test medium (DTM) is a selective medium for dermatophytes and is suitable to diagnose dermatophytosis in rabbits.
• It contains Sabouraud dextrose agar with added gentamycin, chlortetracycline and cyclohexamide which inhibit non-dermatophyte growth. Phenol red is included as a pH indicator.
• Dermatophytes utilize protein in the medium and create an alkaline pH which changes the medium from yellow to red.
• The medium will change color soon as dermatophyte mycelium growth is noted.
• Saprophytic fungi utilize carbohydrates first, so no color change occurs initially. Once carbohydrates are depleted and protein is utilized the medium will then change color after several days.
• It is important to observe the DTM daily as a color change either before colonies are seen or during early visible growth is positive for dermatophytes.
• DTM inhibits conidia formation and makes colony morphology less distinctive. Species identification requires additional culture on plain Sabouraud’s dextrose agar (SDA).

Sampling

Tests

Methodologies

• Culture can be performed at room temperature, but 30°​C/86°​F is considered optimal; a small water dish should be added to the incubator to prevent insufficient humidity.
• Avoid direct light as UV light inhibits fungal growth. Placing samples and cultures in a dark area is advised.
• Positive results can be obtained after 5 days but culture must be continued for 14 days before a sample can be considered negative.
• Species identification requires additional culture on plain Sabouraud’s dextrose agar:
  • Gross colony morphology is less distinctive on DTM.
  • Microscopic examination of conidia is required for definitive species identification and DTM inhibits growth.
• On SDA species morphology is very distinctive:
  • Trichophyton mentagrophytes Trichophyton spp colonies are flat with a powdery to granular surface and may have a raised central tuft or folding. Color varies from white to buff with a yellow-brown underside.
  • Microsporum canis Microsporum canis colonies appear white and resemble cotton wool. They often have an orange-yellow undersurface.
• Once colonies are observed, forceps can be used to gently touch the colony surface with a small section of acetate tape, eg Scotch pressure sensitive tape® (3M). Place samples on a slide with lactophenol cotton blue and examine under x100.
• T. mentagrophytes conidia are long and cigar-shaped whereas M. canis are spindle-shaped, thick-walled and have a terminal knob .

Availability

• Fungal culture can be performed in-house as long as samples are examined daily.
• Commercial point-of-care test kits are available, eg Dermaphyt® (Kruuse).
• Many commercial laboratories will offer the test, but samples should be submitted in paper to avoid false-negative results.

Validity

Sensitivity

•  Careful selection of sample material has a significant impact on successful culture.

Specificity

• Colonies are more easily identified on plain SDA.
• Microscopic analysis of hyphae and macroconidia structure is required to identify species in addition to colony morphology.
• False results can occur if interpretation relies on assessing only medium color change.  

Technique (intrinsic) limitations

• Fastidious cultural requirements.
• Plain SDA plates are prone to overgrowth by bacteria and saprophytes.
• DTM limits ability to identify species morphology.

Technician (extrinsic) limitations

• Interpretation is dependent on experience.

Result Data

Further Reading

Publications

Refereed Papers

• Recent references from PubMed and VetMedResource.
• Coyner K S (2010) How to perform and interpret dermatophyte cultures: Use this guide – put together by a veterinary dermatologist – to maximize your success with this indispensable in-house test. Vet Med 105 (7), 304-307.
• Borman A M, Szekely A, Campbell C K & Johnson E M (2006) Evaluation of the viability of pathogenic filamentous fungi after prolonged storage in sterile water and review of recent published studies on storage methods. Mycopathologia 161 (6), 361-368 PubMed.
• Canny C J & Gamble C S (2003) Fungal diseases of rabbits. Vet Clin North Am Exotic Anim Pract 6 (2), 429-433 PubMed.
• Sinski J T, Wallis B M & Kelley L M (1979) Effect of storage temperature on viability of Trichophyton mentagrophytes in infected guinea pig skin scales. J Clin Microbiol 10 (6), 841-843 PubMed.

Other sources of information

• Miller W H, Griffin C E & Campbell L (2013) Diagnostic Methods. In: Muller & Kirk’s Small Animal Dermatology. 7th edn. Elsevier Inc, USA. pp 86-91.