ISSN 2398-2977      

Dermatology: Wood's lamp examination

pequis

Introduction

  • The tryptophan metabolites of certain dermatophytes, eg Microsporum canis, M. equinum, M. audouinii, M. distortum and Trichophyton schoenleinii fluoresce under ultraviolet light of 253.7 nm wavelength.
  • A Wood's lamp is a source of ultraviolet light filtered through a cobalt or nickel filter.
  • Trichophytin equinum, T. mentagrophytes, T. verrucosum and M. gypseum most commonly affect equine Dermatophytosis whereas the fluorescence-producing species are a very rare cause of pathology in the horse thus Wood's lamp examination is rarely useful in these patients.

Uses

  • Screening for Microsporum canis, M. equinum, M. audouinii, M. distortum and Trichophyton schoenleinni infection Dermatophytosis.
  • Selection of appropriate hairs for culture.
    Positive Wood's lamp examination is only suggestive, not diagnostic of Microsporum canis infection - a negative finding does not rule out infection.

Advantages

  • Rapid screening technique for dermatophyte infections that produce fluorescence.

Disadvantages

  • Fungal culture or the presence of arthrospores on direct microscopic examination of hairs is essential for diagnosis, so Wood's Lamp examination is a complementary aid.
  • Fungal culture is needed to determine the species of the organism and is a more sensitive method of diagnosing dermatophytosis than hair examinations.
  • Not all (but most) cases of Microsporum canis infection fluoresce.
  • Some topical products, soap residue and seborrhoeic material may fluoresce (but lacking the apple green/emerald green of M. canis).
  • Only actively infected hairs fluoresce.
  • If hairs are positive on Wood's lamp examination, these hairs should be submitted for fungal culture or observed under the microscope for fungal arthrospores.

Requirements

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Preparation

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Procedure

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Sloet van Oldruitenborgh-Oosterbaan M M & Grinwis G C M (2018) Clinical pathology in equine dermatology. Equine Vet Educ 30 (7), 377-385 VetMedResource.
  • Moriello K A, Coyner K, Paterson S & Mignon B (2017) Diagnosis and treatment of dermatophytosis in dogs and cats. Clinical Consensus Guidelines of the World Association for Veterinary Dermatology. Vet Dermatol 28 (3), 266-e68 PubMed.
  • Sparkes A H, Werrett G Stokes, C R & Gruffydd-Jones T J (1994) Improved sensitivity in the diagnosis of dermatophytosis by fluorescence microscopy with calcafluor white. Vet Rec 134 (12), 307-308 PubMed.

Other sources of information

  • Scott D W & Miller W H Jr (2011) Diagnostic Methods. In: Equine Dermatology. 2nd edn. Saunders, USA. pp 35-100.
  • Scott D W & Miller W H Jr (2011) Fungal Skin diseases. In: Equine Dermatology. 2nd edn. Saunders, USA. pp 171-211.
  • Knottenbelt D C (2009) Pascoe’s Principles and Practice of Equine Dermatology. 2nd edn. Saunders, USA.

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