Dermatophytosis

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FUNGI



Microsporum canis alopecia and scaling lesions - Courtesy of Bristol BioMed Image Archive
Ringworm in a dog -Courtesy of Bristol BioMed Image Archive
Trichophyton mentagrophytes in a dog - Courtesy of Bristol BioMed Image Archive
Ringworm on a human arm - Wikimedia Commons

General

  • Pigmented, saprophytic organisms called Phaeohyphomycetes
    • Previously called 'Fungi Imperfecti'
  • The two main species of veterinary interest are Microsporum and Trichophton
  • Worldwide
  • They are usually secondary invaders
  • Able to penetrate all layers of skin, but are generally restricted to the keratin layer and its appendages
    • Therefore, most often seen in subcuticular or cutaneous sites
    • Lack of tolerance to body temperature and antifungal activity in serum and body fluids prevent the fungi invading subcutaneously
  • Transmitted by direct or indirect contact
  • Immunuocompromised hosts may develop systemic infections
  • Microsporum - zoophilic
    • Parasites of animals
  • Trichophyton - geophilic
    • Inhabits soil
  • Epidermophyton - anthropophilic
    • Parasites of people
  • Common in many species, especially cats
  • Hot, humid environment predisposes to infection
  • More common in young animals
  • Produce proteolytic enzymes to penetrate surface lipid
  • Fungal hyphae invade keratin -> break into arthrospores
  • Phaeohyphomycosis:
    • Occurs sporadically in cats, horses, cattle, fish, reptiles, amphibians, birds, and rarely in dogs
    • Examples include: Exophiala sp., Phialophora sp., Pseudomicrodochium sp., Bipolaris sp., Moniella sp., Cladosporium sp., Wangiella sp., Curvularia spp., Exserohilum sp., Alternaria sp., Staphylotrichum sp., and Xylohypha sp
    • Culture is necessary for definitive diagnosis

Pathogenesis

  • Superficial perivascular dermatitis -> exocytosis (migration through epidermal layers) -> intracorneal microabscesses
  • Exocytosis -> folliculitis -> furunculosis
  • Highly variable lesions
  • Normal -> eruptive nodular -> pseudomycetoma -> onychomycosis
  • Secondary invasion by Staphylococcus aureus and Staphylococcus intermedius are common and cause pustules in the hair follicles
  • Grossly:
    • Circular or irregular lesion, may coalesce
    • Scaly to crusty patches
    • Alopecia due to broken hair shafts and hairs lost from inflammed follicles
    • Follicular papules and pustules
    • Peripheral red ring (ringworm) due to dead fungi in areas of inflammation at centre of lesions and viable fungi peripherally
  • More common in housed animals, rather than animals turned out to pasture
  • Highest incidence of disease during the winter
    • May resolve spontaneously in the spring and summer

Histology

  • Perifolliculitis, folliculitis or furunculosis
  • Epidermal hyperplasia
  • Intracorneal microabscesses
  • Septate hyphae or spores may be found in stratum corneum and keratin of hair follicles

Diagnosis

  • Wood's Lamp
    • UV light
    • Florourescence if fungi present
  • Samples can be examined in 10-20% KOH for the presence of hyphae or arthrospores
    • Lactophenol Cotton Blue enhances visualisation
  • Sabouraud's Dextrose agar containing cyclohexamide and chloramphenicol at room temperature for a month for culture
  • Dermatophyte Test Medium
    • Saubouraud's Dextrose agar with phenol red indicator
    • Medium changed from yellow to red

Treatment

  • Isolation of infected animal
  • Precautions should be taken to prevent human infection
  • Griseofulvin best method
    • Expensive
    • Oral dosage
    • Prolonged treatment required
  • Whitfield's ointment
    • Salicylic and benzoic acid
  • Other tretment:
    • Aqueous lime sulphur topically for dogs
    • Iodine
    • Antibiotics, e.g. natamycin
    • Imidiazole derivatives

Further Links