Dermatophytosis

FUNGI

• Dermatophytes in dermatophytosis

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)

Dermatiaceous fungi are pigmented, saprophytic organisms - Phaeohyphomycetes

• They usually infect animals secondary to traumatic implantation of the organisms, and are therefore most often seen in subcuticular or cutaneous sites.
• In immunuocompromised hosts they may develop systemic infections.

Phaeohyphomycosis:

• It occurs sporadically in cats, horses, cattle, fish, reptiles, amphibians, and birds, and rarely in dogs.

Fungi implicated in animal phaeohyphomycosis 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.

• Caused by dermatophytes
  • Microsporum - zoophilic
  • Trichophyton - geophilic
  • Epidermophyton - anthropophilic
• Common in many species, especially cats
• Hot, humid environment predisposes and viable fungi peripherally
• More common in young animals
• Produce proteolytic enzymes to penetrate surface lipid
• Fungal hyphae invade keratin -> break into arthrospores
• Epidermal hyperplasia (hyperkeratosis, parakeratosis, acanthosis) and inflammation
• Superficial perivascular dermatitis -> exocytosis (migration through epidermal layers) -> intracorneal microabscesses
• Exocytosis -> folliculitis -> furunculosis
• Highly variable lesions
• Normal -> eruptive nodular -> pseudomycetoma -> onychomycosis
• 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
• Microscopically:
  • Perifolliculitis, folliculitis or furunculosis
  • Epidermal hyperplasia
  • Intracorneal microabscesses
  • Septate hyphae or spores may be found in stratum corneum and keratin of hair follicles