Dermatophytosis

FUNGI

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

• 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

• 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 changes from yellow to red if fungi present

Treatment

• Isolation of infected animal

• Precautions should be taken to prevent human infection

• Griseofulvin best method of treatment
  • Expensive
  • Oral dosage
  • Prolonged treatment required

• Whitfield's ointment
  • Salicylic and benzoic acid

• Other treatments:
  • Aqueous lime sulphur topically for dogs
  • Iodine
  • Antibiotics
  • Natamycin antifungal
  • Imidiazole derivatives

Further Links

• Pathology of dermatophytosis

• Antifungal Drugs

Images

Dermatophyte arthrospores on a hair root Copyright Professor Andrew N. Rycroft, BSc, PHD, C. Biol.F.I.Biol., FRCPath Dermatophytosis mycelium in skin Copyright Professor Andrew N. Rycroft, BSc, PHD, C. Biol.F.I.Biol., FRCPath Dermatophyte in skin using a KOH mount Copyright Professor Andrew N. Rycroft, BSc, PHD, C. Biol.F.I.Biol., FRCPath Copyright Professor Andrew N. Rycroft, BSc, PHD, C. Biol.F.I.Biol., FRCPath Ringworm on a human arm Wikimedia Commons Dermatophytosis in a dog Copyright Professor Andrew N. Rycroft, BSc, PHD, C. Biol.F.I.Biol., FRCPath Dermatophytosis in a cow Copyright Professor Andrew N. Rycroft, BSc, PHD, C. Biol.F.I.Biol., FRCPath Dermatophytosis in a chicken Copyright Professor Andrew N. Rycroft, BSc, PHD, C. Biol.F.I.Biol., FRCPath Dermatophytosis lesion on a dog's leg Copyright Professor Andrew N. Rycroft, BSc, PHD, C. Biol.F.I.Biol., FRCPath Dermatophytosis in a horse Copyright Professor Andrew N. Rycroft, BSc, PHD, C. Biol.F.I.Biol., FRCPath Microsporum canis Copyright Professor Andrew N. Rycroft, BSc, PHD, C. Biol.F.I.Biol., FRCPath Microsporum canis macroconidium Copyright Professor Andrew N. Rycroft, BSc, PHD, C. Biol.F.I.Biol., FRCPath Microsporum canis dysgonic at 3 weeks Copyright Professor Andrew N. Rycroft, BSc, PHD, C. Biol.F.I.Biol., FRCPath Microsporum equinum Copyright Professor Andrew N. Rycroft, BSc, PHD, C. Biol.F.I.Biol., FRCPath