Dermatophytosis

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)

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 and viable fungi peripherally

• 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

Microscopically

• 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

• Pathology of dermatophytosis