VMC 311: Systematic Veterinary Bacteriology and Mycology (2+1)

PATHOGENICITY

Symptoms

• Although not fatal, dermatophytosis can be a cause of significant economic loss and a source of infection for man.
• One of the first clinical signs is loss of hair, followed by an inflammatory reaction of the skin due to the host response.
• Dermatophytosis occurs more commonly in very young, old, or sick animals and most often in stabled rather than pastured animals.
• The peak incidence occurs in the winter. The characteristic lesion is a hyperkeratosis with septate hyphae and arthroconidia in the stratum corneum.
• Invasion of the hair causes the shaft to become weak and break, resulting in circular, scaly areas of alopecia with or without crust formation.
• Arthroconidia within or outside the hairshaft are also referred to as arthrospores.
• Manifestations range from erythema to vesiculopustular reactions and suppuration.
• Mild forms are seen in T. verrucosum infection of calves. Severe reactions are typical in T. mentagrophytes infection of dogs and M. gypseum infec­tion of horses.
• Local plaques (kerion) may resemble certain skin tumors, especially in dogs.
• The inflammatory reaction may arrest the mycotic infection but become the primary problem through secondary suppurative bacter­ia! infection.
• The roughly circular pattern of the lesions and their inflamed margins suggested the terms ringworm and tinea (Latin for worm). Different tineas are
  • Tinea barabe – beard
  • Tinea capitis – scalp
  • Tinea corporis – body
  • Tinea cruris – groin
  • Tinea favosa – favus
  • Tinea imbricata and Tinea manum – hands
  • Tinea pedis – feet
  • Tinea unguium – nails

Lesions

• Lesions are very similar in different animal species.
• The most common areas of infection on dogs and cats are the head and extremities.
• In horses and sheep the neck and girth are most often infected.
• In cattle, the most common sites are the head and neck.
• In fowl, the disease is referred to as fowl favus or white comb, due to the white, moldy crusts that develop on the comb and wattle.
• Hyphae that invade the stratum corneum induce a hyperkeratosis.
• The resulting inflammatory response by the host is most intense at the area of recent invasion.
• The infection spreads in all directions, giving a ring-like appearance.
• The most active site of infection is at the periphery, while the central area begins to heal.
• Therefore, specimens for culture and examination should always be collected from the edge of the lesion.
• The living skin is not infected. The optimal growth temperature of dermatophytes is 30° C; most grow poorly, if at all at 37° C and, therefore cannot infect deeper tissues.
• The host may develop a hypersensitivity to the invading fungus that may result in vesicular lesions developing in various parts of the body.
• These "id" reactions are thought to be due to fungi or their products disseminating in the body and to an immune reaction.
• The resulting hypersensitivity reaction to the fungus may be delayed-type, or immediate.
• Infections due to M. canis are often associated with kerions, which are vesicles in and around the lesion.