Systematic Veterinary Bacteriology and Mycology

PATHOGENESIS

• Proteolytic enzymes (elastase, collagenase, keratinase) may determine virulence, particularly in severe inflammatory disease.
• The dermatophytes are highly specialized for utilizing keratin as food source. Localization in keratinized epidermis has been attributed to the lack of sufficient available iron elsewhere.
• Keratinase is therefore a clearly recognized virulence factor for dermatophytes.
• It is thought that those dermatophytes that are highly species-specific have a keratinase that can only hydrolyze keratin from a particular animal species, whereas those with more broad acting keratinases can invade the skin of many different species (e.g. T. mentagrophytes).
• The infectious unit conidia enters the skin through an abrasion, germinate and hyphae begin to grow in the stratum corneum.
• Portions of mycelium differentiate into arthroconidia. This growth pattern in the hairless skin predominates with some dermatophytes (M. nanum, T. rubrum).
• Hair invasion, which is prominent in most animal ringworm, begins with germination of a spore near a follicular orifice.
• The hyphae invade the hair follicle and enter the cortex of the hair by dissolving the keratin.
• The hyphae and conidia are carried to the surface by the growing hair, which often breaks off.
• Hair invasion may be endothrix (arthroconidia develop within the hair shaft only and the cuticle remains intact) or ectothrix (arthroconidia develop outside the hair shaft and hyphae are within the hair shaft; the cuticle is destroyed).