Species which are pathogenic to dogs include:
- Trichophyton mentagrophytes
- Trichophyton rubrum
- Trichophyton erinacei
- Trichosporon cutaneum
- Trichophyton ajelloi
- Trichophyton terrestre
These dermatophytes commonly cause limited disease, that is more annoyance than pathology, in dogs.
Infection is usually by direct contact, although fomites, mechanical transmission and aerosol are other means of transmission. Dermatophytes reside entirely on the skin surface, feeding on stratum corneal cells. However, the presence of the fungus and its metabolic products usually induces an allergic and inflammatory eczematous response in the host. The type and severity of the host response is often related to the species and strain of dermatophyte causing the infection. Secondly, the dermatophytes are the only fungi that have evolved a dependency on human or animal infection for the survival and dissemination of their species. Proteolytic enzymes have been postulated to be key factors involved in the invasion of the stratum corneum and keratinized epidermal structures. Among these proteases, the secreted subtilisin protease Sub3 was found to be required for adherence of M. canis arthroconidia to feline epidermis.
These properties of dermatophytes distinguish them from saprophytic and opportunistic dematiaceous fungi which usually gain entry to the dermis and subcutaneous tissue via wounds, grass seed-penetrating injuries, etc., and result in phaeohyphomycosis, leading to subcutaneous and systemic diseases and often death.
Dermatophytes must also be distinguished from certain zoophilic yeasts, especially Malassezia spp, which is also common in dogs and reside superficially on the skin. Yeasts cause more generalised dermatological disease (otitis externa, seborrhea, paronychia), often covering the entire skin of the host, whereas dermatophytes are often localised to the head and extremities of the limbs.
Dermatophytes normally reside in wood, soil and plant debris and are found world-wide. There appears to be no seasonal regularity to outbreaks in animals and have been reported year round. The first infection step consists of adherence of arthroconidia to the stratum corneum. After germination of the arthroconidia, dermatophytes invade keratinised structures that have to be digested into short peptides and amino acids to be assimilated.
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