Fungal infections

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Fungi commonly cause superficial skin diseases in horses but are responsible for severe and often fatal systemic infections. The most common and least pathogenic fungus which affects horses is Trichophyton spp and Microsporum spp, which are responsible for causing 'Ringworm'.

Other fungi are cultured included Aspergillus spp, Penicillium spp, Cladosporium spp, Scopulariopsis spp and lipophilic yeasts of the genus Malassezia spp.

Pathology

Exposure to or contact with a dermatophyte does not necessarily result in an infection. Infection may not result in clinical signs. Dermatophytes grow in the keratinized layers of hair, nail and skin. They do not thrive in living tissue or persist in the presence of severe inflammation. The incubation period is 1-4 weeks. An infected animal which is not showing signs may remain in this inapparant carrier state for a prolonged period of time. Some may never become symptomatic. Corticosteroids can modulate inflammation and prolong the infection.

Clinical signs

Signs may range from an inapparent carrier state to alopecia which may be patchy or circular. The classic sign of circular alopecia is more common in cats but often misinterpreted in dogs. Scales, erythema, hyperpigmentation, and pruritus are variable. Paronychitis, granulomatous lesions, or kerions may also be seen. In cats, dermatophytosis should be considered as a differential diagnosis for miliary dermatitis and almost any other dermatitis.

Skin Systemic
Microsporum canis (skin) Cryptococcus spp
Microsporum gypseum (skin) Coccidioidomycosis (Coccioides immitis)
Trichophyton mentagrophytes (skin) Aspergillus spp
Trichophyton rubrum (nasal cavity) Histoplasma spp (Histoplasma capsulatum)
Malassezia spp (otitis externa / otitis media) Phaeohyphomycosis - opportunistic dematiaceous fungi
Trichosporon spp (nasal cavity) Penicillium spp (naso-orbital abscesses)
Prototheca spp (algae in nasal cavity) Sporotrichosis (Sporothrix schenckii)
Aphanoascus fulvescens (commensal) Rhinosporidium spp
Chrysosporium parvum (keratinolytic) Blastomyces dermatitidis
Candida spp Scopulariopsis spp
Microsphaeropsis spp Mucor amphibiorum
Pythium spp - naso-orbital abscesses
Neonatal candidiasis