Subcutaneous mycoses

• Subcutaneous mycoses are diseases caused by fungi and fungus-like microorganisms affecting the skin and subcutaneous tissues.
• Sporothrix schenckii is the cause of sporotrichosis in a variety of animal species, but more frequently in humans, horses, dogs, and cats.
• Histoplasma capsulatum var farciminosum is the cause of epizootic lymphangitis in equids (horses, donkeys, and mules).
• Adult mule showing a multilobulated exophytic mass arising from the nasal mucosa.
• The exophytic mass is composed of a core of fibrovascular tissue that is superficially lined by hyperplastic mucosa.
• The sporangium is filled with a myriad of endospores of Rhinosporidium seeberi.
• The submucosa and stromal tissues contain several large sporangia (arrows).
• Oomycosis (Aphanomyces, Lagenidium, Pythium, and Saprolegnia) causes a variety of diseases in fish and mammals; and miscellaneous conditions involving the skin and subcutaneous tissues, including chromoblastomycosis, phaeohyphomycosis, and mycetoma.
• Sporothrix schenckii is a saprophytic, dimorphic fungus.
• In immune-competent people, sporotrichosis usually manifests as a chronic, ulcerative lymphangitis of skin and subcutaneous tissue.
• In immune-competent horses and dogs, the disease is usually limited to the cutaneous or cutaneolymphatic form, and organisms are typically sparse or rare within the lesions.
• Disseminated disease is very rare in horses and dogs unless the patient is immunosuppressed.
• Cats with sporotrichosis will develop cutaneolymphatic or disseminated disease regardless of their immune status at the time of infection.
• Sporothrix schenckii exhibits a different morphology depending upon the conditions of growth.
• At room temperature (25 ◦ C, on Sabouraud’s agar), S. schenckii grows as a mold.
• At 35 – 37 ◦ C (in tissue or on rich media, e.g., blood agar, incubated at that temperature), S. schenckii exists as budding pleomorphic yeasts (characterized typically by the unique “cigar bodies” but yeasts can also be round shaped).
• The yeast phase of S. schenckii stains with the Gram’s stain, and either phase accepts Romanowsky-type stain (e.g., Wright’s or Giemsa) or fungal stains (periodic acid Schiff, Grocott methenamine silver, and Gridley).
• Sinus tracts in mycetoma carry pus and granules, consisting of microorganisms and inflammatory components, to the surface.
• Brown pigmented, short unbranched hyphal elements and yeast-like fungus cells are found in macrophages and giant cells.
• Hematoxylin and eosin staining is used to identify these elements.
• Diagnosis of rhinosporidiosis is based on gross lesion and microscopic examination of tissue sections or discharge from polyps.
• Formation of cauliflower-like growths, also known as polyps, is very characteristic of infection caused by Rhinosporidium seeberi.
• Abdominal cavity of a cat with fatal systemic phaeohyphomycosis due to Cladophialophora bantiana is characterized by a black granular substance covering the serosal surfaces and necrotic foci scattered throughout the liver.
• Swollen, granule-forming, and discharging sinus tracts are characteristic of mycetoma.
• Treatment for rhinosporidiosis is not effective, and surgical excision is practiced, but many lesions recur.
• Rhinosporidiosis occurs mostly in tropical and subtropical countries, but sporadic cases have been reported in the United States.
• Cladophialophora bantiana is a fungus that causes systemic phaeohyphomycosis in cats.
• Pigmented and septate hyphae in a necrotic area are characteristic of C. bantiana infection.
• Cladophialophora bantiana is a fungus associated with eumycotic mycetoma, which is a disease caused by fungi that enter via a wound.
• Mycetoma may be associated with bacteria, most notably an actinomycete such as members of the genus Nocardia or Actinomyces, resulting in an actinomycotic mycetoma.
• Fungal colonies are surrounded by suppuration bordered by granulomatous reactions in mycetoma.
• Rhinosporidiosis occurs in horses, cattle, mules, dogs, goats, and wild waterfowls, and infection in humans is rare but can occur.
• Excision is the treatment for mycetoma if possible.
• Rhinosporidium seeberi is the cause of rhinosporidiosis, a chronic granulomatous type of infection at mucocutaneous junction.
• Cell wall components of S. schenckii include adhesins for extracellular matrix proteins, lipid that inhibits phagocytosis by monocytes and macrophages, melanin that protects from the effects of reactive oxygen intermediates within phagolysosomes of phagocytic cells, and peptide-rhamnomannan that acts as an immunosuppressive substance by suppressing the liberation of proinflammatory cytokines by phagocytic cells.
• Sialic acids inhibit uptake by phagocytic cells and directs complement proteins toward the degradative pathway, rather than generating effective opsonizing fragments and anaphylatoxins needed to generate an effective inflammatory response.
• Mild cases do not progress beyond the local stage.
• Grows on common laboratory media (SDA) at 25 – 30 ◦ C, taking several weeks to form a cottony, white to brown colony.
• Conversion of mold to yeast on blood-containing agar requires incubation at 37 ◦ C under 15 – 20% CO2.
• Histoplasma capsulatum var Farciminosum causes a painless, freely moveable skin nodule, which overtime enlarges, becomes abscessed, and eventually bursts resulting in an ulcerated lesion.