The Subcutaneous Mycoses

Chapter 46 The Subcutaneous Mycoses


Subcutaneous mycoses comprise a broad range of infections that involve the deeper layers of the skin, muscle, bone, or connective tissue. Common themes include typical association with injuries, etiologic agents usually found in soil or decaying vegetation, and infections of a chronic and insidious nature. The organisms establish themselves in the skin and produce localized infection of the surrounding tissues and lymph nodes; dissemination is rare. Organisms causing subcutaneous mycoses are dematiaceous or hyaline molds and dimorphic fungi, and the common diseases are sporotrichosis, epizootic lymphangitis, chromoblastomycosis, eucomycotic mycetoma, phaeohyphomycosis, and bovine nasal granuloma.


Some bacterial infections may be confused with the subcutaneous mycoses. These infections include actinomycotic mycetoma, botryomycosis, and mycobacteriosis, which are often readily managed by antimicrobial therapy, but subcutaneous mycoses are more difficult to treat. Thus it is important for the veterinarian to correctly establish the etiologic agent.



SPOROTRICHOSIS


Taxonomically, the genus Sporothrix is a member of the phylum Ascomycota, class Euascomycetes, order Ophiostomatales, and family Ophiostomataceae. There are three legitimate species but only one, Sporothrix schenckii, is an important pathogen of animals. Sporothrix schenckii is a thermally dimorphic dematiaceous fungus, existing as a yeast form at 37° C in tissue and in a mycelial or mold form when cultivated at 25° C.



DISEASE AND EPIDEMIOLOGY


Sporotrichosis has been described worldwide but is most common in tropical and subtropical America. The organism is isolated from soil, living and decaying vegetation, peat moss, and wood, and disease follows inoculation of conidia into the skin by puncture wounds from thorns or bites. As a result, most cases present with localized skin and subcutaneous lesions with minimal systemic manifestations. The infection in humans is sometimes called “rose handler’s disease.”


Sporotrichosis is a relatively common disease of humans and animals. Naturally occurring disease has been documented in dogs, cats, horses, donkeys, mules, pigs, fowl, goats, and cattle, and is most common in the dog. Canine disease presents as one of three distinct clinical syndromes; cutaneous and disseminated disease is not rare, but lymphocutaneous is the predominant syndrome. Lesions begin at the point of entry and consist of subcutaneous nodules that ulcerate and heal. As the disease progresses, it follows the course of lymphatic vessels and may eventually involve the lymph nodes. The lesions usually are neither painful nor pruritic.


Feline sporotrichosis develops mainly in intact male cats that are allowed to live outdoors. Initially, lesions appear as small, draining puncture wounds and are commonly seen on the head or at the base of the tail. As the disease progresses, the lesions may become nodular, granulomatous, ulcerative, or necrotic, and systemic spread has been reported.


Hard, cutaneous nodules that develop along the lymphatics, usually on the medial surface of the legs, are commonly observed in horses. Nodules are 1 to 5 cm in diameter and may drain or ulcerate. Visceral or skeletal involvement may occur.


Sporotrichosis is zoonotic, and direct contact with lesions or contaminated bandages may result in human infection. Most human cases have come from contact with cats, often without a history of cat bites or scratches. Infected cats appear to continuously shed fungi in their lesion exudates and feces. In fact, S. schenckii has been isolated from domestic cats without clinical signs of sporotrichosis, reinforcing the zoonotic potential of feline disease. In contrast, canine sporotrichosis is considered to be of minimal zoonotic importance because few organisms are present in the tissues of most affected dogs.



PATHOGENESIS


Sporothrix schenckii conidia or mycelia generally gain access to the host through broken skin, either directly through some traumatic insult or indirectly through contamination of an existing wound. Upon entry, the mycelial or saprophytic form changes to the yeast or parasitic form as a result of the temperature increase and perhaps other in vivo signals. Lymphocutaneous manifestations of disease begin as single or multiple indurated, erythematous nodules that develop at the initial site of contact. The nodules contain microabscesses and granulomas. As the infection spreads along the lymphatics, papules appear, ulcerate, and drain. Regional lymph node involvement is not uncommon. Nodes may ulcerate and discharge pus. The limbs become swollen due to lymphangitis. A cutaneous form with no lymphatic spread also may be seen.


Virulence factors for S. schenckii include thermotolerance, production of extracellular enzymes, and adhesion. Ability to grow at 37° C is a virulence factor, in that it allows the fungus to invade deep tissues. Acid phosphatases are produced by the yeasts, mycelia, and conidia of S. schenckii. These enzymes may interact with macrophages to allow intracellular survival of the organism. Two enzymes, proteinases I and II, hydrolyze human stratum corneum cells in vitro. Adhesion to extracellular matrix proteins may play a crucial role in the invasion process. Both the yeast cells and conidia of S. schenckii adhere to the extracellular protein fibronectin. Cell wall composition may play a role in fungal virulence because the more virulent forms show differences in cell-wall sugar composition with rhamnose-mannose molar ratios when compared with avirulent strains.



DIAGNOSIS


Diagnosis of sporotrichosis may be accomplished by several methods. Microscopic examination of direct mounts of feline specimens may reveal large numbers of small elliptical budding yeasts, referred to as “cigar bodies.” These organisms are not plentiful in clinical materials obtained from infected dogs or horses. Histologic examination of punch biopsy specimens of cutaneous lesions provides a fairly rapid diagnosis, but care must be taken not to confuse S. schenckii with other yeasts. Definitive diagnosis is based on isolation of the organism from specimens.


Sporothrix schenckii forms colonies in 2 to 7 days on Sabouraud dextrose agar at room temperature. The mold colonies are cream colored, wrinkled, and leathery, and turn black or silvery gray with age (Figure 46-1). Microscopically, the mold appears as small, oval, hyaline, or dematiaceous conidia. These are arranged singly along the hyphae, or as “flower petals” at the end of short unbranched conidiophores (Figure 46-2). Conversion from the mold to the yeast phase is required for identification. The mold is plated on brain-heart infusion agar supplemented with 5% blood and incubated at 37° C in 5% to 7% CO2 for 3 to 5 days. Yeast colonies are soft and white to cream colored (Figure 46-3).


image

FIGURE 46-1 Agar plate culture of Sporothrix schenckii, incubated at 20° C.


(Courtesy Public Health Image Library, PHIL #3196, Centers for Disease Control and Prevention, Atlanta, 1969, William Kaplan.)


image

FIGURE 46-2 Micrograph of conidia of yeast phase of Sporothrix schenckii.


(Courtesy Public Health Image Library, PHIL #3063, Centers for Disease Control and Prevention, Atlanta, 1964, Lucille K. Georg.)


image

FIGURE 46-3 Slant culture of yeast colonies of Sporothrix schenckii.


(Courtesy Public Health Image Library, PHIL #3062, Centers for Disease Control and Prevention, Atlanta, 1964, Lucille K. Georg.)

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Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on The Subcutaneous Mycoses

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