Download b. dermatophyte disease

CHAPTER 44
Dermatophytes, Sporothrix and Other Superficial
and Subcutaneous Fungi
The least invasive of fungi are the dermatophytes and other superficial fungi that are
adapted to the keratinized outer layers of the skin. The subcutaneous fungi go a step
further extending to the tissue beneath the skin but rarely invade deeper.
Dermatophytoses are superficial infections of the skin and its appendages, commonly
known as ringworm, athlete’s foot, and jock itch. They are caused by species of three
genera collectively known as dermatophytes. Subcutaneous are introduced traumatically
through the skin and are typically limited to subcutaneous tissues, lymphatic vessels, and
contiguous tissues. They rarely spread to distant organs. The diseases they cause include
sporotrichosis, chromoblastomycosis, and mycetoma.
SUPERFICIAL FUNGI
I. Dermatophytes
A. MYCOLOGY -
1. Form septate hyphae, macroconidia, and microconidia
2. Epidermophyton, Microsporum, and Trichophyton are major genera
3. Grow best at 25°C
B. DERMATOPHYTE DISEASE
a. EPIDEMIOLOGY
1. Reservoir may be human, animal, or soil
2. Transmission requires contact with intact or detached skin or hair
b. PATHOGENESIS
1. Dermatophytoses begin when minor traumatic skin lesions come in
contact with dermatophyte hyphae shed from another infection
2. Initial infection is through minor skin breaks
3. Balance between fungal growth and skin desquamation determines
outcome
4. Sharp advancing margins once believed to be the burrows of worms is the
origin of the common name ringworm and the Latin term tinea (worm)
5. Poor inflammatory response leads to chronic infection
6. Hair shaft is penetrated and broken by hyphae
c. IMMUNITY
1. Delayed hypersensitivity responses occur
2. CMI responses are the most important
3. Widespread infection is associated with T-lymphocyte defects and T.
rubrum
C. DERMATOPHYTOSES: CLINICAL ASPECTS
a. MANIFESTATIONS
1. Range from inapparent colonization to chronic progressive eruptions
that last months or years
2. Various skin sites sometimes labeled as tinea “diseases” e.g., tinea pedis
(feet), tinea manuum (hands), tinea cruris (groin), etc.
3. Hair infection leads to itching and hair loss
4. Skin infection favors moist areas and skin folds
5. Hyperkeratosis can dislodge the nail bed
b. DIAGNOSIS
1. KOH or calcifluor white mounts of skin scrapings and infected hairs
demonstrate hyphae
2. Some species fluoresce under UV (Wood’s) lamp
3. Culture is used when KOH preparations negative
c. TREATMENT AND PREVENTION
1. Topical tolnaftate, allylamines, or azoles usually sufficient
2. Systemic griseofulvin, or azoles used in refractory cases
II. Other Superficial Mycoses
A. Pityriasis (tinea) versicolor
1. Occurs in tropical climates
2. Characterized by discrete areas of hypopigmentation or hyperpigmentation
associated with induration and scaling
3. Members of the genus Malassezia, of which M. furfur is the most common, are
the cause
4. Seen in skin scrapings as clusters of budding yeast cells mixed with hyphae
B. Tinea nigra
1. Tropical infection characterized by brown to black macular lesions, usually on
the palms or soles
2. The cause, Hortaea werneckii, is a black-pigmented fungus found in soil and
other environmental sites
C. Piedra
1. Infection of the hair characterized by black or white nodules attached to the
hair shaft
2. White piedra (Trichosporon cutaneum) infects the shaft in hyphal forms; black
piedra (Piedraia hortae) shows branched hyphae and ascospores in sections of the
hair
SUBCUTANEOUS FUNGI
I. Sporothrix schenckii
A. Mycology
1. Dimorphic fungus that grows as a cigar-shaped, 3- to 5-mm yeast in tissues and
in culture at 37°C
2. The hyphae are thin and septate, producing clusters of conidia at the end of
delicate conidiophores
B. SPOROTRICHOSIS
a. EPIDEMIOLOGY
1. S. schenckii is a ubiquitous saprophyte particularly found in hay, moss,
soil (including potting soil), and decaying vegetation
2. Soil saprophyte is introduced by trauma
3. Occupational disease of gardeners and farmers
4. Outbreaks involve wood and moss
b. PATHOGENESIS
1. Surface binds to extracellular matrix proteins like fibronectin, laminin,
and collagen
2. Melanin in conidia resists oxidative killing
c. IMMUNITY
1. CMI is primary immune mechanism
C. SPOROTRICHOSIS: CLINICAL ASPECTS
a. MANIFESTATIONS
1. Skin papule at site of inoculation eventually ulcerates
2. Lymphatic involvement creates multiple lesions following lymphatic
drainage
3. Deep infection is rare
b. DIAGNOSIS
1. Direct microscopic examination is usually unrewarding due to small
numbers in clinical lesions
2. Definitive diagnosis depends on culture of infected pus or tissue
3. Identification requires demonstration of the typical conidia and of
dimorphism.
c. TREATMENT AND PREVENTION
1. Potassium iodide works for cutaneous disease
2. Amphotericin or itraconazole required for progressive disease
II. Chromoblastomycosis
1. Tropical disease caused by multiple species of Fonsecaea, Phialophora, and
Cladophialophora (Cladosporium)
2. Organisms are found in the soil of endemic areas
3. Appears as papules that develop into scaly, wartlike structures, usually under
the feet
4. Brown pigmented bodies are seen in tissues
II. Mycetoma
1. Clinical term for an infection associated with trauma to the foot which causes
inoculation of any of a dozen fungal species
2. Clinical appearance is of massive induration with draining sinuses
3. Occurs in the tropics most often among those who go barefoot