| MYCOLOGY
SUPERFICIAL MYCOSES |
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Dr Arthur DiSalvo
The superficial (cutaneous)
mycoses are usually confined to the outer layers of skin, hair, and
nails, and do not invade living tissues. The fungi are called
dermatophytes. Dermatophytes, or more properly, keratinophilic fungi,
produce extracellular enzymes (keratinases) which are capable of
hydrolyzing keratin.
A. CLINICAL MANIFESTATIONS
Onychomycosis due to Trychophyton rubrum, right and
left great toe. Tinea unguium. CDC/Dr. Edwin P. Ewing, Jr.
epe1@cdc.gov |
Tinea Versicolor on chest.
CDC/Dr. Gavin Hart |
Tinea means "ringworm"
or "moth-like". Dermatologists use the term to refer to a
variety of lesions of the skin or scalp.
Tinea corporis - small lesions
occurring anywhere on the body.
Tinea pedis - "athlete's
foot". Infection of toe webs and soles of feet.
Tinea unguium (onychomycosis) -
nails. Clipped and used for culture.
Tinea capitis - head.
Frequently found in children.
Tinea cruris - "jock
itch". Infection of the groin, perineum or perianal area.
Tinea barbae - ringworm of the
bearded areas of the face and neck.
Tinea versicolor -
Characterized by a blotchy discoloration of skin which may itch. Up to
25% of the general population may have this lesion at any one time.
Diagnosis is usually possible by direct microscopic examination of KOH-treated
skin scrapings which show a typical aspect of mycelia and spores
described as "spaghetti and meatballs." Caused by Malassezia
furfur
B. ECOLOGY
The dermatophytes (skin plants)
causing human infections may have different natural sources and modes of
transmission:
anthropophilic - usually
associated with humans only; transmission from man to man by close
contact or through contaminated objects.
zoophilic - usually associated
with animals; transmission to man by close contact with animals (cats,
dogs, cows) or with contaminated products.
geophilic - usually found in
the soil, transmitted to man by direct exposure. Knowledge of the
species of dermatophyte and source of infection are important for
proper treatment of the patient and control of the source.
Invasion by zoophilic or
geophilic organisms may cause inflammatory disease in man.
Geographic distribution:
Dermatophytes occur worldwide, but some species have geographically
limited distribution.
C. ETIOLOGIC AGENTS
There are three genera of
dermatophytes:
1. Trichophyton species
(19 species)
Trichophyton mentagrophytes contracted from a dog ©
Bristol Biomedical Image Archive. Used with permission |
Dermatomycosis (ringworm) of hair follicles © Bristol
Biomedical Image Archive. Used with permission |
These infect skin, hair and
nails. Rarely can cause subcutaneous infections, in immunocompromised
individuals. Take 2-3 weeks to grow in culture. The conidia are large (macroconidia),
smooth, thin-wall, septate (0-10 septa), and pencil-shaped; colonies a
re a loose aerial mycelium which grow in a variety of colors.
Identification requires special biochemical and morphological
techniques. Trichophyton rubrum is presently the most common cause of
tinea in South Carolina.
2. Microsporum species (13
species). These may infect skin and hair, rarely nails. Its prevalence
has decreased significantly. When prevalent (15-20 years ago), this
organism could be easily identified on the scalp because infected hairs
fluoresce a bright green color when illuminated with a UV-emitting
Wood's light. The loose, cottony mycelia produce macroconidia which are
thick-walled, spindle-shaped, multicellular, and echinulate (spiny).
Microsporum canis is one of the most common dermatophyte species
infecting humans.
Ringworm, stained preparation, macroconidia of Microsporum
canis © Bristol Biomedical Image Archive. Used with
permission |
Ringworm caused by Microsporum gypseum, culture plate with
Sabouraud's dextrose agar © Bristol Biomedical
Image Archive. Used with permission |
3. Epidermophyton floccosum.
These infect skin and nails and rarely hair. They form yellow-colored,
cottony cultures and are usually readily identified by the thick,
bifurcated hyphae with multiple smooth, club-shaped macroconidia.
D. THERAPY
Skin infections can be treated
(more or less successfully) with a variety of drugs, such as:
Tolfnatate (Tinactin) available
over the counter - Topical
Clotrimazole - Topical
Miconazole - Topical.
Ketoconazole seems to be most
effective for tinea versicolor and other dermatophytes.
Itraconazole - oral
Terbinifine (Lamisil) - oral,
topical.
For skin and Nail infections.
Morpholines - oral
For infections involving the
scalp and particularly the nails, griseofulvin is commonly used. This
antimycotic must be incorporated into the newly produced keratin layer
to form a barrier against further invasion by the fungus. This is a very
slow process requiring oral administration of the drug for long periods
- up to 6 to 9 months for fingernail infections and 12 to18 months for
toenail infections.
Itraconazole and terbinafine are
the drugs of choice for onychomycoses.
E. THE ID REACTION
Patients infected with a
dermatophyte may show a lesion, often on the hands, from which no fungi
can be recovered or demonstrated. It is believed that these lesions,
which often occur on the dominant hand (i.e. right-handed or
left-handed), are secondary to immunological sensitization to a primary
(and often unnoticed) infection located somewhere else (e.g. feet).
These secondary lesions will not respond to topical treatment but will
resolve if the primary infection is successfully treated.
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