In general, humans have a high level of innate immunity to fungi and most of the infections they cause are mild and self-limiting. This resistance is due to:
Fatty acid content of the
skin
pH of the skin, mucosal surfaces and body fluids
Epithelial turnover
Normal flora
Transferrin
Cilia of respiratory tract
When fungi do pass the resistance barriers of the human body and establish infections, the infections are classified according to the tissue levels initially colonized.
A. Superficial
mycoses - infections limited to the outermost layers of the skin and
hair. The superficial mycoses are:
| Disease | Etiological Agent | Symptoms | Identification of organism |
| Pityriasis versicolor | Malassezia furfur | hypopigmented macules | "spaghetti and meatballs" appearance of organism in skin scrapings |
| Tinea nigra | Exophiala werneckii | black macules | black, 2-celled oval yeast in skin scrapings |
| Black piedra | Piedraia hortai | black nodule on hair shaft | black nodule on hair shaft composed of spore sacs and spores |
| White piedra | Trichosporum beigelii | creme-colored nodules on hair shaft | white nodule on hair shaft composed of mycelia that fragment into arthrospores |
B. Cutaneous mycoses - infections that extend deeper into the epidermis, as well as invasive hair and nail diseases.
These diseases are
restricted to the keratinized layers of the skin, hair and nails. Unlike
the superficial mycoses, various cellular immune responses may be evoked,
causing pathologic changes in the host that may be expressed in the deeper
layers of the skin. The agents causing these diseases are termed dermatophytes.
The diseases are referred to as ringwormor tinea.
All of the dermatophytic diseases are caused by members of three genera, Microsporum,
Trichophyton and Epidermophyton, which comprise 41 species. The
cutaneous mycoses are:
| Disease | Etiological Agent | Symptoms | Indentification of organism |
| Tinea capitis | Microsporum sp. Trichophyton sp. Epidermophyton sp. | ringworm lesion of scalp | presence/absence and shape of micro- and macroconidia in scrapings from lesion |
| Tinea corporis | Microsporum sp. Trichophyton sp. Epidermophyton sp | ringworm lesion of trunk, arms, legs | presence/absence and shape of micro- and macroconidia in scrapings from lesion |
| Tinea manus | Microsporum sp. Trichophyton sp. Epidermophyton sp | ringworm lesion of hand | presence/absence and shape of micro- and macroconidia in scrapings from lesion |
| Tinea cruris "jock itch" | Microsporum sp. Trichophyton sp. Epidermophyton sp | ringworm lesion of groin | presence/absence and shape of micro- and macroconidia in scrapings from lesion |
| Tinea pedis"athlete's foot" | Microsporum sp. Trichophyton sp. Epidermophyton sp | ringworm lesion of foot | presence/absence and shape of micro- and macroconidia in scrapings from lesion |
| Tinea unguium | Microsporum sp. Trichophyton sp. Epidermophyton sp | infection of nails | presence/absence and shape of micro- and macroconidia in scrapings from lesion |
| Ectothrix | Microsporum sp. Trichophyton sp. Epidermophyton sp | infection of hair shaft surface | mycelium and spores on hair shaft |
| Endothrix | Microsporum sp. Trichophyton sp. Epidermophyton sp | infection of hair shaft interior | mycelium and spores in hair shaft |
C. Subcutaneous mycoses - infections involving the dermis, subcutaneous tissues, muscle and fascia
These infections initially involve the deeper layers of the dermis,
subcutaneous tissue or bone. Most infections have a chronic or insidious
growth pattern, eventually extending into the epidermis and are expressed
clinically as lesions on the skin surface. They are initiated by trauma to
the skin and are difficult to treat and surgical intervention (excision or
amputation) is frequently employed. The subcutaneous mycoses are:
| Disease | Etiological Agent | Symptoms | Identification of Organism |
| Sporotrichosis | Sporothrix schenckii | Nodules and ulcers along lymphatics at site of inoculation | Budding yeast in tissue exudate that converts to mold with "rosette pattern" of conidiation on culture at 25oC. |
| Chromoblastomycosis | Fonsecaea
pedrosoi
Fonsecaea compacta Wangiella dermatitidis |
Warty nodules that progress to "cauliflower-like" appearance at site of inoculation | Copper-colored spherical yeast called "Medlar bodies" in tissue |
| Mycetoma | Pseudallescheria
boydii
Madurella grisea Madurella mycetomatis |
Draining sinus tracts at site of inoculation | White, brown, yellow or black granules in exudate that are fungal colonies |
D. Systemic mycoses - infections that originate primarily in the lung and may spread to many organ systems.
Unlike most other fungi, the five systemic mycotic agents are inherently virulent. Each species has biochemical and structural features that enable it to evade host defenses. The primary focus of infection is the lung but secondary infection may occur elsewhere in the body. The five etiological agents are identified by their morphology on agar plates (saprobic phase) and in tissue (parasitic phase):
Dimorphic with mold to yeast transition when infecting susceptible
species. Yeast cells are relatively small. Saprobic phase shows
tuberculate macroconidia.
Saprobic Phase Parasitic Phase
Dimorphic with mold to yeast transition when infecting susceptible species. Yeast cells are medium size with thick walls.
Saprobic Phase Parasitic Phase
Dimorphic with mold to yeast transition when infecting susceptible species. Yeast cells have multiple buds.
Saprobic Phase Parasitic Phase
Dimorphic with mold to spherule transition when infecting susceptible species. Spherules are multinucleate.
Saprobic Phase Parasitic Phase
Monomorphic with yeast phase only. This is the only pathogenic yeast with a capsule. The capsule is extremely large.
Saprobic Phase Parasitic Phase
E. Opportunistic mycoses - infections in patients with immune deficiencies who would otherwise not be infected
Opportunistic mycoses are seen in those people with impaired host defenses
such as occurs in
AIDS Alteration of normal flora Diabetes mellitus
Immunosuppressive therapy Malignancy
The major
opportunistic mycoses include:
| Disease | Etiological Agent | Symptoms | Identification of organism |
| Candidiases | Candida albicans | Creamy growth on various areas of body | Budding yeast, septate hyphae, pseudohyphae in tissue. Germ tubeformation in serum |
| Aspergillosis | Aspergillus fumigatus | "Fungus ball" in tissue | Morphology of asexual fruiting structure |
| Zygomycosis | Rhizopus sp. Absidia sp. Mucor sp. | Various | Morphology of asexual fruiting structure and mycelium |
Asexual fruiting structure of Rhizopus species, illustrating sporangium, sporangiophore, sporangiospores, coenocytic hyphae and rhizoids.
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