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:
- 1. the fatty acid content of the skin.
- 2. the pH of the skin, mucosal surfaces and body fluids.
- 3. Epithelial cell turnover.
- 4. Normal flora.
- 5. Transferrin.
- 6. Cilia of the 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.
B. Cutaneous mycoses– infections that extend deeper into the epidermis, as well as invasive hair and nail diseases.
These diseases are restricted to the keritinized layers of the skin, hair, and nails. Unlike the superficial mycoses, host immune responses may be evoked, resulting in pathologic changes expressed in the deeper layers of the skin. The organisms that cause these diseases are called dermatophytes. These diseases are often called ringworm or tinea. All the following diseases are causes by Microsporum, Trichophyton, and Epidermophyton, which comprise 41 species.
C. Subcutaneous mycoses- infections involve the dermis, subcutaneous tissues, muscle, and fascia. These infections are chronic and are initiated by trauma to the skin. These infections are difficult to treat and may require surgical intervention.
D. Systemic mycoses- infections that originate primarily in the lungs and may spread to many organ systems. These organisms are inherently virulent. All but Cryptococcus are dimorphic fungi.
Histoplasma capsulatum– Ohio and Mississippi river valleys, Yeast cells in tissue, Tuberculate macroconidia in mycelial phase.
Blastomyces dermatitidis– Ohio and Mississippi river valleys, Broad Base Budding yeast in tissue, Mycelia= microconidia
Coccidioides immitis– Southwestern US. Spherule in tissue, barrel-shaped Arthroconidia in mycelia phase.
Cryptococcus neoformans– Only yeast phase but unusual in that the cells are encapsulated as demonstrated by an India Ink stain.
E. Opportunistic mycoses– infections of patients with immune deficiencies who would otherwise not be infected. Ex. AIDS, altered normal flora, diabetes mellitus, immunosuppressive therapy, malignancy.
Candidiasis– Candida albicans– Creamy growth on various body surfaces. ex. mouth, skin, vagina. Budding yeast. Form pseudohyphae in tissue. Germ tube when grown in serum.
Aspergillosis– Aspergillus niger.
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