Fungal diseases are called mycoses and those affecting humans can be divided into four groups based on the level of penetration into the body tissues:
- Superficial mycoses are caused by fungi that grow only on the surface of the skin or hair.
- Cutaneous mycoses or dermatomycoses include such infections as athlete’s foot and ringworm, in which growth occurs only in the superficial layers of skin, nails, or hair.
- Subcutaneous mycoses penetrate below the skin to involve the subcutaneous, connective, and bone tissue.
- Systemic or deep mycoses are able to infect internal organs and become widely disseminated throughout the body. This type is often fatal.
The most common type of subcutaneous mycosis seen worldwide is sporotrichosis, which occurs most often in gardeners and farmers who come in direct contact with soil. This is a chronic infection caused by the fungus Sporothrix schenckii, occurring in three forms. The ‘cutaneous lymphatic form’ is characterized by a single pustule or nodule that forms at the site of invasion. This is followed by lymphatic spread and the development of numerous subcutaneous lesions. This ‘disseminated form’ is marked by multiple, painless cutaneous or subcutaneous nodules, which can form into ulcers or abscesses involving the muscles, joints, bones, eyes, gastrointestinal system, mucous membranes, and nervous system. The ‘pulmonary form’ results from the inhalation of spores, but produces much the same forms of the disease.
Other forms of subcutaneous mycoses occur mostly in the tropics and subtropics and are caused by several fungal species. These conditions are called chromomycosis (producing wartlike nodules that can ulcerate) and maduromycosis (or mycetoma – a chronic slowly progressive destructive infection involving several layers of skin, producing abscessing granulomas). Treatment is difficult and often requires surgical removal of the offending tissues.
Dermatomycoses is a superficial fungal infection that penetrates only the epidermis, hair, or nails. About thirty different species of the genera Epidermophyton, Microsporum, and Trichophyton (collectively known as dermatophytes) cause infections commonly known as athlete’s foot, jock itch, and ringworm. The term tinea, along with the area of body involved, is also used when referring to these infections. Tinea is Latin for worm or grub because the infections were originally thought to be caused by worm-like parasites. For example:
- tinea capitis is an infection of the scalp common in school children;
- tinea corporis is an infection on the body such as ringworm;
- tinea pedis occurs on the feet especially the classic athlete’s foot;
- tinea cruris is an infection of the genital area commonly called ‘jock itch;’
- tinea barbae affects the area of the beard;
- tinea unguium or onychomycosis is an infection of the toenail or fingernail;
- tinea manvum affects the hands.
Different species afflict different anatomical sites in different ways. Dermatophyte fungi typically causes scaly lesions that tend to be circular (ringworm). Dermatophyte fungi are molds, but there are yeasts that also cause skin infections. Trichophyton rubrum and T. mentagrophytes are the dermatophytes that cause athelete’s foot. Malassezia furfur causes a type of dandruff, as well as an infection known as Pityriasis versicolor (formerly Pityrosporum ovale andP. orbiculare). M. furfur is actually a complex of seven species that are normally harmless, but, for some reason, turn pathogenic at times. Infection produces a flaking skin rash and either a bleaching of the skin or a deposit of more pigment to the area. Lesions are generally found on the trunk, and sport a golden yellow florescent color when an ultraviolet light is shone on them.
Systemic mycoses occur in two basic forms – respiratory and disseminated tissue. If left untreated, the disseminated form is usually fatal. Surgical removal of large pulmonary lesions have been useful in some cases. Other systemic mycoses include:
- Coccidioidomycosis (valley fever) is caused by Coccidioides immitis, a fungus found in desert areas and prevalent in the southwestern US and in some areas of Central and South America. These spores are carried by desert winds and infect the pulmonary spaces. As many as 80% of the population of the central valleys of California test positive for this fungus (see more below).
- Histoplasmosis is caused by the fungus Histoplasma capsulatum, which grows in soil fertilized by bat droppings. It is most prevalent in the Ohio River and Mississippi River valleys, where positive skin tests reveal that at least 80% of the population have been infected. Mild lung infections are usually the result of the disease and often go unnoticed, but, on healing, small calcified nodules that are often mistaken for tuberculosis lesions on chest X-rays are left on the lungs.
- Blastomycosis is caused by the fungus Blastomycoses dermatitidis. It grows in soil as a mold producing conidia that infect human by the airborne route. Along with common mild respiratory infections, skin lesions can also appear. The fungus grows as a budding yeast in human tissues.
- Cryptococcosis is caused by the yeast Cryptococcus neoformans, normally found in soil and habitats of pigeons. It exists only in the yeast form. Scientists speculate that it is millions of years old. Most infections are of a mild respiratory nature, but, in compromised persons, the infection may spread throughout the body affecting mostly the central nervous system. Cryptococcus neoformans gattii is usually found in tropical regions, but has now made an appearance on Vancouver Island in Canada, with several deaths attributed to it.
The fungus, Coccidioides immitis or cocci, for short, is an infectious, but not contagious, disease contracted by inhaling spores produced in soil. Anything that disturbs the soil will send these spores into the air – construction, earthquakes, off-roading, dust storms, archeological digs, etc. So far, it is found only in the Western Hemisphere, particularly in the American southwest. The strong winds of southern California between Thanksgiving and Christmas produce numerous cases of these deadly cocci illnesses. The fungii grow so rapidly that death can result very quickly. Once inhaled, the spores often migrate to one area and lay dormant in a pocket of tissue, looking like a hanging beehive. The only known drug proven effective has been amphotericin B, but it has horrendous side effects, killing healthy human cells as well as the invasive agent. This leaves the kidneys and bone marrow especially sensitive, and usually results in kidney damage and anemia.
Mucormycosis refers to the fairly rare diseases produced by a variety of common fungi of the order Mucorales. These, too, are seen only in the severely immunocompromised patient. The fungi penetrate the respiratory or intestinal mucosa and can enter through breaks in the skin as well. Localized lesions may develop, followed by a spreading to the blood, which carry it to all organs. Death often results.
Fungi most commonly associated with specific immunocompromised patients:
- Candida species, Aspergillus species, Phycomyces species (Leucopenia or bone marrow failure);
- Candida, Cryptococcus, Coccidioides, Histoplasma (Cellular immunity or tissue transplants);
- Zygomyces, Rhizopus, Mucor, Absidia (Diabetes);
- Zygomyces (steroid therapy);
- Candida, Cryptococcus, Histoplasma (malignancies as in leukemia and lymphoma, and Hodgkin’s disease);
- Candida, Cryptococcus, Histoplasma. (AIDS)
Clinically significant fungi and the areas of disease they cause:
- Malassezia furfur and Exophiala werneckii (superficial skin)
- Piedraia hortae and Trichosporon beigelii (hair)
- Microsporum species, (skin and hair)
- Epidermophyton species (skin and nails)
- Trichophyton species (skin, hair, and nails)
- Sporothrix schenckii, Cladosporium species, Phialophora species, and Fonsecaea species (subcutaneous/lymphatic tissues – chromoblastomycosis)
- Histoplasma capsulatum, Coccidioides immitis, Fusarium species, Penicillium species (systemic respiratory)
- Blastomyces dermatitidis (subcutaneous/respiratory)
- Cryptococcus neoformans (respiratory/CNS)
- Aspergillus species, Mucor species, Candida species, and Rhizopus species (opportunistic involving various body sites)
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