Fungi are eukaryotic (contain a true nucleus) organisms that lack chlorophyll and rely on preformed organic matter as their energy source. Fungi may be grouped into two broad categories—yeasts and molds—based on their physical characteristics (single cells or hyphae) and mode of reproduction (budding or spore formation). Many species are dimorphic and may exist as yeast and as molds depending on the temperature or other characteristics of the physical environment. In general, fungi grow in damp, dark environments. The specific type of fungi present in a particular area depends on environmental conditions and available substrates. Of more than 100,000 fungal species, only several hundred cause infection and disease in humans. Fungi are found ubiquitously in the environment; however, despite this constant exposure, serious fungal diseases are rarely found in immunocompetent individuals.
Fungal infections, called mycoses (myco Latin for fungus), may be superficial, deep, or systemic, and may cause very mild to life-threatening disease.
The most common types of fungal infections are superficial skin infections caused by a variety of fungal species known as dermatophytes. While most fungal infections are not spread from person to person, superficial mycoses often are. The clinical conditions, tinea pedis, tinea manis, tinea corporis, and tinea capitis, affect the feet (athlete’s foot), hands, body, and head, respectively. These infections are most often an itchy nuisance which can usually be easily controlled with topical medications. Pytiriasis versicolor is a fungal skin infection most often found around puberty and causes hyper- or hypopigmentation of the skin. Onychomycosis is a fungal infection of the finger or toe nails which can be chronic and difficult to treat. Piedra is a fungal colonization of the hair shaft in which the color and steadfastness of the fungal nodules depends on the species of fungus. A species of yeast called candida albicans is responsible for superficial infections of mucous membranes such as the vagina or mouth and are known commonly as yeast infections. Candida species can also cause skin infections in warm, moist areas, particularly in babies (diaper rash) or in adults in intertriginous areas or folds of fat in the breasts or abdomen.
In people with HIV/AIDS, on chemotherapy, or with other immunocompromising conditions, superficial mycoses can progress to serious life-threatening conditions. For example, candidal infections can spread from the oral cavity to the esophagus, causing candidal esophagitis which can be very painful and prevent already-ill people from eating or drinking. Fungal infections of the eyes, though superficial, can be very serious. Mycotic keratitis (fungal infection of the cornea of the eye) can take many forms, and is associated with contact lens use, eye surgery, and immunosuppressive illnesses.
Fungal infections that infect deeper skin tissues include sporothrix schenckii, a dimorphic fungi classically contracted from a piercing from a rose bush. Sporotricosis manifests as pain and redness that tracks up lymphatic vessels in the arm or leg and treated with oral antifungal medications. Mycetoma or madura foot is a fungal infection of an extremity, usually the foot, which invades and destroys skin and subcutaneous tissue. It can spread to deeper tissues such as bone and tendon, and cause very serious disability.
Systemic fungal diseases are those that enter through a break in the skin, the lungs, or mucous membranes and progress to infect other parts of the body. These are much more serious infections that are usually only found in specific geographical regions where the fungi are found in the environment. For example, paracocidiomycosis is a fungal infection found only in Latin America. Spores of the fungus are inhaled and immediately invade the body, but the infection may remain dormant for many years. When it manifests, patients, usually men, present with painful ulcers in the mouth, nose, and throat. Paracoccidiomycosis is very difficult to treat and may progress or present as a lung infection resembling tuberculosis.
Histoplasmosis is a systemic fungal infection found throughout the world and usually affects the lungs, but may involve any body organ. It has become much more common since the HIV epidemic and, depending on the individual, it may cause anything from flu-like illness to death. The histoplasma fungus is most often found in soil enriched by bat or bird droppings and is, therefore, classically found in spelunkers. It may also be acquired occupationally by those working in mines, excavation sites, or other activities that may disrupt and aerosolize spores.
Cryptococcosis is caused by an encapsulated yeast, which like histoplasmosis, is inhaled, and is also common in AIDS sufferers. One of its most severe manifestations is cryptococal meningitis which is a fungal infection of the membranes surrounding the brain, often leading to death. Blastomycosis is a fungal disease which is found in southeastern and south-central United States and Canada. This fungi is found near streams or in areas with moist soil where its spores are inhaled by people working or recreating in these areas. The spores then enter the lungs and develop into yeast which causes an asymptomatic infection in over 50 percent of people. In others, it can cause acute lung infection resembling a bacterial pneumonia, or chronic lung infections that may mimic lung cancer on X-ray. Blastomycosis, like other systemic mycoses may also affect other body systems, particularly bones, and the genitourinary tract.
Antifungal medications exploit the fact that fungal cells use ergosterol in their cell walls whereas human cells use cholesterol. Medications from the azole group (ketoconazole, clotrimatzole, etc.) inhibit the synthesis of ergosterol, whereas polyene antimycotics (amphotericin) disrupt the ergosterol cell wall, killing the cells. Newer echinocandins (mycafungin) inhibit the synthesis of glucan, another cell wall component.
Fungal infections may be simple or difficult to treat using these agents, with the success of treatment often depending as much on the host as on the medication or the fungus itself. Serious fungal infections in the developed world became more common during the height of the AIDS epidemic, but since the advent of highly active antiretroviral therapy (HAART), they have become less frequent. Fungal infections are still a cause of significant morbidity and mortality in the developing world, particularly in those with high HIV prevalence.
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