Medical mycology

The study of fungi (molds and yeasts) that cause human disease. Fungal infections are classified according to the site of infection on the body or whether an opportunistic setting is necessary to establish disease. Fungal infections that occur in an opportunistic setting have become more common due to conditions that compromise host defenses, especially cell-mediated immunity. Such conditions include acquired immunodeficiency syndrome (AIDS), cancer, and immunosuppressive therapy to prevent transplant rejection or to control inflammatory syndromes. Additionally, opportunistic fungal infections have become more significant as severely debilitated individuals live longer because of advances in modern medicine, and nosocomial (hospital-acquired) fungal infections are an increasing problem. Early diagnosis with treatment of the fungal infection and control of the predisposing cause are essential. See Opportunistic infections

Antifungal drug therapy is extremely challenging since fungi are eukaryotes, as are their human hosts, leading to problems with toxicity or cross-reactivity with host molecules. Most antifungal drugs target the fungal cell membrane or wall. The “gold standard” for therapy of most severe fungal infections is amphotericin B, which binds to ergosterol, a membrane lipid found in most fungi and some other organisms but not in mammals. Unfortunately, minor cross-reactive binding of amphotericin B to cholesterol in mammalian cell membranes can lead to serious toxicity, especially in the kidney where the drug is concentrated. Recent advances in antifungal therapy include the use of liposomal amphotericin B and newer azoles such as fluconazole and itraconazole, which show reduced toxicity or greater specificity. Conversely, drug resistance in pathogenic fungi is an increasing problem, as it is in bacteria.

Candidiasis is the most common opportunistic fungal infection, and it has also become a major nosocomial infection in hospitalized patients. Candida albicans is a dimorphic fungus with a yeast form that is a member of the normal flora of the surface of mucous membranes. In an opportunistic setting, the fungus may proliferate and convert to a hyphal form that invades these tissues, the blood, and other organs. The disease may extend to the blood or other organs from various infected sites in patients who are suffering from a grave underlying disease or who are immunocompromised. Other important opportunistic fungal diseases include aspergillosis, mucormycosis, and cryptococcus.

Healthy persons can acquire disease from certain pathogenic fungi following inhalation of their fungal spores. The so-called deep or systemic mycoses are all caused by different species of soil molds; most infections are unrecognized and produce no or few symptoms. However, in some individuals infection may spread to all parts of the body from the lung, and treatment with amphotericin B or an antifungal azole drug is essential.

Other fungal infections develop when certain species of soil molds are inoculated deep into the subcutaneous tissue, such as by a deep thorn prick or other trauma. A specific type of lesion develops with each fungus as it grows within the tissue. Proper wound hygiene will prevent these infections.

Ringworm, also known as dermatophytosis or tinea, is the most common of all fungal infections. Some species of pathogenic molds can grow in the stratum corneum, the dead outermost layer of the skin. Disease results from host hypersensitivity to the metabolic products of the infecting mold as well as from the actual fungal invasion. Tinea corporis, ringworm of the body, appears as a lesion on smooth skin and has a red, circular margin that contains vesicles. The lesion heals with central clearing as the margin advances. On thick stratum corneum, such as the interdigital spaces of the feet, the red, itching lesions, known as athlete's foot or tinea pedis, become more serious if secondary bacterial infection develops. The ringworm fungi may also invade the hair shaft (tinea capitis) or the nail (onychomycosis). Many pharmaceutical agents are available to treat or arrest such infections, but control of transmission to others is important. See Fungal infections, Fungi, Yeast

medical mycology