Interesting link on Fungal Infections

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Several thousand species of fungi have been described, but fewer than

100 are routinely associated with invasive diseases of humans. In

general, healthy humans have a very high level of natural immunity to

fungi, and most fungal infections are mild and self-limiting. Intact

skin and mucosal surfaces and a functional immune system serve as the

primary barriers to colonization by these ubiquitous organisms, but

these barriers are sometimes breached.

Unlike viruses, protozoan parasites, and some bacterial species,

fungi do not require human or animal tissues to perpetuate or

preserve the species. Virtually all fungi that have been implicated

in human disease are free-living in nature. However, there are

exceptions, including various Candida spp., which are frequently

found on mucosal surfaces of the body such as the mouth and vagina,

and Malassezia furfur, which is usually found on skin surfaces that

are rich in sebaceous glands. These organisms are often cultured from

healthy tissues, but under certain conditions they cause disease.

Only a handful of fungi cause significant disease in healthy

individuals. Once established, these diseases can be classified

according to the tissues that are initially colonized.

Superficial mycoses

Four infections are classified in the superficial mycoses. Black

piedra, caused by Piedraia hortai, and white piedra, caused by

Trichosporn beigleii, are infections of the hair. The skin infections

include tinea nigra, caused by Exophiala werneckii, and tinea

versicolor, caused by M. furfur. Where the skin is involved, the

infections are limited to the outermost layers of the stratum

corneum; in the case of hairs, the infection is limited to the

cuticle. In general, these infections cause no physical discomfort to

the patient, and the disease is brought to the attention of the

physician for cosmetic reasons.

Cutaneous mycoses

The cutaneous mycoses are caused by a homogeneous group of

keratinophilic fungi termed the dermatophytes. Species within this

group are capable of colonizing the integument and its appendages

(the hair and the nails). In general, the infections are limited to

the nonliving keratinized layers of skin, hair, and nails, but a

variety of pathologic changes can occur depending on the etiologic

agent, site of infection, and immune status of the host. The diseases

are collectively called the dermatophytoses, ringworms, or tineas.

They account for most of the fungal infections of humans.

Subcutaneous mycoses

The subcutaneous mycoses include a wide spectrum of infections caused

by a heterogeneous group of fungi. The infections are characterized

by the development of lesions at sites of inoculation, commonly as a

result of traumatic implantation of the etiologic agent. The

infections initially involve the deeper layers of the dermis and

subcutaneous tissues, but they eventually extend into the epidermis.

The lesions usually remain localized or spread slowly by direct

extension via the lymphatics, for example, subcutaneous

sporotrichosis.

Systemic mycoses

The initial focus of the systemic mycoses is the lung. The vast

majority of cases in healthy, immunologically competent individuals

are asymptomatic or of short duration and resolve rapidly,

accompanied in the host by a high degree of specific resistance.

However, in immunosuppressed patients the infection can lead to life-

threatening disease. See also Fungi; Medical mycology.

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Medical mycology

Home > Library > Science > Sci-Tech EncyclopediaThe 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 also 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 allfungal infections. Some species of pathogenic molds can grow in

the stratumcorneum, the dead outermost layer of the skin. Disease

results from hosthypersensitivity to the metabolic products of the

infecting mold as well asfrom the actual fungal invasion. Tinea

corporis, ringworm of the body, appearsas a lesion on smooth skin and

has a red, circular margin that containsvesicles. The lesion heals

with central clearing as the margin advances. Onthick stratum

corneum, such as the interdigital spaces of the feet, the red,itching

lesions, known as athlete's foot or tinea pedis, become moreserious

if secondary bacterial infection develops. The ringworm fungi may

alsoinvade the hair shaft (tinea capitis) or the nail

(onychomycosis). Manypharmaceutical agents are available to treat or

arrest such infections, butcontrol of transmission to others is

important. See also Fungal infections; Fungi; Yeast.

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