Fungi

Fungi are eukaryotic, heterotrophic organisms with rigid cell walls that typically reproduce by the production of spores. Characteristic of fungi are their cell walls that may contain chitin (a polymer of N-acetylglucosamine), glucans, mannans and cellulose. The fungi are also unique in their ability to reproduce asexually and /or sexually, in either case, with the subsequent production of spores.

There are three groups of fungi – mushrooms, molds and yeasts. Mushrooms do not infect humans and are generally the purview of botanists.

Yeasts are unicellular true microbes that behave much like bacteria in the context of interaction with humans and disease causation.

The familiar fuzzy colonies of molds are, of course, macroscopic. However, they form microscopic spores that are often the infecting form of the organism either by inhalation or by direct traumatic implantation through the skin.

Microbiologists concern themselves with yeasts and molds, a very small number of which are medically important and are discussed below.

Fungal Infections Commonly Seen in Primary Care in Canada

In general, serious infections caused by fungi are rare in temperate climates. However there are several common conditions that are responsible for a large number of visits to primary care providers.

Superficial Skin Infections

Ringworm (Tinea)

Superficial mycoses are those fungal infections that are limited to the skin and its appendages, the hair and nails. Infections are not serious but can be very disconcerting to those affected. The most common of these types of infections are the dermatomycoses, commonly known as ringworm (because of the prototypic ring shaped appearance) or tinea.

Dermatomycoses are caused by three genera referred to as dematophytes – Microsporum spp., Trichophyton spp. and Epidermophyton spp. Most human disease is caused by dermatophyte species that have become “human adapted” and are only found on affected individuals. Transmission is human to human. A good example is the transmission of Trichophyton rubrum causing athlete’s foot in several members of a sports team that share shower facilities.

Tinea pedis (athlete’s foot), tinea unguium (nail), and tinea cruris (groin) are very commonly seen by the primary care physician or dermatologist. Tinea barbae (beard and mustache), and tinea capitis (hair and scalp) are seen less frequently.

Diagnosis is often very apparent clinically and can be confirmed by the identification of fungal elements by microscopic examination of skin or nail scrapings. These fungi can by cultured on specialized artificial media, however it is a laborious process, does not influence clinical management, and therefore reserved for difficult cases.

Tinea versicolor
This is a very common, benign, poorly recognized superficial skin infection characterized by hyper or hypo-pigmented macules predominantly on the chest and back. It is also referred to a Pityriasis versicolor and is caused by Malassezia furfur – an organism found on the normal skin of a large proportion of people, that is, it is part of the “usual flora”. However, some people have a predisposition for this fungus to “overgrow” and become a pathogen. It has also been implicated in the genesis or promotion of seborrheic dermatitis. Affected persons tend to have recurrent problems.

Diagnosis is mostly clinical. The fungus can be confirmed in skin scrapings by direct microscopy. Culture is possible but rarely necessary.

Yeast Infections of Mucous Membranes and Skin
Candida albicans is the most important fungus infecting humans. It is a human associated organism much like Staphylococcus aureus. All of us have C. albicans on our skin/mucous membranes and usually we live together happily (symbiotically). Humans are the normal and only ecologic habitat for C. albicans. However, it is opportunistic pathogen that causes disease when defenses wane. C. albicans can be found in the normal mouth, gut and vagina in a large proportion of individuals but regularly causes several minor conditions.

Thrush
This is the term used to describe oral mucosa infection with Candida characterized by creamy, curdlike patches adherent to the tongue and other surfaces. It is very common in the first few weeks of life. It has also become more common in older children and adults as a result of several factors. The use of inhaled corticosteroids for the treatment of asthma has markedly increased in recent years and thrush is a common unwanted effect. Broad-spectrum antibacterial use also predisposes to thrush. These forms of thrush are usually easily recognized and treated with topical antifungals. Thrush

More serious oral infections occur in patients that are immunosuppressed. These people are at larger risk for extension of infection into the esophagus which requires much more aggressive therapy.

Diagnosis can be confirmed by scraping plaques from the mucosa and microscopically observing characteristic yeast cells. See Candida Gram stain.

Diaper rash
Another common condition. The warm moist environment promotes overgrowth of Candida. The rash is usually distinctive with large areas of confluent, bright red patches surrounded by “satellite” lesions. Diaper Rash

Vaginal Candidiasis
Another common manifestation, vaginal candidiasis or “yeast infections” are frequently associated with recent antibacterial antibiotic use in women of childbearing age. Diagnosis is often obvious with characteristic whitish “curd-like” discharge and plaques adherent to vaginal mucosa. Gram stain findings can confirm.

Intertrigo
The moist areas in skin folds are also commonly overgrown with Candida. This is a particular problem with obese people and the area under the breasts and groin are common locations. Diagnosis is clinical.

Opportunistic Fungal Infections Common in Canadian Hospitals

In the modern medical world, more and more patients are immuno-incompetent. Aggressive immune modulating therapies are employed in the treatment of cancer and an ever-increasing array of “autoimmune diseases”. Hospital care involves necessary use of large amounts of antibacterial antibiotics which provide an environment for proliferation and sustenance of fungi.

Candida spp.
Candida albicans and related species of Candida have increased in importance as pathogens in hospitals. Particularly worrisome are bloodstream infections commonly associated with central intravenous catheters in ICU patients. Diagnosis is made with routine cultures as for bacteria. Candida albicans has become the fourth most common organism recovered in blood cultures in North America.

Aspergillosis
Aspergillus spp. are ubiquitous mycelial fungi (molds) that only cause disease in very immunocompromised people. However, the disease is often very severe and is a particular worry in leukemia and bone marrow transplant patients – the most immunocompromised of all.

Fungal Infections Uncommonly Seen in Canada

Subcutaneous mycoses

Subcutaneous mycoses are self-limiting infections of tissue underlying the skin. These are contracted by the physical traumatic implantation of the fungus and generally remain limited to the area around the inoculation. Examples of subcutaneous fungal infections include: mycetoma, chromoblastomycosis and sporotrichosis.

While mycetoma and chromoblastomycosis are predominantly tropical or sub-tropical infections, sporotrichosis caused by the dimorphic fungus, Sporothrix schenckii occurs in North America. It is characterized by nodular lesions and ulcers found in the skin, subcutaneous tissues and associated lymph nodes most commonly on the hands and arms. The classic injury leading to sporotrichosis is the pricking of a finger with a rose thorn – a common source of this fungus. Specific diagnosis is usually made histologically by the observance of the characteristic “cigar bodies” or elongated yeast cells in infected tissue.

Systemic mycoses

Systemic fungal infections may involve many tissues and organs of the body. These diseases are caused by dimorphic fungi which are found in the soil and produce spores which are inhaled by the human host. Dimorphism is the unusual property of a very few fungi to exist as molds in nature and as yeasts when infecting humans.

Following inhalation of the fungal spores, a primary respiratory infection may occur. In a vast majority of cases the pulmonary infection is asymptomatic or only mildly symptomatic and self-limiting, rarely progressing beyond the lungs. However, in rare cases these infections become systemic and a very distinct tissue phase of the organism is characteristic.

Three types of dimorphic, systemic fungal infections occur in North America – coccidioidomycosis, histoplasmosis and North American blastomycosis. They are geographically limited.

Coccidioidomycosis
Commonly known as valley fever, this infection is endemic to the San Joaquin Valley in California but can also be found in several other south western states, northern Mexico, Honduras, Venezuela, Bolivia, etc. See Coccidiodomycosis map Canadian cases occur in people that have traveled through these areas and have inhaled the arthrospores produced by the mycelial phase of the fungus growing in the soil.

The causative agent is Coccidioides immitis, which can produce a primary, self-limiting respiratory infection. In rare cases the disease may become systemic and affect the meninges, spleen and bones. Microscopic examination of biopsy material from infected tissue reveals the presence of round thick-walled spherules containing endospores, which constitute the yeast phase of the fungus responsible.

Histoplasmosis
Caused by the soil inhabitant, Histoplasma capsulatum, it is the only dimorphic fungus that is endemic in Canada. It is present in the lower St. Lawrence and the Ottawa river valleys. Inhalation of tuberculate macroconidia (spores) of the fungus can result in a primary lung infection and, in extremely rare cases the infection may become systemic involving the reticulo-endothelial system. Very small yeast cells (1-2 ┬Ám) of the dimorphic fungus may be microscopically observed in the macrophages of infected hosts.

A particularly severe form of histoplasmosis, Progressive Disseminated Histoplasmosis (PDH) is a serious concern in AIDS patients residing in endemic areas. It is uniformly fatal if not treated. Patients with PDH may be diagnosed by visually observing yeast cells in peripheral blood leucocytes.

North American Blastomycosis
This infection, caused by the dimorphic fungus, Blastomyces dermatitidis, may occur in three clinical forms: cutaneous, pulmonary and systemic. Inhalation of spores produced by the fungus may result in a primary pulmonary infection which may become acute and chronic and of varying severity. The cutaneous disease is the most common form of extra pulmonary blastomycosis and nodules may appear on the skin and ulcerate on such peripheral areas as the face, hands, wrists or lower legs. Systemic blastomycosis results in patients with extensive and non-resolving pulmonary involvement and can affect the bone, urogenital system including the kidneys. The presence of large, single-budding yeast cells of the tissue phase of B. dermititidis can be determined by microscopic examination of clinical material.

Category: Microorganisms

Other Notes