Classification and Laboratory Diagnosis of Fungal Diseases
Overview of Fungal Disease Classification
Fungal diseases, also known as mycoses, are categorized based on the site of infection and the nature of the fungal agent involved. The classification system primarily divides these infections into four major groups: superficial mycoses, subcutaneous mycoses, systemic mycoses, and opportunistic mycoses. Superficial mycoses involve the outermost layers of the body, including the skin, hair, nails, and mucous membranes. Subcutaneous mycoses represent mycotic infections that affect the skin, subcutaneous tissues, and occasionally the bone; these typically result from the inoculation of saprophytic fungi found in soil or decaying matter. Systemic mycoses are deeper infections that involve multiple internal organs. They are largely caused by saprophytic fungi that are inhaled as spores, leading first to pulmonary infection and subsequently disseminating to cause various systemic manifestations. Finally, opportunistic mycoses are caused by fungi that are normally found as human commensals or environmental saprophytes but which cause disease when the host's immune system is compromised.
Superficial Mycoses and Dermatophytes
Superficial mycoses specifically involve the keratinized layers of the skin, hair, and nails. A primary subgroup within this category is the dermatophytes, which are classified into three distinct genera based on their invasion patterns and colonial characteristics. The genus Microsporium is known to invade both the hair and skin, frequently causing ringworm; when cultured on Sabouraud Dextrose Agar (SDA), its colonies typically appear reddish-brown. The genus Trichophyton facilitates infections in the skin, hair, and nails, and is a common cause of both ringworm and athlete's foot; its colonies on SDA are characterized by white or pink hues. The third genus, Epidermophyton, primarily invades the skin and nails, producing colonies that are green and slightly granular in texture. Beyond dermatophytes, other superficial infections include Tinea versicolor caused by Malassezia furfur, Tinea nigra caused by Hortaea werneckii, and Piedra, which can be black (caused by Piedraia hortae) or white (caused by Trichosporon beigelii).
Subcutaneous Mycoses (Intermediate Deep Infections)
Subcutaneous mycoses are considered intermediate deep infections because they involve tissues deep to the epidermis, such as the dermis and underlying muscles. These infections often localise in areas deeper than simple keratinized bodies but can also manifest in the mouth, vagina, and gastrointestinal tract (GIT). When cultured on Sabouraud Dextrose Agar, these fungi often produce cream-colored colonies. Notable examples of subcutaneous mycoses include Chromoblastomycosis and infections caused by Candida albicans. A comprehensive classification of these diseases include Mycetoma (caused by agents like Madurella mycetomatis or Pseudallescheria boydii), Sporotrichosis (caused by Sporothrix schenckii), Chromoblastomycosis (caused by Phialophora verrucosa or Fonsecaea pedrosoi), and Rhinosporidiosis (caused by Rhinosporidium seeberi).
Systemic Mycoses (Deep Infections)
Systemic mycoses represent deep fungal infections that involve internal organs. The natural habitat for the causative agents of these diseases is generally the soil. Infection occurs when a host inhales fungal spores, which primarily affects the lungs and can extend to other organs such as the eyes. On Sabouraud Dextrose Agar (SDA), these fungi typically form smooth, white or cream-colored colonies. Examples of systemic mycoses include Actinomycosis and infections by Cryptococcus neoformans. Major systemic diseases include Histoplasmosis (caused by Histoplasma capsulatum), Blastomycosis (caused by Blastomyces dermatitidis), Coccidioidomycosis (caused by Coccidioides immitis), and Paracoccidioidomycosis (caused by Paracoccidioides brasiliensis).
Opportunistic Mycoses and Other Fungal Entities
Opportunistic mycoses are caused by saprophytic fungi that usually exist as environmental contaminants or human commensals but rarely cause disease in healthy individuals. These fungi become pathogenic in immunocompromised patients. Examples include Geotrichum, which can occasionally lead to pulmonary, bronchial, or oral diseases. Other prominent opportunistic infections include Candidiasis (caused by Candida albicans and other Candida species), Cryptococcosis (Cryptococcus neoformans), Zygomycosis (caused by Rhizopus, Mucor, or Absidia), and Aspergillosis (caused by Aspergillus flavus, Aspergillus fumigatus, or Aspergillus niger). Additional opportunistic agents include Penicillium marneffei (Penicilliosis), Pneumocystis jirovecii (Pneumocystosis), and various Fusarium species (Fusariosis). It is important to note that superficial and systemic manifestations are also commonly observed in cases of candidiasis, cryptococcosis, aspergillosis, and zygomycosis. Furthermore, some fungi do not cause infection directly but produce health issues through the production of toxins, a condition known as Mycotoxicoses.
Laboratory Diagnosis of Fungal Diseases
The laboratory diagnosis of fungal infections begins with the collection of appropriate specimens based on the site of infection. These include scrapings of the skin, nails, and hair from infected areas, as well as scrapings from mucous membranes. Other clinical samples include sputum, cerebrospinal fluid (CSF), aspirated pus, and tissue biopsies. In cases of suspected systemic mycoses, blood and CSF are critical specimens for analysis.
Direct microscopy is a fundamental diagnostic tool. One standard method is the KOH Mount, where specimens like skin, hair, nails, or tissue are placed on a slide with a drop of KOH preparation. This is covered with a coverslip and incubated for minutes at . The KOH serves to digest keratin and other cellular materials, leaving the fungus intact for observation. Specific stains are also utilized: the Gram stain is commonly used to observe Candida species. Lactophenol Cotton Blue (LPCB) is used for wet mount preparations, where phenol kills live organisms, lactic acid preserves fungal structures, and cotton blue stains the chitin in the fungal cell wall. For detecting fungi within tissue sections, Periodic Acid-Schiff (PAS) stains are employed.