Classifying Mycoses: Types and Examples

Fungal infections affect millions of people worldwide, ranging from minor skin conditions to life-threatening systemic diseases. For nursing students and healthcare professionals, understanding how mycoses are classified helps in recognizing symptoms, making accurate diagnoses, and providing appropriate care. The classification system organizes these infections based on the depth of tissue invasion and the type of tissue affected, creating four main categories that guide clinical decision-making.

How mycoses are classified 🔗

Mycoses are classified based on the site of infection, route of acquisition, and virulence type. The site-based classification divides fungal infections into four main groups depending on tissue involvement depth and host immune response. The primary pathogens possess well-defined geographic ranges, while opportunistic fungi appear ubiquitously, with infection frequency depending on immunocompromised patient populations.

Superficial mycoses: infections of the outermost skin layers 🔗

Superficial mycoses are fungal infections restricted to the stratum corneum and hair shafts, with no penetration into the epidermis. These infections cause minimal or no inflammation because they remain confined to dead, keratinized tissue. They essentially elicit no inflammatory response and are generally more cosmetically concerning than medically serious.

Tinea versicolor (pityriasis versicolor) 🔗

This common condition is caused by yeasts of the Malassezia species, including M. furfur, M. sympodialis, and M. globosa. The infection presents as multiple scaling macular lesions whose colors vary from white to brownish, typically appearing on the trunk, neck, and upper arms. The characteristic appearance under microscopic examination shows a pattern often described as resembling pasta and round objects due to the presence of short hyphae and yeast cells together.

Piedra: nodular hair shaft infections 🔗

Piedra manifests as fungal infection of hair shafts characterized by visible nodules. White piedra, caused by Trichosporon species including T. beigelii, appears as soft, whitish nodules attached to the hair cuticle. In contrast, black piedra, caused by the dematiaceous fungus Piedraia hortae, consists of black-colored, firm, irregular nodules also located on hair cuticles. Both conditions are more prevalent in tropical and subtropical regions.

Tinea nigra 🔗

Tinea nigra, caused by Hortaea werneckii, affects the corneum layer, especially of palmoplantar regions, producing an asymptomatic brownish macula. This infection can be mistaken for melanoma due to its dark appearance, making proper diagnosis essential.

Subcutaneous mycoses: deeper tissue infections 🔗

Subcutaneous mycoses include infections characterized by involvement of subcutaneous tissues usually at the point of traumatic inoculation. These infections typically result from traumatic inoculation of fungi into the skin, often in individuals working outdoors, particularly in tropical and subtropical environments.

Sporotrichosis 🔗

Sporotrichosis is caused by Sporothrix schenckii and involves subcutaneous tissue at the point of traumatic inoculation, usually spreading along cutaneous lymphatic channels. Often called rose gardener’s disease, it commonly occurs after working with plants, soil, or timber when the fungus enters through small wounds like thorn pricks or splinters.

Mycetoma 🔗

Mycetoma is a suppurative and granulomatous subcutaneous mycosis that destroys contiguous bone, tendon, and skeletal muscle. The infection is characterized by draining sinus tracts from which small but grossly visible pigmented grains or granules are extruded. These grains represent microcolonies of the infecting fungi and serve as a diagnostic hallmark of the disease.

Chromoblastomycosis 🔗

Chromoblastomycosis is characterized by verrucoid lesions of the skin, usually of the lower extremities, with histological examination revealing muriform cells. The most common causes are Fonsecaea pedrosoi, Fonsecaea compacta, Cladosporium carionii, and Phialophora verrucosa. Unlike mycetoma, this infection generally remains limited to subcutaneous tissue without involving bone, tendon, or muscle.

Systemic mycoses: soil-derived deep infections 🔗

Systemic mycoses are mainly pulmonary diseases caused by dimorphic pathogenic fungi. These organisms exist as molds in the environment, particularly in soil, but convert to yeast or other forms when they infect human tissues. Most cases are acquired through inhalation of fungal spores, causing localized pneumonia as the primary infection manifestation.

Histoplasmosis 🔗

Histoplasmosis results from inhalation of conidia of Histoplasma capsulatum which convert in vivo into the blastoconidial (budding yeast) form. The disease mainly occurs in the central and eastern United States but likely has nationwide distribution. Dissemination can occur to hilar and mediastinal lymph nodes, spleen, liver, bone marrow, and brain, particularly threatening infants and immunocompromised patients.

Cryptococcosis 🔗

Cryptococcosis most typically causes pneumonia and/or meningitis as an opportunistic infection. The infection is caused by Cryptococcus neoformans or Cryptococcus gattii, found in soil and bird droppings, particularly pigeon excreta. Defective cellular immunity, especially in acquired immunodeficiency syndrome, represents the most common risk factor.

Coccidioidomycosis 🔗

Coccidioidomycosis occurs predominantly in the southwestern United States and is caused by Coccidioides immitis and Coccidioides posadasii. Arthroconidia are inhaled and convert in the lung to spherules. Most infections are clinically mild, but some patients develop progressive pulmonary disease with potential dissemination to brain, bone, and other sites.

Blastomycosis 🔗

Blastomycosis is caused primarily by Blastomyces dermatitidis and Blastomyces gilchristii and mainly occurs in midwestern, south-central, and southeastern states. The infection results from inhalation of conidia from the mycelial phase which convert in vivo to the parasitic yeast phase. The clinical pattern typically presents as chronic pneumonia, with dissemination occurring most commonly to skin, bone, and in males, the prostate.

Opportunistic mycoses: infections in immunocompromised hosts 🔗

Opportunistic mycoses are caused by less virulent agents, and invasive disease almost always occurs in immunocompromised people or those with indwelling foreign bodies. Aspergillus and Candida species are the main organisms isolated most frequently from immunocompromised patients, with other relevant agents including Cryptococcus species, Fusarium species, and Zygomycetes.

Candidiasis 🔗

Candidiasis is the most common opportunistic fungal infection, with Candida albicans being the most common causative agent. The infection can be superficial or deep. Superficial candidiasis may involve epidermal and mucosal surfaces including oral cavity, pharynx, esophagus, intestines, urinary bladder, and vagina. Deep candidiasis affects kidneys, liver, spleen, brain, eyes, and heart, with major risk factors including prolonged broad-spectrum antibiotics, cytotoxic chemotherapy, corticosteroids, and vascular catheters.

Aspergillosis 🔗

Invasive aspergillosis most frequently involves the lungs and paranasal sinuses, potentially disseminating to brain, kidneys, liver, heart, and bones. Quantitative and functional defects in circulating neutrophils are key risk factors for development of invasive aspergillosis, with neutropenia from cytotoxic chemotherapy and systemic corticosteroids being common predisposing factors.

Mucormycosis (Zygomycosis) 🔗

Zygomycosis due to Rhizopus, Rhizomucor, Absidia, Mucor species causes invasive sinopulmonary infections. A particularly life-threatening form called rhinocerebral syndrome occurs in diabetics with ketoacidosis. Besides diabetic ketoacidosis, neutropenia and corticosteroid use represent other major risk factors. These fungi have a strong propensity for invading blood vessels, leading to tissue necrosis.

Clinical significance and recognition 🔗

Understanding mycosis classification enables healthcare providers to approach diagnosis systematically and initiate appropriate treatment promptly. The range of patients at risk for invasive fungal infections continues to expand to encompass patients with acquired immunodeficiency syndrome, those immunosuppressed due to therapy for cancer and organ transplantation, and those undergoing major surgical procedures. These fungal infections are underrecognized and frequently misdiagnosed, often resembling bacterial and viral community-acquired pneumonia.

Recognition patterns differ among the four categories. Superficial mycoses present primarily with cosmetic concerns and minimal symptoms. Subcutaneous infections typically follow traumatic inoculation with slow progression and localized involvement. Systemic mycoses often begin as respiratory infections that can disseminate in susceptible individuals. Opportunistic infections require consideration of the patient’s immune status and risk factors.

What do you think? How might understanding these classification categories change your approach to assessing patients with suspicious skin lesions or respiratory symptoms? What questions would you ask a patient presenting with a chronic, slowly progressing skin nodule to determine if it might be a subcutaneous mycosis?

References

1. https://www.ncbi.nlm.nih.gov/books/NBK7902/
2. https://pmc.ncbi.nlm.nih.gov/articles/PMC5514591/
3. https://pmc.ncbi.nlm.nih.gov/articles/PMC10561858/
4. https://www.sciencedirect.com/topics/medicine-and-dentistry/systemic-mycosis
5. https://www.cdc.gov/mmwr/volumes/71/ss/ss7107a1.htm
6. https://www.actasdermo.org/en-cutaneous-involvement-in-deep-mycoses-articulo-S1578219016302402
7. https://pmc.ncbi.nlm.nih.gov/articles/PMC4001330/
8. https://academic.oup.com/cid/article/78/6/1559/7295325