Mycoses (singular: mycosis) refer to infections caused by fungi. The term originates from the Greek word “mykes,” meaning fungus.
More than 200,000 species of fungi have been scientifically documented worldwide.
Out of these, only about 100–150 fungal species are known to cause disease in humans.
Among the disease-causing fungi, approximately 25 species are responsible for the majority of human fungal infections.
The scientific field dedicated to studying fungi that cause human diseases is known as Medical Mycology.
This field focuses on understanding the epidemiology of fungal infections, including how these infections spread and affect populations.
It also examines the characteristics and pathogenic mechanisms of infections caused by specific fungal species.
Identification and diagnosis of fungal infections in medical mycology involve several laboratory techniques, including histological examination of tissues, direct microscopic observation of fungal structures, culture of fungi in laboratory media, and molecular detection methods such as the scientific technique of Polymerase Chain Reaction.
Epidemiology of Mycoses
Epidemiology of mycoses focuses on factors that influence the occurrence and spread of fungal infections in human populations.
A major contributing factor to the development of mycoses is the alteration of the host’s immune system, which reduces the body’s natural ability to defend against fungal pathogens.
Such immune alterations commonly occur due to underlying diseases or the use of immunosuppressive agents, both of which weaken immune responses.
When these immune changes cause an individual to become immunocompromised, the risk of fungal infections significantly increases.
In immunocompromised individuals, fungal infections may occur as exogenous infections, which originate from external environmental sources.
Exogenous fungal infections may enter the body through inhalation of fungal spores, ingestion of contaminated materials, accidental physical injuries, or unintentional trauma during surgical procedures.
Fungal infections can also occur as endogenous infections, which originate from microorganisms already present within the body.
In endogenous infections, fungi that normally exist as part of the normal microbial flora may transform into opportunistic pathogens when the host’s immune defenses are weakened.
Types of Mycoses
Superficial mycoses
Cutaneous mycoses
Subcutaneous mycoses
Systemic mycoses
Opportunistic mycoses
1. Superficial Mycoses
Superficial mycoses are fungal infections that affect the outermost layers of the body, including the hair, nails, and the skin layer known as the stratum corneum, without spreading into deeper tissues.
Because these infections remain limited to the surface, they generally do not disseminate into internal organs or deeper skin layers.
Infection of the hair shaft is known as piedra, which occurs in two forms.
Black piedra is caused by the fungus Piedraia hortae and is characterized by the formation of black nodules on scalp hair.
White piedra is caused by Trichosporon beigelii and typically affects beard and mustache hair, producing white or light-colored nodules along the hair shaft.
The yeast Malassezia furfur causes a superficial skin infection called pityriasis versicolor, which results in discolored patches on the skin.
Commensal yeasts such as Candida albicans can also cause superficial yeast infections, particularly in the nails and mucous membranes of the mouth and vagina, when conditions favor fungal overgrowth.
2. Cutaneous Mycoses
Cutaneous mycoses are fungal infections that affect the keratinized tissues of the body, including the skin, hair, and nails.
These infections are primarily caused by dermatophytes, which are mould fungi capable of invading keratinized tissues.
Dermatophyte infections are commonly referred to as ringworm or tinea, and the infections collectively are known as dermatomycoses.
Dermatophytes possess the ability to break down keratin, a structural protein found in the skin, nails, and hair, using it as a source of nitrogen for growth.
The major dermatophyte genera responsible for these infections include Trichophyton, Epidermophyton, and Microsporum.
Tinea barbae refers to infection of the beard hair, mainly caused by Trichophyton mentagrophytes.
Tinea capitis is an infection of the scalp hair caused by species of Trichophyton and Microsporum.
It is characterized by hair loss and scaling of the scalp.
In some cases, dermatophyte infection may lead to inflammation and suppurating lesions.
Tinea corporis is an infection of the skin, characterized by itchy, red, circular lesions with vesiculopustular borders.
It is mainly caused by Trichophyton rubrum or Microsporum canis.
Tinea cruris is the infection of the groin region, commonly known as jock itch, and is caused by Trichophyton rubrum, Epidermophyton floccosum, or Trichophyton mentagrophytes.
Tinea unguium (onychomycosis) refers to fungal infection of the nails, which results in thickened, discolored, and crumbly nails.
The main causative agents are Trichophyton rubrum and Trichophyton mentagrophytes.
Tinea pedis, commonly known as athlete’s foot, is characterized by itchy, vesiculopustular or scaly lesions on the feet.
The primary causative agents are Trichophyton rubrum and Epidermophyton floccosum.
3. Subcutaneous Mycoses
Subcutaneous mycoses are fungal infections that affect the subcutaneous tissues and sometimes adjacent bones.
These infections typically occur when saprophytic fungi present in soil, thorns, or wood splinters enter the body through punctures, abrasions, or cuts in the skin.
The disease develops slowly over time, forming subcutaneous nodules that may ulcerate.
These nodules can spread through the lymphatic system, leading to the development of additional nodules along lymphatic channels.
The three major forms of subcutaneous mycoses include mycetoma, chromoblastomycosis, and sporotrichosis.
Mycetoma
Mycetoma is a chronic granulomatous infection characterized by tumor-like swelling of subcutaneous tissue, most commonly affecting the legs, particularly in individuals who walk barefoot.
If untreated, the infection can spread to the fascia and bones, causing severe damage that may eventually require amputation.
Mycetoma can be caused by both bacteria and fungi.
Actinomycotic (bacterial) mycetoma is caused by bacteria such as Nocardia, Streptomyces, and Actinomadura.
Eumycotic (fungal) mycetoma is divided into two categories based on grain color:
White grain mycetoma caused by hyaline septate fungi such as Pseudallescheria boydii, Acremonium, and Fusarium.
Black grain mycetoma caused by dematiaceous fungi such as Madurella mycetomatis, Exophiala jeanselmei, and Curvularia.
Chromoblastomycosis (Chromomycosis)
Chromoblastomycosis is caused by dematiaceous fungi including Cladophialophora carrionii, Phialophora verrucosa, Fonsecaea pedrosoi, and Fonsecaea compacta.
The disease begins as a papule at the site of infection, which slowly progresses into verrucous, cauliflower-like wart lesions.
Sporotrichosis
Sporotrichosis is caused by the dimorphic fungus Sporothrix schenckii.
This fungus is commonly found in soil, barberry shrubs, roses, sphagnum moss, and pine-bark mulch.
Individuals such as gardeners and florists are particularly susceptible to infection.
The disease is characterized by red papules appearing after an incubation period of 1–12 weeks, which eventually develop into sores.
The primary lesion may remain localized, but in immunocompromised individuals, the infection can spread to other organs, resulting in extracutaneous sporotrichosis, which may involve the lungs, blood, lymphatic system, bones, and brain.
4. Systemic Mycoses
Systemic mycoses are fungal infections that usually begin when airborne fungal spores are inhaled into the lungs.
Unlike superficial, cutaneous, or subcutaneous mycoses, these infections can disseminate from the lungs to other organs and tissues throughout the body, leading to systemic disease.
The etiological fungal pathogens responsible for systemic mycoses are generally classified into two categories: true pathogens and opportunistic pathogens.
a. True Pathogens
True pathogenic fungi are typically dimorphic fungi capable of infecting both immunocompromised and healthy individuals.
These fungi exhibit dimorphism, meaning they exist as mould in the environment and convert into yeast forms within the host tissues, which represents their invasive or tissue phase.
An exception is Coccidioides immitis, which forms spherules instead of true yeast cells in host tissues.
Opportunistic fungal pathogens include yeasts and moulds that primarily infect individuals with weakened immune systems, particularly immunocompromised patients.
Important opportunistic fungi include:
Cryptococcus neoformans and Cryptococcus gattii – cause cryptococcosis
Species of Aspergillus – cause aspergillosis
Species of Candida – cause candidiasis
Blastomycosis
Blastomycosis is caused by Blastomyces dermatitidis.
The infection initially presents as acute pulmonary blastomycosis, affecting the lungs after inhalation of fungal spores.
Depending on the immune status of the individual, the disease may progress into chronic pulmonary infection.
Dissemination of the infection can occur, with the skin being the most common secondary site.
Cutaneous lesions are typically suppurative and granulomatous with well-demarcated edges.
In some cases, complications such as osteomyelitis, arthritis, and rarely meningitis may develop.
Coccidioidomycosis
Coccidioidomycosis is caused by Coccidioides immitis.
Infection begins when arthroconidia are inhaled into the respiratory tract.
In many individuals, the infection is asymptomatic or manifests as a mild pulmonary illness similar to common respiratory infections.
In certain cases, the disease progresses into chronic pulmonary disease and may disseminate to other organs.
Disseminated infections are more frequently observed in individuals of Asian or Black ethnicity and in pregnant women.
Paracoccidioidomycosis
Paracoccidioidomycosis is caused by Paracoccidioides brasiliensis.
The disease begins as a primary pulmonary infection, which is usually asymptomatic in early stages.
As the infection progresses, it may lead to chronic granulomatous inflammation.
In children and adolescents, the disseminated form often results in enlargement of superficial or visceral lymph nodes.
In adults, dissemination commonly leads to lesions of mucous membranes, especially affecting the nasal and oral mucosa.
Histoplasmosis
Histoplasmosis is caused by Histoplasma capsulatum.
The infection begins in the lungs after inhalation of aerosolized microconidia.
Approximately 95% of infections are asymptomatic and resolve spontaneously in immunocompetent individuals.
However, high levels of exposure may cause symptoms such as fever, cough, and joint pain.
Dissemination of histoplasmosis occurs through the reticuloendothelial system, primarily affecting the liver, spleen, and lymph nodes.
In immunocompromised individuals, the infection may lead to hepatic involvement and adrenal gland deterioration.
Regardless of immune status, histoplasmosis can produce ulcerative pulmonary lesions resembling those seen in Tuberculosis.
Cryptococcosis
Cryptococcosis is mainly caused by the encapsulated yeast Cryptococcus neoformans.
Infection occurs through the respiratory tract, leading initially to transient pulmonary infection.
The infection may disseminate to other organs, including the bones, skin, and brain.
When the brain becomes involved, the disease manifests as cryptococcal meningitis, which is particularly common in patients with Acquired Immunodeficiency Syndrome.
Cryptococcus gattii differs in that it can infect immunocompetent individuals.
This species is capable of reducing the host inflammatory immune response, allowing it to successfully establish infection.
Both Cryptococcus species are encapsulated yeasts, and the capsule enhances their pathogenicity.
The capsule also contributes to their survival in the gastrointestinal tract of pigeons, which are an important environmental reservoir.
5. Opportunistic Mycoses
Opportunistic mycoses are fungal infections caused by fungi that normally exist as saprophytic organisms in the environment or as normal microbiota in the human body.
These fungi typically do not cause disease in healthy individuals but become pathogenic when the host immune system is weakened.
Individuals with debilitated immune systems, such as those undergoing organ transplantation requiring immunosuppressive therapy, those suffering from underlying diseases, or those exposed to prolonged antibiotic use, become more vulnerable to these infections.
Under such conditions, the host becomes defenseless against opportunistic fungal invasion.
The most common etiological agents responsible for opportunistic mycoses include species of Aspergillus, which cause aspergillosis, and species of Candida, which cause candidiasis.
a. Aspergillosis
Aspergillosis is an opportunistic fungal infection caused by species of the genus Aspergillus.
More than 200 species of Aspergillus have been scientifically documented, but around 20 species are known to cause aspergillosis in humans.
The predominant pathogenic species include:
Aspergillus fumigatus
Aspergillus niger
Aspergillus flavus
Aspergillus nidulans
The primary portal of entry for Aspergillus infection is the respiratory system, where fungal conidia or conidiophores are inhaled into the lungs.
Both atopic and non-atopic individuals may develop allergic reactions or infections after exposure.
Based on clinical features, aspergillosis is classified into several forms:
Invasive aspergillosis
Aspergilloma
Sinusitis
Allergic bronchopulmonary aspergillosis
i. Invasive Aspergillosis
The primary etiological agent is Aspergillus fumigatus.
In immunocompromised individuals, the fungus grows rapidly in the lungs, invading lung tissue and blood vessels.
This invasion can lead to septic emboli formation.
Although fungemia is relatively rare, neutropenic patients are particularly susceptible to systemic infection.
ii. Aspergilloma
Aspergilloma is commonly referred to as a “fungus ball.”
It forms when fungal mycelium colonizes pre-existing cavities in the lungs, producing a dense mass of intertwined hyphae.
The primary causative agent is Aspergillus fumigatus.
Many patients remain asymptomatic, although some may produce sputum that occasionally contains blood (hemoptysis).
iii. Sinusitis
Fungal sinusitis occurs when Aspergillus species infect the paranasal sinuses.
The most common causative agents are Aspergillus flavus and Aspergillus fumigatus.
iv. Allergic Bronchopulmonary Aspergillosis
Allergic bronchopulmonary aspergillosis (ABPA) is primarily caused by Aspergillus fumigatus.
It is most frequently observed in atopic individuals, particularly those with hypersensitivity reactions.
In this condition, the fungus proliferates within the airways, leading to the formation of mucus plugs.
Other infections caused by Aspergillus species include external otomycosis, mycotic keratitis, and onychomycosis.
b. Candidiasis
Candidiasis is caused by yeasts belonging to the genus Candida.
The two most common etiological agents are:
Candida albicans
Candida glabrata
Other emerging pathogenic species include:
Candida tropicalis, particularly common in neutropenic patients
Candida parapsilosis, associated with hospital-acquired infections in neonatal intensive care units
Candida krusei
Based on the site of infection, candidiasis is divided into:
Superficial candidiasis
Invasive candidiasis
Superficial Candidiasis
Superficial candidiasis affects the skin, nails, and mucous membranes, particularly those of the mouth and vagina, where Candida normally exists as part of the normal microbiota.
Oropharyngeal candidiasis, also known as oral thrush, is characterized by white patches or flecks on the tongue and palate.
Vaginal candidiasis presents with yellow-white curd-like discharge, white sores on the vulva, and intense itching.
Cutaneous candidiasis is less common than dermatophyte infections but typically occurs in moist areas of the body, such as the groin.
Invasive Candidiasis
Invasive candidiasis is a systemic infection that may be localized to a single organ or disseminated throughout the body via the bloodstream, a condition known as candidemia.
Dissemination can lead to infections in deep organs and tissues, producing conditions such as meningitis, osteomyelitis, and endocarditis.
A significant clinical manifestation is Candida endophthalmitis, particularly observed in neutropenic patients.
In this condition, white lesions develop in the eye, leading to eye pain and impaired vision.
References
Hugo and Russell’s Pharmaceutical Microbiology by Stephen P. Denyer, Norman A. Hodges, and Sean P. Gorman. Published in 2004 by Blackwell Science. Relevant information referenced from page 47.
Prescott, Harley and Klein’s Microbiology authored by Joanne M. Willey, Linda M. Sherwood, and Christopher J. Woolverton. Published in 2008 by McGraw-Hill.
Medical Microbiology written by David Greenwood, Mike Barer, Richard Slack, and Will Irving. Published in 2012 by Churchill Livingstone Elsevier.
Microbiology by Michael J. Pelczar Jr., E. C. S. Chan, and Noel R. Krieg. Published in 1993 by Tata McGraw-Hill Education. Relevant content referenced from pages 676–685.
Bailey and Scott’s Diagnostic Microbiology authored by Patricia M. Tille. Published in 2014 by Elsevier Mosby. Relevant material referenced from pages 705–757.
Prescott, Harley and Klein’s Microbiology by Joanne M. Willey, Linda M. Sherwood, and Christopher J. Woolverton. Published in 2008 by McGraw-Hill. Relevant content referenced from pages 997–1018.
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