Mycosis: Etiology, Types, and Classification

Mycosis (plural mycoses) is a fungal infection or disease caused by fungi. It varies in severity from mild symptoms such as rash to life-threatening infections. Severity depends on the site of infection, the extent of infection, and also on the immune status of the host. Both yeast and molds are involved to cause mycosis.

Why this classification matters

Before memorising the classification scheme, understand why it exists: the site-based classification of mycoses directly predicts which organism you should suspect, which specimen to collect, and which test to order.

A clinician who sees a patient with a chronic skin lesion approaches the diagnostic workup completely differently than one managing a neutropenic patient with pulmonary infiltrates even though both might ultimately be fungal infections. The classification framework below is not academic categorisation; it is the clinical decision tree microbiologists and physicians use every day.

Mycoses can be classified based on the

1. site of the infection
2. route of acquisition of the pathogen, and
3. type of virulence exhibited by the fungus

Site of infection

Based on site of infection, mycoses are of 4 types;

Superficial or Cutaneous mycoses

Superficial or cutaneous mycoses are fungal infections of the skin, hair, and nails that are restricted to the keratinized layers of the skin and its appendages. They cause little or no inflammation and there is no direct invasion of deeper tissues. Most cutaneous mycoses are caused by dermatophytes (agents of ringworm, athlete’s foot, etc), but may also be caused by yeasts of the Candida genus and other non-dermatophytic fungi, like tinea and piedra.

Superficial/Cutaneous Mycoses

Signs and symptoms of cutaneous mycoses vary depending on the infectious agent, site of the infection, and severity of the infection. For example, an itchy rash, and ring-like lesions at the site of infection is present with dermatophyte infections (tineas), while cutaneous candidiasis can present with a localized rash in skinfold areas.

Figure: Common Fungal Pathogens and Major Sites of Infections (image source cell.com)

Subcutaneous mycoses

Subcutaneous mycoses are localized infections of the skin and underlying tissues. It occurs when the fungus from the environment gains access to deeper tissues via a cut or puncture wound on the skin.  An inflammatory response develops in the subcutaneous tissue frequently with extension into the epidermis. Symptoms vary among the different diseases, but the common ones are the presence of a localized nodule, ulcerations, granulomatous tissue, subcutaneous mass with abscesses, and fistulae.

Systemic mycoses

Systemic mycoses are fungal infections affecting internal organs such as the lungs, brain eyes, etc. It occurs when fungi enter the body and are disseminated via the bloodstream to multiple organs. Systemic mycoses are common in immune-compromised individuals but may affect immunocompetent individuals too. Symptoms may vary according to the disease but fever, cough, and loss of appetite are common symptoms.

Systemic mycosis can be categorized into two types;

• endemic respiratory infections (affecting both immunocompetent and immunosuppressed) and
• opportunistic infections (greater risk in immunocompromised).

Opportunistic mycoses

Opportunistic mycoses are caused by fungi that are normally found as human commensals or in the environment. They can cause infections in immuno-suppressed individuals (people infected with HIV infections, undergoing cancer therapy or organ transplant recipients, etc).

Candidosis, cryptococcosis, aspergillosis, and zygomycosis can also show superficial and systemic manifestations.

Mycosis Type to Organism to Article

→ For diagnostic methodology applicable across all these categories, see Laboratory Diagnosis of Fungal Infections

Route of acquisition of the pathogen

On the basis of the route of acquisition, fungal infections may be exogenous or endogenous in origin.

Exogenous

When the pathogen is transmitted through an external source (either airborne, cutaneous or percutaneous), the mycoses are called exogenous mycoses. Examples include histoplasmosis, coccidioidomycosis, and blastomycosis.

Endogenous

When the pathogen is acquired from colonization or reactivation of a fungus from latent infection, it is called endogenous mycoses. Examples include Cryptococcus neoformans and Candida spp.

Type of virulence exhibited by the fungus

According to virulence exhibited by the pathogen, fungal infections are classified as primary or opportunistic infections.

Primary infection

Primary infection is when a pathogen establishes infection in an immunologically normal host. Example: histoplasmosis.

Opportunistic infection

An opportunistic infection is when the pathogen requires some compromise of host defenses in order for infection to become established. This may be secondary to an underlying disease process, such as diabetes mellitus, or due to an immunosuppressive agents.

Any fungi could be considered an opportunistic pathogen in the appropriate clinical settings. Fungi previously thought to be non-pathogenic may be the cause of infections. The most common encountered opportunistic fungal pathogens are Aspergillus, Candida, Cryptococcus, Pneumocystis jirovecii, etc.

How to Learn and Remember Mycosis Classification

"The site of a fungal infection is not just where it happens to be — it tells you which organism to suspect and which test to order first."

Key exam facts in one table

References

1. Walsh TJ, Dixon DM. Spectrum of Mycoses. In: Baron S, editor. Medical Microbiology. 4th edition. Galveston (TX): University of Texas Medical Branch at Galveston; 1996. Chapter 75. Available from: https://www.ncbi.nlm.nih.gov/books/NBK7902/
2. Bailey & Scott’s Diagnostic Microbiology, Forbes, 11th edition