Medical Classification of Fungi

Introduction

Fungal infections of the skin are also known as ‘mycoses’. They are common and generally mild. However,fungi can sometimes cause serious disease in very sick or otherwise immune suppressed people.

Medically important fungi can be classified into four categories on the basis of types of infection they cause. They are as follows:

a) Fungi associated with superficial mycoses/cutaneous mycoses.

b) Fungi associated with subcutaneous mycoses.

c) Fungi associated with systemic mycoses.

d)Fungi associated with opportunistic mycoses.

A) Fungi associated with superficial /cutaneous mycoses

The fungi in this group cause superficial or cutaneous fungal infection that involves hair , skin or nail without direct invasion of the deeper tissues.The fungi in this category include the dermatophytes, all of which infect the keratinized tissue. The infection by dermatophytes is commonly characterized by itching ,scaling skins, patches that become inflamed and weeping .The dermatophytes fall into three genera:Trichophyton, Epidermophyton, and Microsporum.

Since superficial mycosis affects the outer layers of the skin, the nails, and hair, it is known as cutaneous infection.The main groups of fungi causing superficial fungal infections are:

• Dermatophytes
• Yeasts like. candida,piedra, Malassezia
• Moulds

B) Fungi associated with subcutaneous mycoses

The fungi in this category produce or cause an infection that is confined to the subcutaneous tissues without dissemination to distant sites .The organisms are usually introduced by a puncture wound.Example include:

Sporotrichosis: caused bySporothrix schenckii

Chromomycosis: caused byPhialophoraandCladospora

Mycetoma : caused byMadurella mycetomatis

Fungi associated with subcutaneous mycosis infects the deeper layers of the skin such as:

• the dermis
• subcutaneous tissue and
• even bone.

The causative organisms of subcutaneous mycoses normally live in the soil living on rotting vegetation. They can get pricked into the skin as a result of an injury .Deeper skin infections include:

• Mycetoma
• Chromoblastomycosis

C) Fungi associated with systemic mycoses

Most of the fungi causing systemic mycoses are dimorphic fungi. Systemic mycoses are fungal infections affecting internal organs. .Only the mold form produces spore ,capable of infecting humans.The spores of some of these pathogens are so infectious that inhalation of single spore can initiate infection .Infection caused by this organism primarily involve the lungs but may become widely disseminated and involve any organ system.The respiratory infections in most cases are asymptomatic and may result without any therapy .However , the infection may turn life threatening in immune-compromised individuals if treatment is not instituted in time.Example includes as follow :

Histoplasmosis :which is caused by Histoplasma capsulatum

Blastomycosis : which is caused by Blastomyces dermatitidis

Coccidioidomycosis : which is caused by Coccidioides immitis

Paracoccidioidomycosis : which is caused by Paracoccidioides brasilensis.

Firstly fungi enter the body via the lungs than through the gut, paranasal sinuses or skin. The fungi can then slowly spread via the bloodstream and causing infection to multiple organs of a host including the skin, often causing multiple organs to fail and eventually resulting in the death of the patient. These fungi are mostly found in soil and wood debris. Histoplasma is prevalent in America, Africa and Asia. Coccidioides and paracoccidioidal are prevalent in the Southern USA and Central America.

Causes of systemic mycoses

• Patients who are immuno-compromised are more predisposed to systemic mycoses, but they can develop in otherwise healthy patients.
• Systemic mycoses can be split between two main varieties, endemic respiratory infections and opportunistic infections.

People who are in risk of systemic mycosis

Immune compromised people are at risk of systemic mycoses. Immunodeficiency result from:

• low white blood cell count
• Human immunodeficiency virus(HIV) infection
• After a major surgical operation
• Systemic malignancy (cancer)
• Organ transplant recipients
• Poorly controlled diabetes mellitus
• Very old or very young.

The prognosisof infection depends on the patient’s immune function Unfortunately, despite treatment many patients die of their infection

Diagnosis of systemic infection

The most reliable tests to confirm infection are as follows:

• skin biopsy for histological (microscopic) analysis,
• fungal culture.
• blood can also be cultured.

Treatment of infection

Systemic antifungal medications can also be given for treatment of systemic infection. Suitable drugs for treatment are as follows:

• Caspofungin
• Amphotericin B
• Voriconazole
• Fluconazole
• Itraconazole

D)Fungi associated with opportunistic mycosis

Any of the fungi could be considered as an opportunistic pathogen in the appropriate clinical settings.The infection with these organisms primarily occurs in patients with some type of compromised to their immune system. This may be secondary to the underlying disease processes such as diabetes mellitus or due to an immunosuppressive agent. Although any fungus could potentially cause diseases in these patients, the most commonly encountered organisms in this group includeAspergillus , Candida , and Cryptococcus. All of these organisms may cause disseminated diseases or systemic disease.

Other systemic mycoses infect those people who are already sick or with animmunodeficiencydisorder i.e. they are ‘opportunists’.

Risks for systemic mycoses

• Infection with (HIV)
• Serious illness and debility
• Diabetes mellitus
• Cancer or leukaemia
• Drug addiction
• Transplant
• Massive doses of antibiotics
• Parenteral nutrition

REFERENCE

D, Grenwood, Slack RCB, and Peutherer J. Medical Microbiology. Dunclude Livingstone: ELBS, 2001.

Cheesbrough, M.Medical Laboratory Manual for Tropical Countries.Vol 2. ELBS London, 2007.

Tille, P.Diagnostic Microbiology.13th. Elsevier, 2014.