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Mycology
Pathogenesis of fungal diseases (Mycoses):
Most fungi are saprophytic or parasitic to plants and animals are adapted to
their natural environment. Infection in humans is a chance event, occurring
only when conditions are favorable. Except for few fungi such as the
dimorphic fungi that cause systemic mycoses and dermatophytes, which
are primary pathogens, the rest are only opportunistic pathogens.
The complex interplay between fungal virulence factors and host defense
factors will determine if a fungal infection will cause a disease. Infection
depends on inoculums size and the general immunity of the host.
The two major physiologic barriers to fungal growth within the human body
are temperature and redox potential. Most fungi are mesophilic and cannot
grow at 37oC. In general, the development of human mycoses is related
primarily to the immunological status of the host and environmental
exposure, rather than to the infecting organism. A small number of fungi
have the ability to cause infections in normal healthy humans.
Fungal Pathogenicity (virulence factors)
Candida albicans
elastase, collagenase
sm
-mediated immune defines of the host.
Host defiance factors:
turnover
(monocytes and macrophages) .
Factors predisposing to fungal infections:
CLINICAL GROUPINGS FOR FUNGAL INFECTIONS
1- SKIN MYCOLOGY
Superficial Mycoses
Cutaneous Mycoses
Subcutaneous Mycoses
2- INFECTIOUS DISEASE MYCOLOGY
Dimorphic Systemic Mycoses
Opportunistic Systemic Mycoses
SKIN MYCOLOGY
The Superficial Mycoses
These are superficial fungal infections of the skin or hair. No living tissue is
invaded and there is no cellular response from the host. Essentially no
pathological changes. These infections are often so innocuous that patients
are often unaware of their condition.
Pityriasis (tinea) versicolor
A chronic, superficial fungal disease of the skin characterized by white,
pink, or
brownish lesions, , and covered with thin scales. The color varies according
to the normal pigmentation of the patient, exposure of the area to sunlight,.
Lesions occur on the trunk, shoulders and arms, rarely on the neck and
face, and fluoresce a pale greenish color under Wood's ultra-violet light.
Young adults are affected most often, but the disease may occur in children
and old age.
The Cutaneous Mycoses
These are superficial fungal infections of the skin, hair or nails. No living
tissue is invaded, however a pathological changes occur in the host
because of the presence of the infectious agent and its metabolic products.
Dermatophytosis - Ringworm or Tinea
Ringworm of scalp, glabrous skin, and nails caused by a closely related
group of fungi known as dermatophytes which have the ability to utilize
keratin as a nutrient source. they have a unique enzymatic capacity keratinase. (e.g. Microsporum
Canis and Trichophyton rubrum).
The Subcutaneous Mycoses
These are chronic, localized infections of the skin and subcutaneous tissue.
The causative fungi are all soil saprophytes of regional epidemiology
whose ability to adapt to the tissue environment and elicit disease is
extremely variable.
Sporotrichosis and Chromoblastomycosis
Primarily a chronic mycotic infection of the cutaneous or subcutaneous
tissues and
lymphatics characterized by nodular lesions which may
suppurate and ulcerate. Infections are caused by the traumatic implantation
of the fungus into the skin, or very rarely, by inhalation into the lungs. And
the infection may also involve the central nervous system, lungs or
genitourinary tract.
INFECTIOUS DISEASE MYCOLOGY
Dimorphic Systemic Mycoses
These are fungal infections of the body caused by dimorphic fungal
pathogens which can overcome the physiological and cellular defences of
the normal human host by changing their morphological form.
They are geographically restricted and the primary site of infection is
usually pulmonary, following the inhalation of conidia.
Histoplasmosis
An intracellular mycotic infection of the reticuloendothelial system caused
by the inhalation of the fungus. Approximately 95% of cases of
histoplasmosis are subclinical . 5% of the cases have chronic progressive
lung disease, chronic cutaneous or systemic disease .
All stages of this disease may mimic tuberculosis.
Coccidioidomycosis
Initially, a respiratory infection, resulting from the inhalation of conidia, that
typically resolves rapidly leaving the patient with a strong specific immunity
to re-infection. However, in some individuals the disease may progress to a
chronic pulmonary condition or as a systemic disease involving the
meninges, bones, joints and subcutaneous and cutaneous tissues.
Opportunistic Systemic Mycoses
Opportunistic fungal infections of the body occur almost exclusively in
debilitated patients whose normal defence mechanisms are impaired. The
organisms involved are cosmopolitan fungi which have a very low inherent
virulence. The increased incidence of these infections and the diversity of
fungi causing them, has paralleled the emergence of AIDS and the use of
antibiotics, cytotoxins, immunosuppressives, steroids and other macro
disruptive procedures that result in lowered resistance of the host.
Candidiasis
A primary or secondary mycosis infection caused by members of the genus
Candida. The clinical manifestations may be acute, sub acute or chronic to
episodic. Involvement may be localized to the mouth, throat, skin, scalp,
vagina, fingers, nails, bronchi, lungs, or the gastrointestinal tract, or
become systemic as in septicemia, endocarditis and meningitis. In healthy
individuals, Candida infections are usually due to impaired epithelial barrier
functions and occur in all age groups, but are most common in the newborn
and the old ages. They usually remain superficial and respond readily to
treatment.
Systemic candidiasis is usually seen in patients with cell-mediated immune
deficiency, and those receiving aggressive cancer, immunosuppression, or
transplantation therapy.
Several species of Candida may be a etiological agents, most commonly,
Candida
albicans
and
rarely
C.
tropicalis,
C.
krusei.
And
C.
pseudotropicalis.. All are ubiquitous and occur naturally on humans.
Aspergillosis
Aspergillosis is a spectrum of diseases of humans and animals caused by
members of the genus Aspergillus. These include (1) mycotoxicosis due to
ingestion of contaminated foods; (2) allergy (3) colonization
(4) necrotizing disease of lungs, and other organs; and rarely (5) systemic
disease. The type of disease and severity depends upon the physiologic
state of the host
and the species of Aspergillus involved. The a etiological agents are
include
Aspergillus fumigatus, A. flavus, A. niger, and A. terreus
Laboratory diagnosis of mycoses:
Specimen collection: specimen collection depends on the site affected.
Different specimens include hair, skin scrapings, nail clippings, sputum,
blood, CSF, urine, corneal scraping, discharge or pus from lesions and
biopsy.
to prevent bacterial overgrowth. In case of delay specimens except skin
specimen, blood and CSF may be refrigerated for a short period.
under Wood’s lamp may be selectively plucked. Hairs may be collected in
sterilized paper envelopes.
lesion is scraped by forceps and collected in sterilized paper envelopes.
Discolored or hyperkeratotic areas of nail may be scraped or diseased
nail clipping may be collected in sterilized paper envelopes.
scraping and transported to laboratory in sterile tube containing saline.
positive microscopic results.
-mouthed container.
In certain cases, pus or exudates must be looked for presence of granules.
Microscopy: Microscopy is used to observe clinical specimens for the
presence of fungal elements or to identify the fungus following culture. In
the latter case, lacto phenol cotton blue is stain of choice, which stains the
fungal elements blue. Direct examination of clinical specimens could be
stained or unstained.
-20% KOH mount: Several specimens are subjected to KOH mount
for direct examination
. Addition of Dimethyl sulphoxide (DMSO) .
t dye, which binds selectively to
chitin of the fungal cell wall. The specimen then can be observed under
fluorescent microscope.
Cryptococcus neoformans.
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