their animal host. These zoophilic species are transmitted from infected animals to humans by direct
and indirect contacts with domestic animals (e.g., cat and dog) and occasionally wild animals.
Examples are Trichophyton violaceum and Microsporum canis.
(iii) Geophilic species: These are saprophytic fungi found in
soil or in dead organic substances. They occasionally cause infection in humans and animals.
Examples are Microsporum gypseum and Trichophyton ajelloi. Dermatophytes usually grow only on
fungus antigen may cause secondary
reaction occurs as a result of hypersensitivity response to circulating fungal antigen, and these
lesions do not contain any fungal hyphae.
Laboratory diagnosis is based on demonstration of fungal element in clinical specimen by
microscopy and confirmation by culture. The specimens include skin scrapings and nail clippings or
hair taken from the areas suspected to be infected by dermatophytes. These entire specimens are
for diagnosis, while culture is always carried out to identify the specific causative fungal agent.
Examination of 10% direct KOH mount may show fungal
hyphae. Three types of hair infections can be demonstrated in
microscopy of 10% KOH wet mount as follows (Fig. 4-1):
hair shaft . It is caused by M. audouinii, M. canis, and Trichophyton mentagrophytes.
Section IV - Medical Mycology By Dr. Kareem Lilo
Endothrix: The clusters of arthrospores are found entirely
The clinical specimens are cultured by inoculation on SDA containing antibiotics like cycloheximide.
The media after inoculation are incubated at 25–30°C for 3 weeks. At 25°C most of the pathogenic
fungi grow well, while saprophytic fungi and bacteria are inhibited.
The cultures are examined at regular intervals, and dermatophytes
are identified based on (a) colony morphology, ( b) pigment
production, and (c) presence of microconidia and macroconidia.
Subcutaneous mycosis is defined as fungal infection associated
with development of characteristic lesion in subcutaneous tissue and overlying skin with or without
extension to bone and muscle. This is caused by a heterogeneous group of fungal infection of low
pathogenic potential introduced in the body percutaneously from a trivial trauma. shows the
classification of subcutaneous mycoses.
Mycetoma is a slowly progressive, chronic granulomatous infection of skin and subcutaneous tissues
with occasional involvement of underlying fascia and bone usually affecting
Figure (4-1) Endothrix and Ectothrix infection
Section IV - Medical Mycology By Dr. Kareem Lilo
extremities are most commonly involved. Microabscesses are formed in subcutaneous tissues
surrounded by polymorphonuclear inflammatory reaction. The center of the lesion consists of
tangled filaments of these organisms.
During the course of infection, microabscesses burst open
with the formation of chronic multiple sinuses discharging copious, seropurulent fluid containing
granules. The color and consistency of these granules vary depending on the fungi that cause the
disease.The condition is characterized by formation of painless, localized, swollen lesions on the
affected limbs.Multiple discharging sinuses are present.
Systemic mycoses are caused by fungi of soil, which are inherently virulent and cause disease in
healthy humans. The systemic ycoses include coccidioidomycosis, paracoccidioidomycosis,
histoplasmosis, blastomycosis, and cryptococcosis.
soil and as yeast at body temperature in mammals. On SDA medium at 37°C, this fungus produces
cottony mycelial growth. The colony is characterized by thin, branching, septate hyphae that produce
tuberculate macroconidia and microconidia.
Blastomycosis is a granulomatous fungal infection caused by B. dermatitidis. B. dermatitidis is a
dimorphic fungus, which occurs in two stages: as mold in soil and as yeast in tissue.
On culture at 37°C and in tissue, the yeast is a round structure
from the Cryptococcus neoformans yeast, which has a narrow-based bud. On culture at 25°C, the
fungus produces a mycelial growth showing typical pyriform microconidia, which measure 2–4 _m
Cryptococcosis, also called European blastomycosis, is an acute to chronic disease caused by an
encapsulated yeast, C. neoformans.
Cryptococcosis is the most common life-threatening fungal disease in patients with AIDS Of the 19
species that comprise the genus Cryptococcus, human disease is associated with only C. neoformans.
Section IV - Medical Mycology By Dr. Kareem Lilo
C. neoformans is a true yeast.
It is an oval and budding cyst and measures 3–6 _m in diameter. The yeast may be single or
may have a single budding daughter cell.
Within the host and in certain culture media, the yeast is urrounded by a wide
polysaccharide capsule. The polysaccharide capsule is composed of mannose, xylose, and
C. neoformans on SDA medium forms smooth, convex, cream-colored colonies at 20–37°C.
Lactophenol cotton blue (LPCB) wet mount of the colony shows budding yeast cells.
C. neoformans has two varieties: C. neoformans var neoformans and C. neoformans var gattii. Based on
antigenic specificity of the capsular polysaccharide, the species has been classified into four
serotypes. These are serotypes A and D (C. neoformans var neoformans) and serotypes B and C (C.
The immune status of the host is the crucial factor in pathogenesis
of cryptococcosis. C. neoformans usually causes most serious infections in patients with
patients undergoing corticosteroid treatment,
patients undergoing organ transplantation,
patients with reticuloendothelial malignancy, and
into the pulmonary alveoli, in which they survive before they are phagocytosed by alveolar
macrophages. Glucosylceramide synthase has been identified as an essential factor in the survival of
C. neoformans in pulmonary alveoli.
Cryptococcal polysaccharide capsule has antiphagocytic properties. Hence, unencapsulated yeast are
readily phagocytosed and destroyed than the encapsulated organisms, which are more resistant to
phagocytosis. The antiphagocytic properties of the capsule prevent recognition of the yeast by
phagocytes and inhibit leukocyte migration into the area of fungal replication.
The host immunity in cryptococcal infection is mediated by both cellular and humoral responses.
the tissues indicates a successful host response against the fungus. Humoral immunity is mediated
Section IV - Medical Mycology By Dr. Kareem Lilo
and the complement play a crucial
role in facilitating the macrophage- and lymphocyte-mediated immune response to the organism.
C.neoformans causes (a) pulmonary cryptococcosis in immunocompetent hosts and in
immunocompromised hosts, (b) CNS
cryptococcosis, and (c) disseminated nonpulmonary non-CNS cryptococcosis.
The clinical manifestations of pulmonary cryptococcosis are widely variable. Pulmonary disease
ffecting immunocompromised hosts. It depends on the immune status of the host
Both the brain and meninges are involved in cryptococcal infection of the CNS infections. Meningitis
and meningoencephalitis are the most common manifestations. These are usually subacute or chronic
in nature. Without specific therapy, the infection is invariably fatal. The patient dies due to the
disease 2 weeks to several years after the symptom onset.
o Disseminated nonpulmonary non-CNS cryptococcosis
to the lungs and CNS, and it occurs most commonly in patients with AIDS and other
serotypes A and D. C. neoformans var gattii is the most common variety that causes disease in
patients, e.g., AIDS. C.neoformans is primarily transmitted by inhalation
(Figure 4-2). Human-to-human transmission does not occur.
Section IV - Medical Mycology By Dr. Kareem Lilo
Laboratory diagnosis of cryptococcal infection is made by demonstration of the yeast in CSF,
of the capsule of C. neoformans. Fixed tissue may also be stained
with mucicarmine, which preferentially stains C. neoformans.
India ink preparation is commonly used to detect budding yeast cells in the CSF (Figure 4-3). The
capsule appears as a clear halo around the yeast cells. By this method, cryptococci can be
demonstrated in 25–50% of patients with cryptococcal meningitis. Gram-stained smear of the CSF
shows Gram-positive yeast cells . The culture of centrifuged CSF specimens confirms diagnosis of the
and ability to grow at 37 C° .
Figure (4-2) Transmission of Cryptococcus neoformans
Section IV - Medical Mycology By Dr. Kareem Lilo
Opportunistic Fungal infections
The opportunistic fungi usually cause infections in persons with impaired host defense, but do not
cause disease in most of the immunocompetent hosts. Since these fungi become pathogens in
individuals with impaired immunity by taking
advantage of the host’s debilitated conditions, they are called opportunistic fungi.
and various Zygomycetes (Table 4-4).
Candida species are the most common fungal pathogens that affect humans. These species are true
circulation and deep tissues. The genus Candida includes more than 100 species, of which only few
cause disease in humans. C. albicans and occasionally other species cause candidiasis, a major
infection in immunocompromised .
Figure(4-3) India ink preparation showing capsule of
Cryptococcus neoformans (_400).
Section IV - Medical Mycology By Dr. Kareem Lilo
Table (4-4 ) Common opportunistic infections
C. albicans is the most common Candida species, which causes
opportunistic infections in immunocompromised hosts. It
forms the part of the normal flora of the mucous membrane of
No comments:
Post a Comment
اكتب تعليق حول الموضوع