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The class
Zygomycetes is divided into two orders, Mucorales and Entomophthorales.
These two orders produce dramatically different infections.
Genera from the
order Mucorales (Rhizopus, Mucor, Rhizomucor, Absidia, Apophysomyces,
Cunninghamella and Saksenaea) cause an angioinvasive infection called
mucormycosis.
Mucormycosis
presents with rhino-orbito-cerebral, pulmonary, disseminated,
cutaneous, or gastrointestinal involvement.
Immunocompromising states such as haematological malignancy, bone
marrow or peripheral blood stem cell transplantation, neutropenia,
solid organ transplantation, diabetes mellitus with or without
ketoacidosis, corticosteroids, and deferoxamine therapy for iron
overload predispose patients to infection.
Mucormycosis in
immunocompetent hosts is rare, and is often related to trauma.
Mortality rates
can approach 100% depending on the patient's underlying disease and
form of mucormycosis.
Mode of infection:
These saprophytic fungi are
widely distributed in nature and infection is acquired by exposure to
their sporangiospores. Although relatively uncommon,
mucormycosis is the third most frequent opportunistic mycosis in
patients with neoplastic diseases.
Appearance of the hyphae:
The hyphae
of the mucoraceous zygomycetes have a characteristic appearance in
tissue sections.
Typical hyphae are broad (6 to 25
micrometer wide), thin-walled and pleomorphic, with irregular,
non-parallel contours.
Branches arise haphazardly, often at
right angles to the parent hyphae. Septa can be found in some of the
hyphae, though most of the hyphae appear nonseptate (cenocytic).
Because the hyphae have little
structural stability, they are often folded, twisted, wrinkled, or
collapsed.
The thin hyphal walls stain as well
with hematoxylin as with the special stains for fungi.
Thick-walled, ovoid chlamydoconidia are
rarely formed in tissues , but sporangia are almost never produced.
Clinical presentation:
Several clinical forms of mucormycosis are recognized.
Rhinocerebral mucormycosis
begins as a fulminant
infection of the nasal cavity, paranasal sinuses, and soft tissues of
the orbit.
The infection, often unilateral, may
extend directly from these sites to involve the meninges and brain,
and it may be complicated by thrombosis of the cavernous sinus and
internal carotid artery.
Patients with diabetic acidosis or
leukemia are predisposed to rhinocerebral infection, which is most
often caused by Rhizopus oryzae. Once established, this infection is
difficult to treat and is rapidly fatal.
Invasive
pulmonary mucormycosis
and disseminated mucormycosis occur preferentially in patients with
acute leukemia or lymphoma.
A more limited
endobronchial form,
which occurs in diabetics, causes bronchial obstruction and may
eventually lead to serious hemorrhage after hyphal invasion into
adjacent blood vessels.
Gastrointestinal
mucormycosis
usually begins as a secondary infection of preexisting ulcers in
malnourished persons, but it may also be a manifestation of
disseminated infection. The stomach is involved most frequently,
followed by the colon and small intestine.
Cutaneous mucormycosis
can be a manifestation of disseminated mucormycosis or can occur as a
primary infection of burned patients or patients whose surgical wounds
are dressed with contaminated elastic bandages. Many of the
bandage-associated nosocomial infections have been caused by Rhizopus
rhizopodiformis.
Disseminated mucormycosis
can involve almost any organ, most frequently the lungs, central
nervous sysytem, spleen, kidneys, heart, and gastrointestinal tract.
Septic thrombosis of the
coronary arteries produces
mycotic myocardial infarction.
Pathological features:
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Pulmonary mucormycosis is a progressive
infection characterized by pulmonary vascular invasion, parenchymal
infarction, and hematogenous dissemination.
Image Link
In
gastrointestinal
mucormycosis mucosal lesions are blackened, necrotic ulcers, and local
vascular invasion may lead to ischemia of adjacent segments of the
stomach or intestine.
Cutaneous lesions are necrotic nodules
that ulcerate and become covered with blackened exudates.
Invasive opportunistic infections
caused by the mucoraceous zygomycetes are characterized by tissue
infarction and, in the non-granulocytopenic host, acute suppurative
inflammation.
Infarcts are caused by thrombosis
complicating hyphal invasion of arteries and veins.
Angioinvasion may also lead to
hematogenous dissemination.
Microscopically the lesions are
characterized by coagulative necrosis and neutrophilic infiltration.
Rarely a granulomatous reaction is
observed in localized, indolent, or partially treated infections.
Treatment:
Successful treatment of mucormycosis
depends on early recognition of the infection and control of the
underlying disease coupled with radical debridement or excision of
devitalized tissue and systemic antifungal chemotherapy with
amphotericin B.
Prognosis:
Greater awareness of
this opportunistic infection and rapid clinical intervention have
resulted in improved prognosis.
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