Pseudallescheria boydii is
the most frequent etiologic agent of eumycotic mycetoma
and is an occasional agent of otomycosis, keratitis, endophthalmitis,
meningitis, brain abscess, and osteomyelitis.
This fungus can be
isolated from clinical specimens in either of the two forms: the
asexual (anamorphic) form, designated Scedosporium (formerly
Monosporium) apiospermum, and the sexual (telemorphic) form,
designated Pseudallescheria (formerly Allescheria and
Petriellidium) boydii.
By convention, the disease is named for
the sexual form.
P. boydii
can colonize preexisting pulmonary cavities and form intracavitary
fungus balls that resemble those produced by the Aspergillus
spp.
The intracavitary mycelium consists of amorphous hyphal
aggregates or of contact, concentrically laminated and intertwined
hyphae.
Invasive pulmonary and disseminated pseudallescheriosis are
rare opportunistic infections that
occur preferentially in patients treated for acute leukemia who are
granulocytopenic.
Invasive lesions are infarctive as a consequence of
mycelial invasion of blood vessels, but abscesses have also been
reported.
Disseminated pseudallescheriasis frequently involves the
brain, thyroid gland, heart, and kidneys.
In tissue
sections, the hyphae of P. boydii resemble those of the Aspergillus
spp.
The septate hyphae, 2 to 5 micrometer wide, branch in a dichotomous
but haphazard pattern and may produce vesicles, truncated terminal
conidia, and terminal or intercalated chlamydoconidia .
Hyphal vascular
invasion is conspicuous in the invasive and disseminated lesions.
Because the morphologic features of the hyphae in tissue sections
closely resemble those of
aspergillosis, the diagnosis is best confirmed
by culture or immunoflourescence.
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