Introduction to Fungi, Third Edition

292 PLECTOMYCETES
Histoplasma capsulatum grows as a mycelium
at room temperature, but at 37°C it develops
small budding yeast cells (3 5 mm) which can
spread throughout the patient under favourable
conditions. The normal route of infection is by
inhalation, especially of the small microconidia.
One common way to become infected is by
cleaning buildings of bird or bat excreta, or by
exploring caves in which these animals dwell;
histoplasmosis is therefore also called the
‘spelunker’s disease’ (Woods, 2002). Three strains
of H. capsulatum are distinguished, and they are
present in soil, especially when contaminated
with bird or bat guano. Endemic areas are North
America (var. capsulatum) or equatorial Africa
(var. duboisii). Histoplasma capsulatum var. farciminosum
infects horses sporadically in Africa, Asia
and Eastern Europe (Weeks et al., 1985).
Ajellomyces dermatitidis (anamorph
Blastomyces dermatitidis)
The gymnothecia of A. dermatitidis are similar to
those of A. capsulatus, but asexual reproduction is
by means of stalked or sessile conidia which are
smooth or spiny, but not tuberculate. Yeast cells
are formed at 37°C, and these are much larger
(10 12 mm in diameter) and have a thicker wall
than those of H. capsulatum. The disease (blastomycosis)
starts as an infection of the lung
which can spread systemically to other sites,
especially skin and bones. As in H. capsulatum,
considerable efforts are currently being made to
characterize the cell surface properties in
B. dermatitidis (Hogan et al., 1996; Brandhorst
et al., 2002). Crucial roles are probably played by
the presence or absence of a-(1,3)-glucan in the
cell wall, and by surface adhesins which are
proteins that mediate the recognition of yeast
cells by the host’s immune system.
The fungus is soil-borne, especially in moist
soil such as the banks of rivers. It is of North
American origin, being particularly prevalent in
the Mississippi and Ohio river valleys. Isolates
from Africa represent a genetically different
subpopulation (Guého et al., 1997).
Paracoccidioides brasiliensis
No teleomorph has been described as yet, but
DNA sequence data predict that it will be an
Ajellomyces if it is found (Guého et al., 1997).
Conidia (arthroconidia) are seldom formed in
culture, but the fungus is readily recognizable
in the yeast state at 37°C because it produces a
large, thick-walled central cell (30 mm in diameter)
from which several smaller daughter cells
bud off, often several at the same time. This type
of budding is therefore called the ‘pilot-wheel
stage’ (Fig. 11.3e). The first organ affected by
paracoccidioidomycosis is the lung, but the
infection may spread, causing grossly deforming
lesions on mouth, nose and in gastrointestinal
regions. The fungus is thought to occur in forest
soils in areas with heavy rainfall in Central and
South America. As with the other serious pathogens
described in this section, the precise
ecological niche of P. brasiliensis is still obscure,
but it is possibly spread by the nine-banded
armadillo, Dasypus novemcinctus (Restrepo et al.,
2001).
Coccidioides immitis
In culture, C. immitis reproduces by Malbrancheatype
arthroconidia (Fig. 11.3f). In infected tissues,
arthrospores swell and give rise to large, thickwalled
spherical cysts or spherules (50 100 mmin
diameter) which produce endospores. Endospores
are very small (3 4 mm) and are readily disseminated
in the bloodstream. Each endospore can
develop into a new spherule. The disease (coccidioidomycosis)
can be benign with influenza-like
symptoms, but infections of the lung may spread
to other organs such as the skin, brain and bones.
Coccidioidomycosis can be fatal even to immunocompetent
humans (Dixon, 2001).
DNA sequence comparisons indicate a relationship
with the teleomorphic genus Uncinocarpus
(Sigler et al., 1998). Further, detailed studies of
the distribution of specific DNA sequences in the
genomes of isolates from various patients have
revealed that sexual recombination must occur
in nature, even though no teleomorph is known
(Burt et al., 1996). Coccidioides immitis is a soil-borne
fungus present as arthrospores, especially in arid
regions of the south-western United States and
in localized places in South America. It can infect
wild mammals, but outbreaks occur especially
among farmers and building workers, and after
dust storms, earthquakes and other events that