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Ecology
The genus
Microsporum includes
anthropophilic,
geophilic,
and
zoophilic
species. Some species are cosmopolitan while others have
geographically restricted distributions. Geophilic
Microsporum species inhabit the soil, the zoophilic species
affect animals, and the anthropophilic species primarily affect
humans. However, there are some species that are isolated from
both soil and animals. Microsporum is the asexual
state of the fungus and the teleomorph phase is referred to as
genus Arthroderma.
Species
There are seventeen
conventional species under genus Microsporum and among
these, there are nine significant species namely, anthropophilic
Microsporum audouinii, zoophilic Microsporum canis
isolated from cats and dogs, geophilic Microsporum
cookei isolated from furs of cats, dogs, and rodents,
Microsporum distortum, anthropophilic Microsporum
ferrugineum, zoophilic Microsporum gallinae mainly
isolated from fowl, zoophilic Microsporum gypseum isolated
from fur of rodents, Microsporum nanum which is both
geophilic and zoophilic in nature are isolated from swine, and
zoophilic Microsporum persicolor isolated from vole and
field mouse. Molecular studies for taxonomic re –
classification of Microsporum species are still in
progress.
Pathogenicity and Health Effects
Microsporum
is one of the three genera that cause
dermatophytosis
which is the general term referring to the infection that occurs
in hair, skin or nails caused by any
dermatophyte
species. Microsporum species has the ability to
degrade keratin and thus, can dwell on skin and its appendages and
still remains non – invasive. The fungus is bestowed with
virulence factors such as its keratinase enzyme, proteinases, and
elastases. Microsporum species particularly infect
the hair and skin, except for Microsporum persicolor which
does not infect the hair. The pathogenesis of the infection
depends on the natural reservoir of the species in such a way that
the geophilic species are acquired through contact with soil,
zoophilic species are transmitted from the infected animal, and
direct or indirect human – to – human transmission is of concern
for anthropophilic species. Infections involving the nails are
rare. Immunocompromised patients are infected as well as the
otherwise healthy hosts.
Macroscopic Appearance
Ø
Growth
rate may range from being slow to rapid, and colonies are
glabrous, downy, or wooly, and diameter varies within 1 – 9 cm
after seven – day incubation at 25°C;
Ø
Surface
colony color ranges from white, beige, to cinnamon or rusty while
the reverse may be yellow to red – brown; and
Ø
The
ability to grow on rice grains, or referred to as the hair
perforation test, and also at 37°C help in the differentiation of
Microsporum species from one another.
Microscopic Appearance
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Septate hyphae, microaleurioconidia, macroaleurioconidia, and
hyphae – like
conidiophores are present;
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Microaleurioconidia are hyaline, solitary, smooth, oval to
clavate in shape, unicellular, and thin – walled;
Ø
Macroaleurioconidia are hyaline, typically fusiform (spindle
in shape), echinulate to roughened, thin – to thick – walled,
multicellular with 2 to 15 cells, often with annular frill; and
Ø
Inoculation on specific media, such as potato dextrose agar or
Sabouraud dextrose agar, added with 3 – 5% sodium chloride may be
necessary to promote macroconidia production of some strains; and
Ø
Differences in
macroconidia shape and abundance in
microconidia aid in species differentiation.
Laboratory Precautions
General laboratory
precautions are required, no special safety measures needed.
Susceptibility
Availability data on
in vitro susceptibility tests for dermatophytes are limited
and susceptibility tests are not yet standardized.
Terbinafine and itraconazole show active in vitro against
Microsporum species. However, in one of the vitro studies it
was observed that this fungus was fungus was found to the least
terbinafine – susceptible dermatophyte.
Griseofulvin was once
the drug of choice for treating infections caused by
Microsporum as well as other dermatopyhte – induced
infections. However, safer and more effective alternatives
are now available and preferred such as oral therapy with
terbinafine and itraconazole which are commonly used for treatment
of Microsporum infections.
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