BLASTOMYCES DERMATITIDIS is the causal agent of blastomycosis , an
invasive and often serious fungal infection found occasionally in
humans and other animals in regions where the fungus is endemic . The
causal organism is a fungus living in soil and wet, decaying wood,
often in an area close to a waterway such as a lake, river or stream.
Indoor growth may also occur, for example, in accumulated debris in
damp sheds or shacks. The fungus is endemic to parts of eastern North
America, particularly boreal northern
Ontario , southeastern Manitoba
Quebec south of the
St. Lawrence River
St. Lawrence River , parts of the U.S.
Appalachian mountains and interconnected eastern mountain chains, the
west bank of
Lake Michigan , the state of
Wisconsin , and the entire
Mississippi Valley including the valleys of some major tributaries
such as the
Ohio River . In addition, it occurs rarely in Africa both
north and south of the
Sahara Desert , as well as in the Arabian
Peninsula and the Indian subcontinent. Though it has never been
directly observed growing in nature, it is thought to grow there as a
cottony white mold, similar to the growth seen in artificial culture
at 25 °C (77 °F). In an infected human or animal, however, it
converts in growth form and becomes a large-celled budding yeast .
Blastomycosis is generally readily treatable with systemic antifungal
drugs once it is correctly diagnosed; however, delayed diagnosis is
very common except in highly endemic areas.
* 1 Morphology and phylogeny
* 2 Geographic distribution and variants
* 3 Ecology
* 4 Preventive measures
* 5 References
* 6 External links
MORPHOLOGY AND PHYLOGENY
Blastomyces dermatitidis is the causal agent of blastomycosis , a
potentially very serious disease that typically begins with a
characteristically subtle pneumonia-like infection that may progress,
after 1–6 months, to a disseminated phase that causes lesions to
form in capillary beds throughout the body, most notably the skin,
internal organs, central nervous system and bone marrow. Blastomyces
dermatitidis is the name applied to the ascomycetous fungus ,
Ajellomyces dermatitidis. In 2013, a second species was described in
the genus Blastomyces, B. gilchristii, which subsumes certain strains
previously assigned to B. dermatitidis. Despite widespread use, the
Blastomyces is currently invalid under the International Code of
Botanical Nomenclature. Along with two other important
Histoplasma capsulatum , Paracoccidioides
Polytolypa hystricis , species of
to a recently recognized fungal family, the
Ajellomycetaceae . The
three principal pathogens in this family are all grouped
physiologically as “dimorphic fungi ”: fungi that switch from a
mold-like (filamentous) growth form in the natural habitat to a
yeast-like growth form in the warm-blooded animal host. Blastomyces
dermatitidis itself is a sexual organism, occurring in nature as both
a + mating type and a − mating type. This is epidemiologically
important for two reasons: firstly, it implies that the organism will
be genetically variable, potentially leading to variations in disease
severity, treatment response and habitat preference; secondly, it
implies that a suitable, stable habitat must exist for the complex
process of sexual reproduction to take place. This habitat is as yet
unknown. In its asexual form, the fungus grows as a typical colonial
microfungus, comparable to
Rhizopus mold forms commonly
seen on mouldy bread.
In nature, the fungus forms a network of thread-like mycelium that
penetrates the substratum on which it grows, and then after 3–5 days
of growth begins to reproduce asexually with small (2–10 µm)
conidia (asexual spores). These conidia are probably the main
infectious particles produced by the fungus. They form on individual
short stalks and readily become airborne when the colony is disturbed;
their size places them well within the respirable size range for
particles, meaning that they can deposit deeply in the lungs when
inhaled. Sexual reproduction by the fungus requires the meeting of
colonies of + and – mating type , probably a relatively rare event,
and results in the production of small ascomata (sexual fruiting
bodies) 200–350 µm, looking, to the naked eye, similar to a woollen
fuzz ball, and in microscopic view consisting of a layer of
spiralling, springy guard hairs surrounding a fertile core in which
groups of 8 ascospores (sexual spores) are produced in small round
reproductive sacs (asci). The ascospores, at 1.5–2.0 µm, are among
the smallest reproductive particles produced by fungi, and are within
the respirable size range. The budding yeast cells seen in infected
tissues and bodily fluids are generally relatively large (ca. 8–15
µm) and characteristically bud through a broad base or neck, making
them highly recognizable to the pathologist. A small (“nanic”)
form is rarely seen with cells under 6 µm.
GEOGRAPHIC DISTRIBUTION AND VARIANTS
One of the unexplained regularities of nature is that there are
several fungi of different phylogenetic ancestry that show a similar
pattern of existence: dimorphism (conversion from a filamentous form
in the environment to a yeast form in warm-blooded host tissues),
virulent pathogenesis (ability to cause a significant infection in an
animal host that is otherwise in good health), pulmonary infectivity
(infection mainly via the lungs) and sharply delimited endemism
(occurrence in only a limited geographic range.). Blastomyces
dermatitidis is one of these fungi; the others are Histoplasma
Paracoccidioides brasiliensis ,
Coccidioides immitis , C.
Penicillium marneffei .
The geographic range of B. dermatitidis is largely focused around the
waterways of the St. Lawrence and Mississippi River systems of North
America. There is a widely distributed and much republished, partially
erroneous map that shows the U.S. portion of this range accurately,
inclusive of occurrence in Minnesota, Wisconsin, Ohio, Kentucky,
Arkansas, Tennessee, North and South Carolina, the Virginias,
Mississippi, Louisiana, and a few regions of states adjacent to those
named. The Canadian range of B. dermatitidis shows an abundance of
blastomycosis in broad areas north and south of the St. Lawrence River
in Quebec, as well as high endemicity along the north shore of Lake
Erie and the low endemicity in southeastern corner of Manitoba. Though
Quebec distribution is reasonably accurate, the rest of Canada is
Blastomyces dermatitidis is absent or nearly
so from the
Lake Erie area, but occurs sporadically on the north shore
Ontario , including metropolitan Toronto, and, most notably,
has areas of high endemicity throughout northern Ontario. Remarkably
high incidence is noted for some parts of the Kenora area and
climatologically similar areas of northwestern Ontario. To the west,
the range of endemic blastomycosis extends across southern Manitoba
and into adjacent Saskatchewan. A few cases have been reported from
north central Alberta, e.g., the Edmonton area, though in these cases
an atypical genetic group of the fungus may be involved.
In the rest of the world, B. dermatitidis occurs at low levels in
various parts of Africa, from Algeria to South Africa, as well as in
and near the Arabian Peninsula. The African isolates are divided into
two biologically different antigen groups: isolates from north of the
Sahara are similar to North American isolates in having A and K
antigens, while southern African isolates lack the A antigen.
Isolates from the middle east possess both antigens. The sub-Saharan
African isolates differ in the laboratory from other isolates by being
exceedingly difficult to convert to the yeast phase, and they also
show some enzymatic distinctions.
Blastomyces dermatitidis is one of the most ecologically mysterious
organisms causing human and animal disease. Prediction of disease risk
and prevention of disease are both made extraordinarily difficult by
our very poor understanding of where and how this organism normally
grows in nature. Despite decades of attempts at isolating organisms
from epidemiological foci, B. dermatitidis has only been isolated from
the environment 21 times. Most of these isolations have been based on
the arduous isolation techniques involving the suspension of soil or
other environmental materials in aqueous medium with antibacterial
antibiotics, and injection of mice with these materials, followed by
sacrifice of the animals when they appear ill or at the end of six
weeks. The internal organs of the mice are then checked
microscopically for evidence of blastomycosis. Needless to say, the
cost and complexity of performing such studies is imposing, especially
as the ethical clearance procedures for work involving animals become
ever more involved. More direct and economical mycological techniques
for environmental isolation, such as dilution plating, have never
yielded positive results for
Blastomyces growth. Since B. dermatitidis
will grow readily from clinical samples on common laboratory media,
the lack of success in isolating it from environmental materials is
generally ascribed to the inhibitory effects of co-occurring common
molds and antibiotic-resistant bacteria.
In just one experiment, a single positive B. dermatitidis culture was
gained via use of a novel enrichment broth technique. Recently, in an
important breakthrough, a specific
PCR technique was developed that
was able to detect B. dermatitidis in three environmental samples from
a dog kennel that had been experiencing problems with blastomycosis.
What has been learned from direct isolation and recent
PCR studies is
that B. dermatitidis tends to be associated with soils and wood debris
in areas “characterized by an acidic pH, high organic content (due
to rotting or decayed wood or vegetation and animal or bird
droppings), abundant moisture, and proximity to waterways”. Recent
PCR detections, for example, concerned a Kentucky dog kennel where 35
of 100 dogs had contracted blastomycosis. Previous isolations have
been from comparable sites such as soil and wood debris from an
Wisconsin beaver dam , and woody materials from a Wisconsin
woodpile. Isolation of B. dermatitidis was also accomplished from an
earthen floor indoors on one occasion.
There has been a long history of justifiable speculation that B.
dermatitidis may associate in nature with one or more indigenous North
American mammalian host species. To date, however, all the animal
species that have been subjected to focused investigation have been
exonerated of this specific connection. Unsubstantiated suspicion has
particularly focused on the beaver, but the shrew, the bat and
the prairie dog have also been focal points of interest, with no
conclusive interspecies association being demonstrated to date.
Interestingly, the closely related pathogenic fungus P. brasiliensis
in South America has a well substantiated, though not well understood,
ecological link with the nine-banded armadillo,
Dasypus novemcinctus .
This member of the mammalian order
Edentata has no close relatives in
the geographic range of B. dermatitidis.
Avoidance of exposure in endemic areas is the principal means of
disease prevention. Because the agent is known to distribute in dusts,
the minimization of dust-generating activities, such as digging,
sweeping, etc., is key. Although a method of soil decontamination has
been described and demonstrated to be effective, it uses hazardous
chemicals and its use is best reserved for situations that cannot be
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