''Blastomyces dermatitidis'' is a
dimorphic fungus that causes
blastomycosis, an invasive and often serious fungal infection found occasionally in humans and other animals.
[ It lives 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
Endemism is the state of a species being found only in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also foun ...
to parts of eastern North America, particularly boreal northern Ontario
Ontario is the southernmost Provinces and territories of Canada, province of Canada. Located in Central Canada, Ontario is the Population of Canada by province and territory, country's most populous province. As of the 2021 Canadian census, it ...
, southeastern Manitoba
Manitoba is a Provinces and territories of Canada, province of Canada at the Centre of Canada, longitudinal centre of the country. It is Canada's Population of Canada by province and territory, fifth-most populous province, with a population ...
, Quebec
Quebec is Canada's List of Canadian provinces and territories by area, largest province by area. Located in Central Canada, the province shares borders with the provinces of Ontario to the west, Newfoundland and Labrador to the northeast, ...
south of the St. Lawrence River
The St. Lawrence River (, ) is a large international river in the middle latitudes of North America connecting the Great Lakes to the North Atlantic Ocean. Its waters flow in a northeasterly direction from Lake Ontario to the Gulf of St. Lawren ...
, parts of the U.S. Appalachian mountains
The Appalachian Mountains, often called the Appalachians, are a mountain range in eastern to northeastern North America. The term "Appalachian" refers to several different regions associated with the mountain range, and its surrounding terrain ...
and interconnected eastern mountain chains, the west bank of Lake Michigan
Lake Michigan ( ) is one of the five Great Lakes of North America. It is the second-largest of the Great Lakes by volume () and depth () after Lake Superior and the third-largest by surface area (), after Lake Superior and Lake Huron. To the ...
, the state of Wisconsin
Wisconsin ( ) is a U.S. state, state in the Great Lakes region, Great Lakes region of the Upper Midwest of the United States. It borders Minnesota to the west, Iowa to the southwest, Illinois to the south, Lake Michigan to the east, Michig ...
, and the entire Mississippi Valley
The Mississippi River is the main stem, primary river of the largest drainage basin in the United States. It is the second-longest river in the United States, behind only the Missouri River, Missouri. From its traditional source of Lake Ita ...
including the valleys of some major tributaries such as the Ohio River
The Ohio River () is a river in the United States. It is located at the boundary of the Midwestern and Southern United States, flowing in a southwesterly direction from Pittsburgh, Pennsylvania, to its river mouth, mouth on the Mississippi Riv ...
. In addition, it occurs rarely in Africa both north and south of the Sahara Desert
The Sahara (, ) is a desert spanning across North Africa. With an area of , it is the largest hot desert in the world and the list of deserts by area, third-largest desert overall, smaller only than the deserts of Antarctica and the northern Ar ...
, as well as in the Arabian Peninsula
The Arabian Peninsula (, , or , , ) or Arabia, is a peninsula in West Asia, situated north-east of Africa on the Arabian plate. At , comparable in size to India, the Arabian Peninsula is the largest peninsula in the world.
Geographically, the ...
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 . 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 drug
An antifungal medication, also known as an antimycotic medication, is a pharmaceutical fungicide or fungistatic used to treat and prevent mycosis such as athlete's foot, ringworm, candidiasis (thrush), serious systemic infections such as c ...
s once it is correctly diagnosed; however, delayed diagnosis is very common except in highly endemic areas.
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. The sexual form of this fungus was formerly known as '' Ajellomyces dermatitidis''.
In 2013, a second species was described in the genus ''Blastomyces'', '' B. gilchristii'', which subsumes certain strains previously assigned to ''B. dermatitidis''.[ Three more species have been described: '' Blastomyces emzantsi'', '' Blastomyces parvus'' and '' Blastomyces percursus''.
Along with two other important human-pathogenic fungi, '' Histoplasma capsulatum'', '' Paracoccidioides brasiliensis'' and '' Polytolypa hystricis'', species of ''Blastomyces'' belong to the family Ajellomycetaceae.][ The three principal pathogens in this family are all grouped physiologically as "]dimorphic fungi
A dimorphic fungus is a fungus that can exist in the form of both mold and yeast
Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom (biology), kingdom. The first yeast originated hundreds of mill ...
": 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 ''Penicillium
''Penicillium'' () is a genus of Ascomycota, ascomycetous fungus, fungi that is part of the mycobiome of many species and is of major importance in the natural environment, in food spoilage, and in food and drug production.
Some members of th ...
'' or '' Rhizopus ''mold forms commonly seen on mouldy bread.
In nature, the fungus forms a network of thread-like mycelium
Mycelium (: mycelia) is a root-like structure of a fungus consisting of a mass of branching, thread-like hyphae. Its normal form is that of branched, slender, entangled, anastomosing, hyaline threads. Fungal colonies composed of mycelium are fo ...
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
A conidium ( ; : conidia), sometimes termed an asexual chlamydospore or chlamydoconidium (: chlamydoconidia), is an asexual, non- motile spore of a fungus. The word ''conidium'' comes from the Ancient Greek word for dust, ('). They are also ...
(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 eight 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 (c. 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 capsulatum'', '' Paracoccidioides brasiliensis'', ''Coccidioides immitis
''Coccidioides immitis'' is a pathogenic fungus that resides in the soil in certain parts of the southwestern United States, northern Mexico, and a few other areas in the Western Hemisphere.
Epidemiology
''C. immitis'', along with its relativ ...
'', '' C. posadasii'' and '' Talaromyces 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
Lake Erie ( ) is the fourth-largest lake by surface area of the five Great Lakes in North America and the eleventh-largest globally. It is the southernmost, shallowest, and smallest by volume of the Great Lakes and also has the shortest avera ...
and the low endemicity in southeastern corner of Manitoba. Though the Quebec distribution is reasonably accurate, the rest of Canada is strongly misrepresented. ''Blastomyces dermatitidis'' is absent or nearly so from the Lake Erie area, but occurs sporadically on the north shore of Lake Ontario
Lake Ontario is one of the five Great Lakes of North America. It is bounded on the north, west, and southwest by the Canadian province of Ontario, and on the south and east by the U.S. state of New York (state), New York. The Canada–United Sta ...
, 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.][
]
Ecology
''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 abandoned Wisconsin ]beaver dam
A beaver dam or beaver impoundment is a dam built by beavers; it creates a pond which protects against predators such as coyotes, alligators, cougars, foxes, eagles, wolves and bears, and holds their food during winter. These structures modify th ...
,[ 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
Prairie dogs (genus ''Cynomys'') are herbivorous burrowing Marmotini, ground squirrels native to the grasslands of North America. There are five recognized species of prairie dog: black-tailed prairie dog, black-tailed, white-tailed prairie dog ...
[ have also been focal points of interest, with no conclusive interspecies association being demonstrated to date. 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''.
]
Preventive measures
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 managed otherwise.[
]
References
External links
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*
* http://pathmicro.med.sc.edu/mycology/mycology-6.htm
{{Authority control
Onygenales
Fungi described in 1898
Fungal pathogens of humans
Fungus species