Histoplasma Capsulatum
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''Histoplasma capsulatum'' is a species of dimorphic fungus. Its sexual form is called ''Ajellomyces capsulatus''. It can cause pulmonary and disseminated histoplasmosis. ''Histoplasma capsulatum'' is "distributed worldwide, except in Antarctica, but most often associated with river valleys" and occurs chiefly in the " Central and
Eastern United States The Eastern United States, often abbreviated as simply the East, is a macroregion of the United States located to the east of the Mississippi River. It includes 17–26 states and Washington, D.C., the national capital. As of 2011, the Eastern ...
" followed by " Central and
South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a considerably smaller portion in the Northern Hemisphere. It can also be described as the southern Subregion#Americas, subregion o ...
, and other areas of the world". It is most prevalent in the
Ohio Ohio ( ) is a U.S. state, state in the Midwestern United States, Midwestern region of the United States. It borders Lake Erie to the north, Pennsylvania to the east, West Virginia to the southeast, Kentucky to the southwest, Indiana to the ...
and
Mississippi Mississippi ( ) is a U.S. state, state in the Southeastern United States, Southeastern and Deep South regions of the United States. It borders Tennessee to the north, Alabama to the east, the Gulf of Mexico to the south, Louisiana to the s ...
River valleys. It was discovered by Samuel Taylor Darling in 1906.


Growth and morphology

''Histoplasma capsulatum'' is an ascomycetous fungus closely related to '' Blastomyces dermatitidis''. It is potentially sexual, and its sexual state, ''Ajellomyces capsulatus'', can readily be produced in culture, though it has not been directly observed in nature. ''H. capsulatum'' groups with ''B. dermatitidis'' and the
South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a considerably smaller portion in the Northern Hemisphere. It can also be described as the southern Subregion#Americas, subregion o ...
n pathogen '' Paracoccidioides brasiliensis'' in the recently recognized fungal family Ajellomycetaceae. It is dimorphic and switches from a mould-like (filamentous) growth form in the natural habitat to a small, budding
yeast Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom (biology), kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are est ...
form in the warm-blooded animal host. Like ''B. dermatitidis'', ''H. capsulatum'' has two mating types, "+" and "–". The great majority of North American isolates belongs to a single genetic type, but a study of multiple genes suggests a recombining, sexual population. A recent analysis has suggested that the prevalent North American genetic type and a less common type should be considered separate phylogenetic species, distinct from ''H. capsulatum'' isolates obtained in Central and South America and other parts of the world. These entities are temporarily designated NAm1 (the rare type, which includes a famous experimental isolate designated "the Downs strain") and NAm2 (the common type). As yet, no well-established clinical or geographic distinction is seen between these two genetic groups. In its asexual form, the fungus grows as a colonial microfungus strongly similar in macromorphology to ''B. dermatitidis''. A microscopic examination shows a marked distinction: ''H. capsulatum'' produces two types of conidia, globose macroconidia, 8–15 μm, with distinctive tuberculate or finger-like cell wall ornamentation, and ovoid microconidia, 2–4 μm, which appear smooth or finely roughened. Whether either of these conidial types is the principal infectious particle is unclear. They form on individual short stalks and readily become airborne when the colony is disturbed. Ascomata of the sexual state are 80–250 μm, and are very similar in appearance and anatomy to those described above for ''B. dermatitidis''. The ascospores are similarly minute, averaging 1.5 μm. The budding yeast cells formed in infected tissues are small (about 2–4 μm) and are characteristically seen forming in clusters within phagocytic cells, including histiocytes and other macrophages, as well as monocytes. An African phylogenetic species, '' H. duboisii'', often forms larger yeast cells to 15 μm.


Geographic distribution

''Histoplasma capsulatum'' is "distributed worldwide, except in Antarctica, but most often associated with river valleys" and occurs chiefly in the "Central and Eastern United States" followed by "Central and South America, and other areas of the world" It is most prevalent in the
Ohio Ohio ( ) is a U.S. state, state in the Midwestern United States, Midwestern region of the United States. It borders Lake Erie to the north, Pennsylvania to the east, West Virginia to the southeast, Kentucky to the southwest, Indiana to the ...
and
Mississippi Mississippi ( ) is a U.S. state, state in the Southeastern United States, Southeastern and Deep South regions of the United States. It borders Tennessee to the north, Alabama to the east, the Gulf of Mexico to the south, Louisiana to the s ...
river valleys. The enzootic and
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 ...
zones of ''H. capsulatum'' can be roughly divided into core areas, where the fungus occurs widely in soil or on vegetation contaminated by bird droppings or equivalent organic inputs, and peripheral areas, where the fungus occurs relatively rarely in association with soil, but is still found abundantly in heavy accumulations of bat or bird guano in enclosed spaces such as caves, buildings, and hollow trees. The principal core area for this species includes the valleys of the Mississippi, Ohio, and
Potomac River The Potomac River () is in the Mid-Atlantic (United States), Mid-Atlantic region of the United States and flows from the Potomac Highlands in West Virginia to Chesapeake Bay in Maryland. It is long,U.S. Geological Survey. National Hydrography D ...
s in the USA, as well as a wide span of adjacent areas extending from
Kansas Kansas ( ) is a landlocked U.S. state, state in the Midwestern United States, Midwestern region of the United States. It borders Nebraska to the north; Missouri to the east; Oklahoma to the south; and Colorado to the west. Kansas is named a ...
,
Illinois Illinois ( ) is a U.S. state, state in the Midwestern United States, Midwestern United States. It borders on Lake Michigan to its northeast, the Mississippi River to its west, and the Wabash River, Wabash and Ohio River, Ohio rivers to its ...
,
Indiana Indiana ( ) is a U.S. state, state in the Midwestern United States, Midwestern region of the United States. It borders Lake Michigan to the northwest, Michigan to the north and northeast, Ohio to the east, the Ohio River and Kentucky to the s ...
, and
Ohio Ohio ( ) is a U.S. state, state in the Midwestern United States, Midwestern region of the United States. It borders Lake Erie to the north, Pennsylvania to the east, West Virginia to the southeast, Kentucky to the southwest, Indiana to the ...
in the north to
Mississippi Mississippi ( ) is a U.S. state, state in the Southeastern United States, Southeastern and Deep South regions of the United States. It borders Tennessee to the north, Alabama to the east, the Gulf of Mexico to the south, Louisiana to the s ...
,
Louisiana Louisiana ( ; ; ) is a state in the Deep South and South Central regions of the United States. It borders Texas to the west, Arkansas to the north, and Mississippi to the east. Of the 50 U.S. states, it ranks 31st in area and 25 ...
, and
Texas Texas ( , ; or ) is the most populous U.S. state, state in the South Central United States, South Central region of the United States. It borders Louisiana to the east, Arkansas to the northeast, Oklahoma to the north, New Mexico to the we ...
in the south. In some areas, such as
Kansas City The Kansas City metropolitan area is a bi-state metropolitan area anchored by Kansas City, Missouri. Its 14 counties straddle the border between the U.S. states of Missouri (9 counties) and Kansas (5 counties). With and a population of more t ...
, skin testing with the histoplasmin antigen preparation shows that 80–90% of the resident population have an antibody reaction to ''H. capsulatum'', probably indicating prior subclinical infection. Northern U.S. states such as
Minnesota Minnesota ( ) is a U.S. state, state in the Upper Midwestern region of the United States. It is bordered by the Canadian provinces of Manitoba and Ontario to the north and east and by the U.S. states of Wisconsin to the east, Iowa to the so ...
,
Michigan Michigan ( ) is a peninsular U.S. state, state in the Great Lakes region, Great Lakes region of the Upper Midwest, Upper Midwestern United States. It shares water and land boundaries with Minnesota to the northwest, Wisconsin to the west, ...
, New York and
Vermont Vermont () is a U.S. state, state in the New England region of the Northeastern United States. It borders Massachusetts to the south, New Hampshire to the east, New York (state), New York to the west, and the Provinces and territories of Ca ...
are peripheral areas for histoplasmosis, but have scattered counties where 5–19% of lifetime residents show exposure to ''H. capsulatum''. One New York county, St. Lawrence county (across 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 ...
from the
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Preston
Brockville Brockville is a city in Eastern Ontario, Canada, in the Thousand Islands region. Although it is the seat of the United Counties of Leeds and Grenville, it is politically Independent city, independent of the county. It is included with Leeds and ...
area of
Ontario, Canada 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 ...
) shows exposures over 20%. The distribution of ''H. capsulatum'' in Canada is not as well documented as in the US. The St. Lawrence Valley is probably the best known endemic region based both on case reports and on a number of skin test reaction studies that were done between 1945 and 1970. The
Montreal Montreal is the List of towns in Quebec, largest city in the Provinces and territories of Canada, province of Quebec, the List of the largest municipalities in Canada by population, second-largest in Canada, and the List of North American cit ...
area is a particularly well documented endemic focus, not just in the agricultural regions surrounding the city but also within the city itself. The
Mount Royal Mount Royal (, ) is a mountain in the city of Montreal, immediately west of Downtown Montreal, Quebec, Canada. The city's name is derived from the mountain's name. The mountain is part of the Monteregian Hills situated between the Laurentian M ...
area in central Montreal, especially the north and east sides of Mt. Royal Park, showed exposure rates between 20 and 50% in schoolchildren and locally lifetime-resident university students. A particularly high rate of 79.3% exposure was shown in
St. Thomas, Ontario St. Thomas is a city in southwestern Ontario, Canada. It gained its city charter on March 4, 1881. The city is also the seat for Elgin County, although it is independent of the county. At the time of the 2021 Census, the population of the city w ...
, south of
London, Ontario London is a city in southwestern Ontario, Canada, along the Quebec City–Windsor Corridor. The city had a population of 422,324 according to the 2021 Canadian census. London is at the confluence of the Thames River (Ontario), Thames River and N ...
, after 7 local residents had died of histoplasmosis in 1957. Based on numerous small regional studies, histoplasmin skin test reactors form ca. 10–50 % of the population in much of southern Ontario and in Quebec’s St. Lawrence Valley, ca. 5% in southern Manitoba and some northerly parts of
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, ...
(e.g.,
Abitibi-Témiscamingue Abitibi-Témiscamingue () is an administrative region located in western Québec, Canada, along the border with Ontario. It became part of the province in 1898. It has a land area of and its population was 147,082 people as of the 2021 cens ...
), and ca. 1% in
Nova Scotia Nova Scotia is a Provinces and territories of Canada, province of Canada, located on its east coast. It is one of the three Maritime Canada, Maritime provinces and Population of Canada by province and territory, most populous province in Atlan ...
. Exposure of aboriginal Canadians occurs remarkably far north in Quebec, but has not been reported in similar boreal biogeoclimatic zones in many other parts of Canada. Recently and remarkably, a cluster of four indigenously acquired cases of histoplasmosis was shown to be associated with a golf course in suburban
Edmonton, Alberta Edmonton is the capital city of the Provinces and territories of Canada, Canadian province of Alberta. It is situated on the North Saskatchewan River and is the centre of the Edmonton Metropolitan Region, which is surrounded by Central Alberta ...
. Examination suggested that local soil was the source.


Spectrum of disease

Histoplasmosis is usually a subclinical infection that does not come to the attention of the person involved. The organism tends to remain alive in the scattered pulmonary calcifications; therefore, some cases are detected by emergence of serious infection when a patient becomes immunocompromised, perhaps decades later. Frank cases are most often seen as acute pulmonary histoplasmosis, a disease that resembles acute pneumonia but is usually self-limited. It is most often seen in children newly exposed to ''H. capsulatum'' or in heavily exposed individuals. Erythematous skin conditions arising from antigen reactions may complicate the disease, as may myalgias, arthralgias, and rarely, arthritic conditions. Emphysema sufferers may contract chronic cavitary pulmonary histoplasmosis as a disease complication; eventually the cavity formed may be occupied by an ''
Aspergillus ' () is a genus consisting of several hundred mold species found in various climates worldwide. ''Aspergillus'' was first catalogued in 1729 by the Italian priest and biologist Pier Antonio Micheli. Viewing the fungi under a microscope, Miche ...
'' fungus ball ( aspergilloma), potentially leading to massive hemoptysis. Another uncommon form of histoplasmosis is a slowly progressing condition known as granulomatous mediastinitis, in which the lymph nodes in the mediastinal cavity between the lungs become inflamed and ultimately necrotic; the swollen nodes or draining fluid may ultimately affect the bronchi, the superior vena cava, the esophagus or the pericardium. A particularly dangerous condition is mediastinal fibrosis, in which a subset of individuals with granulomatous mediastinitis develop an uncontrolled fibrotic reaction that may press on the lungs or the bronchi, or may cause right heart failure. There are a number of other rare pulmonary manifestations of histoplasmosis. Histoplasmosis, like blastomycosis, may disseminate haematogenously to infect internal organs and tissues, but it does so in a very low proportion of cases, and half or more of these dissemination cases involve immunocompromisation. Unlike blastomycosis, histoplasmosis is a recognized
AIDS The HIV, human immunodeficiency virus (HIV) is a retrovirus that attacks the immune system. Without treatment, it can lead to a spectrum of conditions including acquired immunodeficiency syndrome (AIDS). It is a Preventive healthcare, pr ...
-defining illness in people with HIV infection; disseminated histoplasmosis affects approximately 5% of AIDS patients with
CD4 In molecular biology, CD4 (cluster of differentiation 4) is a glycoprotein that serves as a co-receptor for the T-cell receptor (TCR). CD4 is found on the surface of immune cells such as helper T cells, monocytes, macrophages, and dendritic c ...
+ cell counts <150 cells/μL in highly endemic areas. The incidence of this condition dropped significantly after introduction of current anti-HIV therapies. Other conditions very uncommonly associated with ''H. capsulatum'' include
endocarditis Endocarditis is an inflammation of the inner layer of the heart, the endocardium. It usually involves the heart valves. Other structures that may be involved include the interventricular septum, the chordae tendineae, the mural endocardium, o ...
and
peritonitis Peritonitis is inflammation of the localized or generalized peritoneum, the lining of the inner wall of the abdomen and covering of the abdominal organs. Symptoms may include severe pain, swelling of the abdomen, fever, or weight loss. One pa ...
.


Ecology and epidemiology

''Histoplasma capsulatum'' appears to be strongly associated with the droppings of certain bird species as well as bats. A mixture of these droppings and certain soil types is particularly conducive to proliferation. In highly endemic areas there is a strong association with soil under and around chicken houses, and with areas where soil or vegetation has become heavily contaminated with faecal material deposited by flocking birds such as
starling Starlings are small to medium-sized passerine (perching) birds known for the often dark, glossy iridescent sheen of their plumage; their complex vocalizations including mimicking; and their distinctive, often elaborate swarming behavior, know ...
s and blackbirds. Bird roosting areas that are ''Histoplasma''-free appear to be lower in nitrogen, phosphorus, organic matter and moisture than contaminated roosting areas. The guano of gulls and other colonially nesting water-associated birds is rarely connected to histoplasmosis. Bat dwellings, including caves, attics and hollow trees, are classic ''H. capsulatum'' habitats. Histoplasmosis outbreaks are typically associated with cleaning guano accumulations or clearing guano-covered vegetation, or with exploration of bat caves. In addition, however, outbreaks may be associated with wind-blown dust liberated by construction projects in endemic areas: a classic outbreak is one associated with intense construction activity, including subway construction, in Montreal in 1963. As with blastomycosis, a good understanding of the precise ecological affinities of ''H. capsulatum'' is greatly complicated by the difficulty of isolating the fungus directly from nature. Again, the mouse passage procedure originally devised by Emmons must be used. A direct PCR technique for detection of ''H. capsulatum'' in soil has been published. ''H. capsulatum'' appears particularly likely to cause clinical disease in young children, persons working in sites contaminated by conducive bird or bat droppings, persons exposed to construction dust raised from contaminated sites, immunocompromised patients, and emphysema sufferers. Elimination of the agent from contaminated soils typically involves the use of toxic fumigants with limited success.


Etymology

In 1905, Samuel Taylor Darling serendipitously identified a protozoan-like microorganism in an autopsy specimen while trying to understand
malaria Malaria is a Mosquito-borne disease, mosquito-borne infectious disease that affects vertebrates and ''Anopheles'' mosquitoes. Human malaria causes Signs and symptoms, symptoms that typically include fever, Fatigue (medical), fatigue, vomitin ...
, which was prevalent during the construction of the
Panama Canal The Panama Canal () is an artificial waterway in Panama that connects the Caribbean Sea with the Pacific Ocean. It cuts across the narrowest point of the Isthmus of Panama, and is a Channel (geography), conduit for maritime trade between th ...
. He named this microorganism ''Histoplasma capsulatum'' because it invaded the cytoplasm (plasma) of histiocyte-like cells (Histo) and had a refractive halo mimicking a capsule (capsulatum), a misnomer.


Additional images

File:Histopathology of Histoplasma capsulatum, GMS stain.jpg, Histopathology of ''Histoplasma capsulatum'', GMS stain, showing narrow budding yeast File:Histoplasmosis capsulatum.jpg, ''H. capsulatum''. Methenamine silver stain. Histoplasma pas-d.jpg, ''Histoplasma'' (bright red, small, circular). PAS diastase stain File:Histoplasma pas-d small.jpg, ''Histoplasma''. PAS diastase stain. File:Histoplasma in granuloma pas-d.jpg, ''Histoplasma'' in a
granuloma A granuloma is an aggregation of macrophages (along with other cells) that forms in response to chronic inflammation. This occurs when the immune system attempts to isolate foreign substances that it is otherwise unable to eliminate. Such sub ...
. PAS diastase stain. File:Histoplasma in granuloma gms.jpg, ''Histoplasma'' in a granuloma. GMS stain.


See also

* '' Blastomyces'' * '' Coccidioides''


References

{{Taxonbar, from=Q143747 Onygenales Fungi described in 1906 Fungi and humans Fungal pathogens of humans Fungus species