Leishmania tropica
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''Leishmania tropica'' is a
flagellate A flagellate is a cell or organism with one or more whip-like appendages called flagella. The word ''flagellate'' also describes a particular construction (or level of organization) characteristic of many prokaryotes and eukaryotes and their ...
parasite Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has ...
and the cause of anthroponotic
cutaneous leishmaniasis Cutaneous leishmaniasis is the most common form of leishmaniasis affecting humans. It is a skin infection caused by a single-celled parasite that is transmitted by the bite of a phlebotomine sand fly. There are about thirty species of ''Leishm ...
in
humans Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...
. This parasite is restricted to
Afro-Eurasia Afro-Eurasia (also Afroeurasia, Eurafrasia or the Old World) is a landmass comprising the continents of Africa, Asia, and Europe. The terms are compound words of the names of its constituent parts. Its mainland is the largest and most populou ...
and is a common cause of infection in
Afghanistan Afghanistan, officially the Islamic Emirate of Afghanistan,; prs, امارت اسلامی افغانستان is a landlocked country located at the crossroads of Central Asia and South Asia. Referred to as the Heart of Asia, it is bordere ...
,
Iran Iran, officially the Islamic Republic of Iran, and also called Persia, is a country located in Western Asia. It is bordered by Iraq and Turkey to the west, by Azerbaijan and Armenia to the northwest, by the Caspian Sea and Turkmeni ...
,
Syria Syria ( ar, سُورِيَا or سُورِيَة, translit=Sūriyā), officially the Syrian Arab Republic ( ar, الجمهورية العربية السورية, al-Jumhūrīyah al-ʻArabīyah as-Sūrīyah), is a Western Asian country loc ...
,
Yemen Yemen (; ar, ٱلْيَمَن, al-Yaman), officially the Republic of Yemen,, ) is a country in Western Asia. It is situated on the southern end of the Arabian Peninsula, and borders Saudi Arabia to the Saudi Arabia–Yemen border, north and ...
,
Algeria ) , image_map = Algeria (centered orthographic projection).svg , map_caption = , image_map2 = , capital = Algiers , coordinates = , largest_city = capital , relig ...
,
Morocco Morocco (),, ) officially the Kingdom of Morocco, is the westernmost country in the Maghreb region of North Africa. It overlooks the Mediterranean Sea to the north and the Atlantic Ocean to the west, and has land borders with Algeria to ...
, and northern
India India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the so ...
.


History

The first description of ''Leishmania tropica'' was done in 1903 by
James Homer Wright James Homer Wright (April 8, 1869 – January 3, 1928) was an early and influential American pathologist, who was chief of pathology at Massachusetts General Hospital from 1896 to 1926. Wright was born in Pittsburgh, Pennsylvania. In 1915, he j ...
, an American
pathologist Pathology is the study of the causal, causes and effects of disease or injury. The word ''pathology'' also refers to the study of disease in general, incorporating a wide range of biology research fields and medical practices. However, when us ...
. In 1914, it was suggested that ''L. tropica'' should be divided into two
subspecies In biological classification, subspecies is a rank below species, used for populations that live in different areas and vary in size, shape, or other physical characteristics (morphology), but that can successfully interbreed. Not all species ...
, namely ''L. tropica minor'' and ''L. tropica major'', based on the size of the parasites found in skin
lesion A lesion is any damage or abnormal change in the tissue of an organism, usually caused by disease or trauma. ''Lesion'' is derived from the Latin "injury". Lesions may occur in plants as well as animals. Types There is no designated classifi ...
s. Later, these two subspecies turned out to be epidemiologically different and were correlated to different types of lesions. ''L. tropica minor'' causes dry nodular lesions in urban environments, while ''L. tropica major'' causes wet ulcerating lesions in rural regions. Bray et al. therefore proposed in 1973 that the subspecies should be considered as two separate species, ''L. tropica major'' became ''L. major'', and ''L. tropica minor'' became ''L. tropica'', which is the naming that is still being used.


Biology

The parasite has an interesting biology since it is very
heterogeneous Homogeneity and heterogeneity are concepts often used in the sciences and statistics relating to the uniformity of a substance or organism. A material or image that is homogeneous is uniform in composition or character (i.e. color, shape, siz ...
biochemically, serologically, and genetically compared to other ''
Leishmania ''Leishmania'' is a parasitic protozoan, a single-celled organism of the genus '' Leishmania'' that are responsible for the disease leishmaniasis. They are spread by sandflies of the genus ''Phlebotomus'' in the Old World, and of the genus '' ...
'' species. The different strains have been shown to cause different patterns of
pathogenicity In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...
and
humoral immune response Humoral immunity is the aspect of immunity that is mediated by macromolecules - including secreted antibodies, complement proteins, and certain antimicrobial peptides - located in extracellular fluids. Humoral immunity is named so because it in ...
s in BALB/c mice models. Fusion and hybridization of different ''L. tropica'' strains can be efficiently induced ''
in vitro ''In vitro'' (meaning in glass, or ''in the glass'') studies are performed with microorganisms, cells, or biological molecules outside their normal biological context. Colloquially called "test-tube experiments", these studies in biology an ...
'' by exposure of
promastigote Trypanosomatida is a group of kinetoplastid excavates distinguished by having only a single flagellum. The name is derived from the Greek ''trypano'' (borer) and ''soma'' (body) because of the corkscrew-like motion of some trypanosomatid species. ...
s (a stage of the life cycle) to
DNA damage DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as radiation can cause DNA da ...
stress.


Life cycle

''
Leishmania ''Leishmania'' is a parasitic protozoan, a single-celled organism of the genus '' Leishmania'' that are responsible for the disease leishmaniasis. They are spread by sandflies of the genus ''Phlebotomus'' in the Old World, and of the genus '' ...
'' species alternate between two main life forms: intracellular
amastigote An amastigote is a protist A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryot ...
s in the
sandfly Sandfly (or sand fly) is a colloquial name for any species or genus of flying, biting, blood-sucking dipteran (fly) encountered in sandy areas. In the United States, ''sandfly'' may refer to certain horse flies that are also known as "greenhea ...
– the
vector Vector most often refers to: *Euclidean vector, a quantity with a magnitude and a direction *Vector (epidemiology), an agent that carries and transmits an infectious pathogen into another living organism Vector may also refer to: Mathematic ...
– and extracellular motile
promastigote Trypanosomatida is a group of kinetoplastid excavates distinguished by having only a single flagellum. The name is derived from the Greek ''trypano'' (borer) and ''soma'' (body) because of the corkscrew-like motion of some trypanosomatid species. ...
s in the
mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
– the
host A host is a person responsible for guests at an event or for providing hospitality during it. Host may also refer to: Places * Host, Pennsylvania, a village in Berks County People *Jim Host (born 1937), American businessman * Michel Host ...
. In the mammalian host, promastigotes are introduced into the skin by the bite of a sandfly. After being taken up by
phagocyte Phagocytes are cells that protect the body by ingesting harmful foreign particles, bacteria, and dead or dying cells. Their name comes from the Greek ', "to eat" or "devour", and "-cyte", the suffix in biology denoting "cell", from the Greek ...
s, they transform into intracellular amastigotes and stay in this form during the remaining life cycle in the mammalian host. Through simple
division Division or divider may refer to: Mathematics *Division (mathematics), the inverse of multiplication *Division algorithm, a method for computing the result of mathematical division Military *Division (military), a formation typically consisting ...
, they can multiply and proceed to infect other phagocytotic cells. Later, depending partly on the
immunity Immunity may refer to: Medicine * Immunity (medical), resistance of an organism to infection or disease * ''Immunity'' (journal), a scientific journal published by Cell Press Biology * Immune system Engineering * Radiofrequence immunity desc ...
of the host, the infection can become symptomatic and result in leishmaniasis. Sand flies become infected by ingesting phagocytes with ''
Leishmania ''Leishmania'' is a parasitic protozoan, a single-celled organism of the genus '' Leishmania'' that are responsible for the disease leishmaniasis. They are spread by sandflies of the genus ''Phlebotomus'' in the Old World, and of the genus '' ...
'' from a mammalian host. Then, in the sandfly stage, the process differs between ''
Leishmania ''Leishmania'' is a parasitic protozoan, a single-celled organism of the genus '' Leishmania'' that are responsible for the disease leishmaniasis. They are spread by sandflies of the genus ''Phlebotomus'' in the Old World, and of the genus '' ...
'' species. In the life cycle of ''L. tropica'', it develops back into the promastigote stage inside the
midgut The midgut is the portion of the embryo from which most of the intestines develop. After it bends around the superior mesenteric artery, it is called the "midgut loop". It comprises the portion of the alimentary canal from the end of the foregut ...
of the sandfly vector and migrates to the
proboscis A proboscis () is an elongated appendage from the head of an animal, either a vertebrate or an invertebrate. In invertebrates, the term usually refers to tubular mouthparts used for feeding and sucking. In vertebrates, a proboscis is an elong ...
of the sandfly, whereafter the life cycle can repeat itself.


Hosts and vectors

Humans are the main reservoir hosts of ''L. tropica''.
Rock hyrax The rock hyrax (; ''Procavia capensis''), also called dassie, Cape hyrax, rock rabbit, and (in the King James Bible) coney, is a medium-sized terrestrial mammal native to Africa and the Middle East. Commonly referred to in South Africa as the das ...
es (''Procavia capensis'') are a possible reservoir in
Israel Israel (; he, יִשְׂרָאֵל, ; ar, إِسْرَائِيل, ), officially the State of Israel ( he, מְדִינַת יִשְׂרָאֵל, label=none, translit=Medīnat Yīsrāʾēl; ), is a country in Western Asia. It is situated ...
. Natural infection by ''L. tropica'' has also been demonstrated in
domestic dogs The dog (''Canis familiaris'' or ''Canis lupus familiaris'') is a domesticated descendant of the wolf. Also called the domestic dog, it is derived from the extinct Pleistocene wolf, and the modern wolf is the dog's nearest living relative. Do ...
,
red fox The red fox (''Vulpes vulpes'') is the largest of the true foxes and one of the most widely distributed members of the Order (biology), order Carnivora, being present across the entire Northern Hemisphere including most of North America, Europe ...
es,
golden jackal The golden jackal (''Canis aureus''), also called common jackal, is a wolf-like canid that is native to Southeast Europe, Southwest Asia, South Asia, and regions of Southeast Asia. The golden jackal's coat varies in color from a pale creamy y ...
s,
gundi Gundis or comb rats (family Ctenodactylidae) are a group of small, stocky rodents found in Africa. They live in rocky deserts across the northern parts of the continent. The family comprises four living genera and five species ( Speke's gundi, ...
s, and other species of wild
rodent Rodents (from Latin , 'to gnaw') are mammals of the order Rodentia (), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents. They are na ...
s. The main sandfly vector for ''L. tropica'' is ''
Phlebotomus ''Phlebotomus'' is a genus of " sand flies" in the Diptera family Psychodidae. In the past, they have sometimes been considered to belong in a separate family, Phlebotomidae, but this alternative classification has not gained wide acceptance. E ...
sergenti''. Other reported vector species are '' P. arabicus, P. guggisbergi'', '' P. chabaudi'', '' P. rossi'', and '' P. saevus''.


Clinical manifestations

''L. tropica'' causes a broad spectrum of leishmaniasis forms in humans. Most common is a variant called dry-type
cutaneous leishmaniasis Cutaneous leishmaniasis is the most common form of leishmaniasis affecting humans. It is a skin infection caused by a single-celled parasite that is transmitted by the bite of a phlebotomine sand fly. There are about thirty species of ''Leishm ...
. After an
incubation period Incubation period (also known as the latent period or latency period) is the time elapsed between exposure to a pathogenic organism, a chemical, or radiation, and when symptoms and signs are first apparent. In a typical infectious disease, the i ...
lasting more than 2 months, a small brownish nodular lesion will appear with a slowly extending plaque reaching a size of after 6 months. This will heal after about 1 year but leaves a scar. Other forms of the disease, which occur more rarely in humans, include
visceral leishmaniasis Visceral leishmaniasis (VL), also known as kala-azar (Hindi: kālā āzār, "black sickness") or "black fever", is the most severe form of leishmaniasis and, without proper diagnosis and treatment, is associated with high fatality. Leishmaniasis ...
,
post-kala-azar dermal leishmaniasis Post-kala-azar dermal leishmaniasis (PKDL) is a complication of visceral leishmaniasis (VL); it is characterised by a macular, maculopapular, and nodular rash in a patient who has recovered from VL and who is otherwise well. The rash usually start ...
(a variant of visceral leishmaniasis), viscerotropic leishmaniasis, and leishmaniasis recidivans (a variant of cutaneous leishmaniasis). Dogs are known to rarely suffer from
visceral In biology, an organ is a collection of tissues joined in a structural unit to serve a common function. In the hierarchy of life, an organ lies between tissue and an organ system. Tissues are formed from same type cells to act together in a ...
,
skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other cuticle, animal coverings, such as the arthropod exoskeleton, have diffe ...
, and
mucosa A mucous membrane or mucosa is a membrane that lines various cavities in the body of an organism and covers the surface of internal organs. It consists of one or more layers of epithelial cells overlying a layer of loose connective tissue. It is ...
l infection with this species. In cats
asymptomatic In medicine, any disease is classified asymptomatic if a patient tests as carrier for a disease or infection but experiences no symptoms. Whenever a medical condition fails to show noticeable symptoms after a diagnosis it might be considered asy ...
infection is thought to be common. Skin and/or mucosal infection is the most common form, with or without visceral infection. Feline visceral infection may occur alone.


Leishmaniasis recidivans

This rare variant of cutaneous leishmaniasis is caused solely by ''L. tropica'' in the
Old World The "Old World" is a term for Afro-Eurasia that originated in Europe , after Europeans became aware of the existence of the Americas. It is used to contrast the continents of Africa, Europe, and Asia, which were previously thought of by the ...
and by '' L. braziliensis'' in the
New World The term ''New World'' is often used to mean the majority of Earth's Western Hemisphere, specifically the Americas."America." ''The Oxford Companion to the English Language'' (). McArthur, Tom, ed., 1992. New York: Oxford University Press, p. 3 ...
. It causes a slowly progressing
lesion A lesion is any damage or abnormal change in the tissue of an organism, usually caused by disease or trauma. ''Lesion'' is derived from the Latin "injury". Lesions may occur in plants as well as animals. Types There is no designated classifi ...
, usually on the face, and is characterized by the development of
papules A papule is a small, well-defined bump in the skin. It may have a rounded, pointed or flat top, and may have a dip. It can appear with a stalk, be thread-like or look warty. It can be soft or firm and its surface may be rough or smooth. Some h ...
or nodules which form mostly around or in the site of primary healed lesions. Lesions of leishmaniasis recidivans stay many years and rarely respond to treatment, thus causing
disfigurement Disfigurement is the state of having one's appearance deeply and persistently harmed medically, such as from a disease, birth defect, or wound. General societal attitudes towards disfigurement have varied greatly across cultures and over time, ...
and becoming destructive with the years.


References


Further reading

* * * * *


External links

*
Leishmaniasis – General Information
US Center for Disease Control and Prevention
Cancer Web
{{Taxonbar, from=Q1950818 Parasitic excavates Trypanosomatida Species described in 1903