''Dracunculus'' is a
genus
Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...
of spiruroid
nematode
The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-Parasitism, parasitic nematodes also known as eelworms. They are a diverse animal phylum inhab ...
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 ...
s in the family
Dracunculidae
Dracunculidae is a family of parasitic nematodes belonging to the order Camallanida
The Camallanida are an order of nematodes.
* Parasites of terrestrial and aquatic vertebrates
* Copepods as obligatory secondary hosts
They are sometime ...
.
The worms can reach a metre in length. If one simply pulls off the protruding head of the worm, the worm will break and leak high levels of foreign
antigen
In immunology, an antigen (Ag) is a molecule or molecular structure or any foreign particulate matter or a pollen grain that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune response. ...
which can lead to
anaphylactic shock
Anaphylaxis is a serious, potentially fatal allergic reaction and medical emergency that is rapid in onset and requires immediate medical attention regardless of use of emergency medication on site. It typically causes more than one of the follow ...
and fast death of the host. Hence it is important to remove the worm slowly (over a period of weeks). This is typically undertaken by winding the worm onto a stick (say, a matchstick), by a few centimetres each day.
Life cycle
All members of ''Dracunculus'' are obligate parasites of mammals or reptiles. Adult females reside just under the skin, and eventually form a blister in the host's skin through which they access the environment. When the blister comes into contact with water, the female releases several hundred thousand first-stage ("L1") larvae. L1 larvae must be ingested by a
cyclopoid copepod
Copepods (; meaning "oar-feet") are a group of small crustaceans found in nearly every freshwater and saltwater habitat (ecology), habitat. Some species are planktonic (inhabiting sea waters), some are benthos, benthic (living on the ocean floor) ...
, which serves as an
intermediate host
In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist ''guest'' (symbiont). The guest is typically provided with nourishment and shelter. Examples include a ...
. Inside the copepod, the larvae develop to the third-stage ("L3").
Definitive host
In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist ''guest'' (symbiont). The guest is typically provided with nourishment and shelter. Examples include a ...
s acquire ''Dracunculus'' by incidentally ingesting infected copepods while drinking water, or by consuming a
paratenic host
In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist ''guest'' (symbiont). The guest is typically provided with nourishment and shelter. Examples include a ...
(e.g. a frog or fish) that has itself consumed a copepod. Inside the definitive host, the L3 larvae leave the digestive tract and migrate to deeper tissues, where within 60–70 days they undergo their final two
molt
In biology, moulting (British English), or molting (American English), also known as sloughing, shedding, or in many invertebrates, ecdysis, is the manner in which an animal routinely casts off a part of its body (often, but not always, an outer ...
s to form sexually mature adults. Male and female adult worms then mate, and pregnant females migrate back to the host's skin – typically to an extremity – and form a blister to repeat the cycle.
[ Following the release of her larvae, the female worm dies, and is either extracted by the affected animal, or falls back into the tissue and is calcified.][
]
Description
Once released, the L1 larvae measure 0.3–0.9 millimeters in length and feature a very long tapered tail. As they develop into L3 larvae, they lose the tapered tail, broaden, and develop a tri-lobed tail; the lengths of most L3 larvae are unknown.[ Adult female ''Dracunculus'' worms are noted for their extraordinary length, with some growing up to 100 ]centimeter
330px, Different lengths as in respect to the Electromagnetic spectrum, measured by the Metre and its deriveds scales. The Microwave are in-between 1 meter to 1 millimeter.
A centimetre (international spelling) or centimeter (American spellin ...
s long. Males are much smaller (16 – 40 millimeter
330px, Different lengths as in respect to the electromagnetic spectrum, measured by the metre and its derived scales. The microwave is between 1 meter to 1 millimeter.
The millimetre (American and British English spelling differences#-re, -er, ...
s) and are relatively rare – in some species the male has never been described. Adults of both sexes are narrow yellow-white colored worms, with a rounded front-end, and a conical tail-end with a pointed tip. Females of different species within the genus tend to look similar, and can rarely be distinguished on morphology alone.[ The body of a fertilized adult female is almost completely filled by its uterus, distended with L1 larvae.][
]
Distribution
''Dracunculus'' worms are distributed globally, though each species has a narrower range. The majority of ''Dracunculus'' species described infect reptiles – predominantly snakes.[ These are spread across the globe, with ''D. ophidensis'' in the ]United States
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...
, ''D. brasiliensis'' in Brazil, ''D. coluberensis'' and ''D. alii'' in India, ''D. houdemeri'' in Vietnam, ''D. doi'' in Madagascar, ''D. dahomensis'' in Benin, ''D. oesophageus'' in Italy, and ''D. mulbus'' in Australia and Papua New Guinea. The only species known to infect a non-snake reptile is ''D. globocephalus'' which has been described in snapping turtle
The Chelydridae is a family of turtles that has seven extinct and two extant genera. The extant genera are the snapping turtles, ''Chelydra'' and '' Macrochelys''. Both are endemic to the Western Hemisphere. The extinct genera are '' Acherontem ...
s in the United States and Costa Rica.[
Most mammal-infecting species are in the Americas, with ''D. insignis'' infecting several wild and domestic mammals in the ]United States
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...
and Canada
Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by tot ...
, ''D. lutrae'' infecting river otters in the United States and Canada, and ''D. fuelleborni'' infecting big-eared opposums in Brazil
Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...
.[ The major exception is ''D. medinensis'', by far the most studied ''Dracunculus'' as it infects humans. ''D. medinensis'' was historically widespread in Africa and South Asia,][ but is now limited to dozens of cases annually in humans and domestic dogs, and may soon be driven to extinction.
]
Species
Reptile-infecting species
There are 14 accepted ''Dracunculus'' species, 10 of which infect reptiles. Eurasia hosts several reptile-infecting ''Dracunculus'' species. ''D. oesophageus'' was originally described from the esophagus of a viperine snake ''Natrix maura
''Natrix maura'' is a natricine water snake of the genus ''Natrix''. Its common name is viperine water snake or viperine snake. Despite its common names, it is not a member of the subfamily Viperinae. This nonvenomous, semiaquatic, fish-eating s ...
'', and has been described several times since. The remaining three Eurasian reptile-infecting species have been described a single time each: ''D. coluberensis'' from an Indian trinket snake
The common trinket snake (''Coelognathus helena'') is a nonvenomous constrictor species of colubrid snake native to south Central Asia.
Etymology
The specific name of this snake, ''helena'', is thought to be a reference to Helen of Troy, cons ...
, and ''D. alii'' and ''D. houdemeri'' from Checkered keelback snake
Check (also checker, Brit: chequer) is a pattern of modified stripes consisting of crossed horizontal and vertical lines which form squares. The pattern typically contains two colours where a single checker (that is a single square within the chec ...
s in India and Vietnam respectively.[
The only snake-infecting ''Dracunculus'' species known in North America is ''D. ophidensis''. It was originally described in ]garter snakes
Garter snake is a common name for generally harmless, small to medium-sized snakes belonging to the genus ''Thamnophis'' in the family Colubridae. Native to North and Central America, species in the genus ''Thamnophis'' can be found from the sub ...
in Michigan and Minnesota by Sterling Brackett in 1938, and has since been reported in blackbelly garter snakes from Mexico, as well as northern water snakes and a plain-bellied water snake in Michigan.[ ''D. brasiliensis'' is the only described snake-infecting ''Dracunculus'' in South America. It was described in 2009 based on a single female worm from an ]anaconda
Anacondas or water boas are a group of large snakes of the genus ''Eunectes''. They are found in tropical South America. Four species are currently recognized.
Description
Although the name applies to a group of snakes, it is often used to re ...
in Brazil, and has since also been found in a Brazilian brown-banded water snake.[ Several worms that appear to be from the genus ''Dracunculus'' have been described in Central America, South America, and the Caribbean; however, they have not been described in sufficient detail to assign them to a species.][ The Americas are also home to the only known turtle-infecting ''Dracunculus'' (also the only species that infects a non-snake reptile), ''D. globocephalus''. First described in 1927 in Oklahoma and Illinois, it is now found in ]snapping turtle
The Chelydridae is a family of turtles that has seven extinct and two extant genera. The extant genera are the snapping turtles, ''Chelydra'' and '' Macrochelys''. Both are endemic to the Western Hemisphere. The extinct genera are '' Acherontem ...
s across the United States, as well as the South American snapping turtle
The South American snapping turtle (''Chelydra acutirostris'') is a species of turtle in the family Chelydridae. This species, which is endemic to
Central and northwestern South America, was previously considered a subspecies of ''Chelydra serpe ...
in Costa Rica.
Africa hosts two known snake-infecting species. Both male and female ''D. doi'' were described from Madagascar ground boas in 1960 and 1973 respectively. ''D. dahomensis'' has been described only from a captive African rock python
The Central African rock python (''Python sebae'') is a species of large constrictor snake in the family Pythonidae. The species is native to sub-Saharan Africa. It is one of 11 living species in the genus ''Python''.
Africa's largest snake and ...
.[
In Australia, the only known snake-infecting ''Dracunculus'' is ''D. mulbus'', described from numerous water pythons in Northern Australia in 2007. It has since been described in Papua New Guinea's Papuan olive python as well.][
]
Mammal-infecting species
Just four ''Dracunculus'' species are known to infect mammals, of which the best known is the human parasite ''D. medinensis''. Historically spread across Africa and South Asia, a major eradication effort has restricted ''D. medinensis'' to just Chad, Ethiopia, Mali, and South Sudan.[ Case numbers have similarly fallen, from an estimated 3.5 million per year at the 1986 start of the eradication program, to just 15 in 2021. ''D. medinensis'' is now most common in dogs, particularly in Chad, where it may spread via fish or frogs as ]paratenic host
In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist ''guest'' (symbiont). The guest is typically provided with nourishment and shelter. Examples include a ...
s.
''D. insignis'' infects dog
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 ...
s and wild carnivore
A carnivore , or meat-eater (Latin, ''caro'', genitive ''carnis'', meaning meat or "flesh" and ''vorare'' meaning "to devour"), is an animal or plant whose food and energy requirements derive from animal tissues (mainly muscle, fat and other sof ...
s, causing cutaneous lesions, ulcers
An ulcer is a discontinuity or break in a bodily membrane that impedes normal function of the affected organ. According to Robbins's pathology, "ulcer is the breach of the continuity of skin, epithelium or mucous membrane caused by sloughing o ...
, and sometimes heart
The heart is a muscular organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide t ...
and vertebral column
The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordata, ...
lesions. Like ''D. medinensis'', it is also known as Guinea worm
''Dracunculus medinensis'', or Guinea worm, is a nematode that causes dracunculiasis, also known as guinea worm disease. The disease is caused by the female which, at up to in length, is among the longest nematodes infecting humans. In contra ...
, as well as ''Dragon'' or ''Fiery Dragon''. The range of ''D. insignis'' is limited to North America
North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...
.
''D. fuelliborni'' parasitizes opossum
Opossums () are members of the marsupial order Didelphimorphia () endemic to the Americas. The largest order of marsupials in the Western Hemisphere, it comprises 93 species in 18 genera. Opossums originated in South America and entered North ...
, ''D. lutrae'' parasitizes otter
Otters are carnivorous mammals in the subfamily Lutrinae. The 13 extant otter species are all semiaquatic, aquatic, or marine, with diets based on fish and invertebrates. Lutrinae is a branch of the Mustelidae family, which also includes wea ...
s, and ''D. ophidensis'' parasitizes reptiles.
References
External links
*
{{Authority control
Secernentea genera
Parasitic nematodes of mammals