| Dracunculus | |
|---|---|
 | |
| Dracunculus medinensis larvae | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Nematoda |
| Class: | Secernentea |
| Order: | Camallanida |
| Family: | Dracunculidae |
| Genus: | Dracunculus Reichard, 1759 |
| Species | |
Dracunculus alii | |
Dracunculus is agenus of spiruridnematodeparasites in the familyDracunculidae.
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 foreignantigen which can lead toanaphylactic shock 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.
All members ofDracunculus 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 acyclopoidcopepod, which serves as anintermediate host. Inside the copepod, the larvae develop to the third-stage ("L3").Definitive hosts acquireDracunculus by incidentally ingesting infected copepods while drinking water, or by consuming aparatenic host (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 twomolts 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.[1] 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.[1]
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.[1] 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.[1] The body of a fertilized adult female is almost completely filled by its uterus, distended with L1 larvae.[1] Adult femaleDracunculus worms are noted for their extraordinary length, with some growing up to 100centimeters long. Males are much smaller (16 – 40millimeters) and are relatively rare – in some species the male has never been described.[1]
Dracunculus worms are distributed globally, though each species has a narrower range. The majority ofDracunculus species described infect reptiles – predominantly snakes.[1] These are spread across the globe, withD. ophidensis in theUnited States,D. brasiliensis in Brazil,D. coluberensis andD. alii in India,D. houdemeri in Vietnam,D. doi in Madagascar,D. dahomensis in Benin,D. oesophageus in Italy, andD. mulbus in Australia and Papua New Guinea. The only species known to infect a non-snake reptile isD. globocephalus which has been described insnapping turtles in the United States and Costa Rica.[1]
Most mammal-infecting species are in the Americas, withD. insignis infecting several wild and domestic mammals in theUnited States andCanada,D. lutrae infectingriver otters in the United States and Canada, andD. fuelleborni infectingbig-eared opossums inBrazil.[1] The major exception isD. medinensis, which is by far the most studiedDracunculus since it infects humans – causingGuinea Worm Disease.[2][3]D. medinensis was historically widespread in sub-saharan Africa and South Asia,[1] but is now limited to dozens of cases annually in humans and domestic dogs, and may soon be driven to extinction due to eradication efforts.[4][5]
There are 14 acceptedDracunculus species, 10 of which infect reptiles. Eurasia hosts several reptile-infectingDracunculus species.D. oesophageus was originally described from the esophagus ofthe viperine water snake, 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 Indiantrinket snake, andD. alii andD. houdemeri fromCheckered keelback snakes in India and Vietnam respectively.[1]
The only snake-infectingDracunculus species known in North America isD. ophidensis. It was originally described ingarter snakes in Michigan and Minnesota by Sterling Brackett in 1938, and has since been reported inblackbelly garter snakes from Mexico, as well asnorthern water snakes and aplain-bellied water snake in Michigan.[1]D. brasiliensis is the only described snake-infectingDracunculus in South America. It was described in 2009 based on a single female worm from ananaconda in Brazil, and has since also been found in a Brazilianbrown-banded water snake.[1] Several worms that appear to be from the genusDracunculus 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.[1] The Americas are also home to the only known turtle-infectingDracunculus (also the only species that infects a non-snake reptile),D. globocephalus. First described in 1927 in Oklahoma and Illinois, it is now found insnapping turtles across the United States, as well as theSouth American snapping turtle in Costa Rica.
Africa has two known snake-infecting species. Both male and femaleD. doi were described fromMadagascar ground boas in 1960 and 1973 respectively.D. dahomensis has been described only from a captiveAfrican rock python.[1]
In Australia, the only known snake-infectingDracunculus isD. mulbus, described from numerouswater pythons in Northern Australia in 2007. It has since been described in Papua New Guinea'sPapuan olive python as well.[1]
Just fourDracunculus species are known to infect mammals, of which the best known is the human parasiteD. medinensis. Historically spread across Africa and South Asia, a major eradication effort has restrictedD. medinensis to just Chad, Ethiopia, Mali, and South Sudan.[1] 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.[6][7]D. medinensis is now most common in dogs, particularly in Chad, where it may spread via fish or frogs asparatenic hosts.[8]
D. insignis infectsdogs and wildcarnivores, causing cutaneous lesions,ulcers, and sometimesheart andvertebral column lesions. LikeD. medinensis, it is also known asGuinea worm, as well asDragon orFiery Dragon. The range ofD. insignis is limited toNorth America.
D. fuelliborni parasitizesopossum,D. lutrae parasitizesotters, andD. ophidensis parasitizes reptiles.
