Dracunculus (nematode)

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, which serves as an intermediate host. Inside the copepod, the larvae develop to the third-stage ("L3"). Definitive hosts acquire Dracunculus by incidentally ingesting infected copepods while drinking water, or by consuming a paratenic 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 two molts 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 female Dracunculus worms are noted for their extraordinary length, with some growing up to 100 centimeters long. Males are much smaller (16 – 40 millimeters) 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 of Dracunculus species described infect reptiles – predominantly snakes.[1] These are spread across the globe, with D. ophidensis in the United States, 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 turtles in the United States and Costa Rica.[1]

Most mammal-infecting species are in the Americas, with D. insignis infecting several wild and domestic mammals in the United States and Canada, D. lutrae infecting river otters in the United States and Canada, and D. fuelleborni infecting big-eared opposums in Brazil.[1] 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,[1] but is now limited to dozens of cases annually in humans and domestic dogs, and may soon be driven to extinction.[2]

• "Dracunculus". Integrated Taxonomic Information System. Retrieved 6 February 2017.