Gordius medinensis Linnaeus, 1758
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 800 mm (31 in) in length, is among the longest nematodes infecting humans. In contrast, the longest recorded male Guinea worm is only 40 mm (1.6 in).
In the mid-19th century, the nematode Camallanus lacustris, which infects freshwater fish, was discovered to develop in copepods. This led to the discovery in 1870 by Russian naturalist Alexei Fedchenko of the means of transmission of D. medinensis, via copepod intermediate hosts.
D. medinensis larvae are found in fresh water, where they are ingested by copepods of the genus Cyclops. Within the copepod, the D. medinensis larvae develop to an infective stage within 14 days. When the infected copepod is ingested by a mammalian host, the copepod is dissolved by stomach acid and the D. medinensis larvae migrate through the wall of the mammalian intestine, and mature into adults. About 100 days after infection, a male and female D. medinensis meet and sexually reproduce within the host tissue. The male dies in the host tissue, while the female migrates to the host's subcutaneous tissue. Around a year after infection, the female causes the formation of a blister on the skin's surface, generally on the lower extremities, though occasionally on the hand or scrotum. When the blister ruptures, the female slowly emerges over the course of several days or weeks. This causes extreme pain and irritation to the host. When the host submerges the affected body part in water, the female expels thousands of larvae into the water. From here, the larvae infect copepods, continuing the lifecycle.
As of 2016, more than 500 dogs in Chad, 13 in Ethiopia, and one dog in Mali and South Sudan, respectively, have been diagnosed with guinea worm. Furthermore, it has been possible to infect frogs under laboratory conditions, and recently, natural infection has been reported in wild frogs in Chad. These findings are a potential problem for the eradication program.
D. medinensis is most commonly found in the subtropic to tropical regions, especially in India, south-west Asia (Iraq, Iran, Pakistan, etc.), and rural areas of Africa, where temperatures between 25 and 30°C are best for larval development. The parasite relies on people accidentally consuming microcrustaceans of the genus Cyclops (copepods), that dwell in stationary bodies of water such as ponds, large, open wells (with stairs), or rain-filled cisterns. The infection occurs most during times of drought or the “dry-season” in humid climates, or during or just after the rain season in the “semiarid, wet-and-dry-climates.” This is due to the lower surface water of the stationary bodies of water, which are prime for the growth of the infected copepods, and main source of water for many.
D. medinensis causes dracunculiasis as a result of the emergence of the female worm, nonemergence of adult worms (usually the male), and secondary bacterial infections. As it emerges to the subcutaneous tissue, the female releases a toxic chemical that may result in nausea, rash at site, diarrhea, dizziness, localized edema, reddish papule, blister, and itching. Arthritis or paraplegia can result from a worm that fails to reach the skin and gets calcified in or along the joint or finds its way into the central nervous tissue. Aseptic abscesses and cystic swelling can also occur when worms rupture before emerging, causing an acute inflammatory response from the host's immune system. 
The female guinea worm slowly starts to emerge from the host's skin after the blister ruptures. The most common method for removing the worm involves submerging the affected body part in water to help coax the worm out. The site is then cleaned thoroughly. Then, slight pressure is applied to the worm as it is slowly pulled out of the wound. To avoid breaking the worm, pulling should stop when resistance is met. Full extraction of the female guinea worm usually takes several days. After each day's worth of extraction, the exposed portion of the worm is wrapped around a piece of rolled-up gauze or small stick to maintain tension. This method of wrapping the worm around a stick or gauze is speculated to be the source for the Rod of Asclepius, the symbol of medicine. Once secure, topical antibiotics are applied to affected region to help prevent secondary infections due to bacteria and then is wrapped in gauze to protect the wound. The same steps are repeated each day until the whole worm has been removed from the lesion.
In the 1980s, the Carter Center initiated a program to eradicate guinea worm. The campaign began in 1980 at the US Centers for Disease Control and Prevention. In 1984, the CDC was appointed as the World Health Organization Collaborating Center for research, training, and eradication of D. medinensis . Twenty countries in 1986 were affected by guinea worms. That year, WHO started the eradication program and the Carter Center took the lead on the effort. The program included education of people in affected areas that the disease was caused by larvae in drinking water, isolation and support for sufferers, and – crucially – widespread distribution of net filters and pipe filters for drinking water, and education about the importance of using them.
As of 2015, the species has been reported to be near eradication. The International Commission for the Certification of Dracunculus Eradication has certified 198 countries, territories, and other WHO represented areas. In January 2015, eight countries remained to be certified as D. medinensis free. These eight countries include Angola, the Democratic Republic of the Congo, Kenya, Sudan, Chad, Ethiopia, Mali, and South Sudan. Of these, Chad, Ethiopia, Mali, and South Sudan are the only remaining endemic countries. Not coincidentally, all four are affected by civil wars which affect the safety of health workers.
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