Dracunculus medinensis

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). The common name 'guinea worm' is derived from the Guinea region of Western Africa. Dracunculus medinensis (little dragon from Medina) was described in Egypt as early as the 15th century BC and possibly was the 'fiery serpent' of the Israelites described in the Bible. 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 each of Mali and South Sudan, 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.