Guinea Worm Disease
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Humans are hosts to nearly 300 species of parasitic worms and over 70 species of protozoa, some derived from our primate ancestors and some acquired from the animals we have domesticated or come into contact with during our history (History of human parasitology. Clin Microbiol Rev 2002 15: 595-612). The best-documented parasitic disease known from ancient times is caused by the nematode worm Dracunculus medinensis. The earliest description is from an Egyptian papyrus from 1500 BC that refers to both the nature of the infection and to techniques for removing the worm. Confirmation of the presence of this worm in ancient Egypt comes from the finding of a well-preserved worms in Egyptian mummies. Dracunculiasis, or Guinea worm disease, is one of the few diseases unambiguously described in the Bible, and most parasitologists accept that the “fiery serpents” that struck down the Israelites in the region of the Red Sea after the Exodus from Egypt somewhere between 1250 to 1200 BC were actually Guinea worms.
The adult worms live in the subcutaneous connective tissues of their victims, from which the females emerge to release thousands of larvae into water, where they are taken up by intermediate hosts, tiny aquatic crustaceans called Cyclops. In these hosts they mature into infectious larvae that infect humans when the crustaceans are accidentally swallowed in contaminated drinking water. On maturity, the large female worm, up to nearly a metre in length, protrudes from the skin, usually of the leg, and causes intense inflammation and irritation. The effects of the disease are crippling. Its victims develop large ulcers, usually in the lower leg. The ulcers swell, sometimes to the size of a tennis ball, and burst, releasing the spaghetti-like parasitic worm. Victims experience a pain so excruciating that they say it feels as if their leg is on fire. The searing pain compels people to jump into water, often the community’s only source of drinking water, to relieve the pain. When the infected person immerses his or her leg in the water, the worm in the leg releases thousands of larvae. The larvae are then ingested by Cyclops that live in the water. Thus the cycle begins again – when people drink the water, they are in effect drinking in the disease.
The most common way to treat Guinea worm disease involves wrapping the worm around a stick. This treatment has been employed for millennia and may have inspired the Rod of Asclepius which historically has symbolized the medical profession. As the adult worm begins to emerge from the patient’s skin, it is wound around a stick, then further extracted by a few centimeters per day. This slow process can take days or even weeks, but it is required to avoid breakage and leaving behind a portion of the worm. Leaving a portion of the dead worm remain within the host’s body increases the risk of infection, and can trigger immune responses resulting in pain and swelling. In many countries, a broken worm is immediately removed surgically, or the worm can be excised surgically from the very beginning if health care facilities are available. Antihelminthic drugs such as metronidazole or thiabendazole are sometimes used in conjunction with physical extraction. However, one study found that antihelminthic therapy was associated with aberrant migration of worms, resulting in infection in areas other than the lower extremity.
Dracunculiasis is a classic example of a neglected tropical disease, a symptom of poverty and disadvantage. Those most affected are the poorest populations often living in remote, rural areas, urban slums or in conflict zones. With little political voice, neglected tropical diseases have a low profile and status in public health priorities. In 1997 the World Health Assembly pledged to completely eradicate Guinea worm disease. This is no small task, but there are several factors which make eradication a possibility. Dracunculiasis is the first parasitic disease targeted for eradication because:
- Diagnosis is easy and unambiguous (presence of an emerging adult worm).
- The transmission agent, Cyclops, is not a mobile vector as is a mosquito.
- The incubation period in both Cyclops and humans is of limited duration.
- Interventions are effective, low cost, and relatively simple to implement.
- The disease has a limited geographic distribution and is seasonal in nature.
- Success in eliminating the disease has been demonstrated in several countries in Asia and the Middle East.
- There is no known animal reservoir.
Is Dracunculiasis eradication close? In 2007 the WHO announced that Guinea worm disease now affects around 25,000 people in nine countries, compared with an estimated 3 million people were infected in over 20 countries in the early 1980s. Twelve countries were declared Guinea worm-free in early March. If progress continues at this rate, the disease could be eradicated in less than two years. It is probable that complete eradication will take quite a few years yet, although it should be possible to eliminate the disease from seven countries in a couple of years, leaving only two endemic countries, Sudan and Ghana (Dracunculiasis eradication by 2009: will endemic countries meet the target? Tropical Medicine & International Health 2007 12: 1403-1408). One lesson to be drawn from the problems of local ownership and the experience of cash rewards is that there are dangers in throwing money at the problem. While the eradication initiative badly needs additional resources, it needs them at such a level and managed in such a way that they do not distort the priorities of the health care system, or exceed the capacity of local staff to manage them. The amounts needed are not large, but their continuity and flexibility is important. Given the highly seasonal transmission of dracunculiasis, the resources must be available at very specific times of the year, which is not always achieved. In spite of the difficulties, complete worldwide eradication of this ancient disease is drawing nearer.
See also: Guinea Worm: Countdown to Zero
Tags: Biology, Environment, Health, Medicine, Microbiology, Parasitology, Podcast, Science, Video
