Classification: Taxonomic ranks under review (cf. Encyclopedic Reference of Parasitology, 2001, Springer-Verlag)

Metazoa (Animalia) (multicellular eukaryotes, animals)
Nemathelminthes (nematodes)
Secernentea (Phasmidea) (with chemoreceptors known as phasmids)
Camallanida (copepod intermediate hosts)
Dracunculoidea (weakly-developed buccal cavity, large guinea worms)

Family: Dracunculidae
These worms include some of the largest known nematodes, several species measuring up to 80cm long. They have heteroxenous (two-host) life-cycles involving vertebrate definitive hosts (in which tissue-dwelling worms develop) ingesting aquatic copepodid intermediate hosts (in which infective larvae develop). Female worms do not lay eggs but birth live larvae (ovoviviparous). Infections in humans cause painful blisters through which larvae are released. Infections have been described throughout human history; the iconic ‘staff-with-serpent’ adopted as the official symbol of medicine may depict the traditional means of worm removal by winding it onto a stick.

Dracunculus medinensis [this species causes dracunculiasis in humans]

Parasite morphology: Guinea-worms develop through four larval stages prior to the formation of large adult worms; eggs are not produced. First-stage larvae appear as thin white tubular stages measuring up to 400µm in length and having a rhabditiform pharynx. The third-stage larvae are longer, measuring up to 600 ?m in length, and they have a filariform pharynx. Adult worms exhibit marked sexual dimorphism; males measuring from 2-4 cm in length with unequal spicules, while creamy-white females grow up to 80cm in length by 2mm in width and contain thousands of embryos. In young females, the vulva is located around the midbody but it becomes atrophied and non-functional in adults, as does the intestine due to the high internal pressure generated by the gravid uterus. Although the worms are very long and thin, they are not true filarial worms and are grouped separately.

Host range: Infections of humans by D. medinensis have been recorded many times in history, being described as ‘little snakes’ by Greek and Roman scholars, ‘fiery serpents’ in Biblical texts (Numbers 21:4-8), and colloquially named Medina-worms, guinea-worms or dragon-worms. Infections occur throughout semi-desert areas of sub-Saharan Africa, India, the Middle-East and Brazil, mainly in rural areas where water is drawn from wells or shallow ponds during the rainy season. It has been estimated that the prevalence of infections has decreased markedly (from 15 million in 1980 to 4 million in 1986 and 60,000 in 1997) due mainly to systematic preventive campaigns fostered by the World Health Organization. D. medinensis infections have occasionally been reported in dogs, cats, cattle, horses and other mammals. Other dracunculid species have been described from snakes, turtles, crocodiles and aquatic birds. The species D. insignis has been found in muskrats, opossums, raccoons and other carnivores in the Americas, and possibly sometimes in humans.

Site of infection: Ingested infective larvae penetrate the gut and invade subcutaneous connective tissues, migrating mainly to the axillary and inguinal regions. Maturing female worms migrate from deep connective tissues to peripheral subsurface locations, particularly in the extremities of limbs (legs and arms) although they can occur elsewhere.

Pathogenesis: Despite their eventual enormous size, infections by guinea-worms usually do not produce any clinical signs until the mature female worms migrate to the skin and provoke the formation of a papule then a blister. Migration may sometimes produce vague allergic reactions, including nausea, dizziness, diarrhoea, rash and local oedema. Infections generally produce two types of lesions: subcutaneous or deep abscesses around dead worms (involving many inflammatory cell types) that tend to calcify; or cutaneous papules which rapidly become blisters through which females release live larvae. Skin lesions may involve local erythema, urticaria, inflammation, ulceration and intense burning pain (fiery serpent of biblical times). Patients seek to relieve symptoms by immersing the affected region in cool water. Lesions are initiated by the deposition of larvae in the tissues and the induction of hypersensitivity reactions which ultimately produce blisters through which larvae, and parts of the adult worm, emerge (a unique means for tissue-dwelling parasites to seek egress from their hosts). In uncomplicated cases, lesions may only last for several weeks until the worm is completely expelled. However, many cases involve secondary bacterial infection of the worm track with persistence of the lesion, chronic ulceration and possible sequelae, involving disseminated infections, phlegma of limbs, contractures of tendons, fibrous ankylosis or arthritis in the joints, or even tetanus.

Mode of transmission: The parasites have a unique indirect life-cycle, involving copepods (Cyclops, water fleas) as intermediate hosts. Adult female worms cause skin blisters which eventually rupture, thus releasing any larvae deposited in the tissues and also exposing the anterior portion of the adult worm. The exposed portion may rupture, or the gravid uterus may prolapse from the worm. Muscular contractions of the body wall force thousands of larvae out in periodic spurts (half a million per day); the contractions often being instigated by contact with water. Females usually die within 2-6 weeks of penetrating the skin. Liberated larvae are infective for less than a week and they actively move about in water attracting copepodid crustaceans which ingest them. Copepods breed best in standing waters such as ponds and open wells, so infections are common in remote rural areas reliant on such water supplies. The larvae penetrate into the haemocoel of the copepods, especially dorsal to the gut, and develop into infective third-stage larvae over 12-15 days (at 25°C). Humans become infected by swallowing infected copepods with drinking water. The infective larvae penetrate the intestinal wall and migrate for about 3 months through connective tissues where male and female worms develop and mate. The males die after mating, while the fertilized females migrate to subcutaneous sites and grow to essentially become non-feeding bags full of larvae. Gravid females begin to emerge from the skin around 10-14 months after infection.

Differential diagnosis: Infections become obvious once a blister forms and part of the female worm emerges. Milky clouds of larvae can also be seen under low magnification when the lesion is placed in water. Immunoserological tests have been developed to detect host antibodies formed against parasite antigens during the pre-patent period of infection.

Treatment and control: The traditional means of curing infections involves the slow extraction of worms by winding them onto a stick a few centimetres a day for several weeks. Excessive force should not be used to avoid breaking the worm and complicating lesions and reactions. Surgical removal may be successful when worms are restricted to superficial sites, but can be difficult when worms are threaded through tendons or deep fascia. Chemotherapy with conventional anthelmintics has not proven effective, but various compounds, such as albendazole, mebendazole, niridazole, thiabendazole and metronidazole, appear to act as anti-inflammatory agents, thus allowing worms to be extracted more easily. Preventive measures involve breaking the cycle of transmission by reducing contamination of water supplies and eliminating copepod hosts. Public education programmes have been developed to discourage infected persons from entering ponds or wells to collect drinking water or to bath. Local water supplies can be treated with temephos (Abate, cyanamid) which kills copepods for several weeks. Drinking water can also be purified by boiling or filtering through fine-meshed cloth (<0.15 mm). The World Health Organization has accredited the global decline in the prevalence of infections to the adoption of many of these simple preventive measures.