5.3 The guinea worm transmission cycle
Dracunculiasis is the only human disease in which the causative agent is transmitted exclusively in drinking water.
The life cycle of D. medinensis involves a larval maturation stage in an intermediate host – microscopic freshwater crustaceans referred to in the infectious disease literature as ‘cyclops’. (Until recently, all the species that transmit guinea worm larvae were classified in a single genus, Cyclops, which has since been subdivided, but for simplicity the traditional term will be used.) People become infected when they drink water contaminated with cyclops containing infective Dracunculus larvae. The larvae are released in the drinker’s stomach, where the cyclops are killed by the acid environment.
Each cycle occurs over a period of about one year and the main sequence of events can be summarised as follows (see also Figure 20).
- Over a period of about three months, the ingested larvae migrate out of the human host’s gut into the chest cavity, and mature into sexually differentiated male and female worms a few centimetres long.
- There they mate and the male worms die, but the females continue growing for the next 8–12 months, reaching 0.5–1.0 m in length.
- The female worms migrate along muscle fascia towards the surface of the body, usually somewhere in the lower limbs (often on the ankles or feet), where they slowly emerge during a predictable seasonal period lasting two to three months.
- As a female worm nears the surface of the human tissue from which it is about to emerge, it releases a burst of larvae just under the host’s skin. A mature female worm carries over 3 million immature larvae.
- The larvae are strongly immunogenic and provoke a rapid inflammatory response, which erupts in an intensely painful, fluid-filled ulcer, usually somewhere on the lower limbs.
- A natural response to the burning pain of the ulcer is to cool it by standing in cold water – and in rural communities this is most commonly a village pond, or a shallow ‘step well’ where people step down into the water to fill drinking pots.
- The sudden cooling stimulates the worm to emerge by a few millimetres and release several thousand immature larvae into the water.
- Over the next six weeks, the worm emerges slowly from the host’s skin, releasing another shower of larvae every time it is exposed to cool water.
- The worm larvae are free swimming and can survive for several days in still ponds and shallow wells, which are also ideal habitats for cyclops.
- The immature larvae are eaten by cyclops, initiating another cycle of infection.
Treatment and control
None of the drugs used in the treatment of other parasitic worms has any therapeutic effect against guinea worms. At best, antibiotics or antibacterial ointment rubbed into the worm eruption site may reduce the risk of secondary bacterial infections.
No vaccine has yet been developed to protect people who ingest the larvae, and infected people do not develop a protective immune response to the adult worms, so they can be infected again and again.
Thus, attempts to control dracunculiasis have had to rely entirely on community action and low-cost interventions rather than on medical treatment.