| Opportunities for Control of Dracunculiasis (1982) |
|Overview of Dracunculiasis|
Guinea worms enter the human body when water containing cyclops infected with the third-stage D. medinensis larvae is swallowed. Cyclops are killed by gastric juices in the stomach, and the larvae are released. The larvae migrate quickly to the duodenal wall and proceed to the abdominal and thoracic cavities where they begin maturing in connective tissue. Male and female worms mate about 3 months after ingestion. The male worms die at 6 months of age and then become encysted, calcified, or are absorbed. The adult female worm, which measures about 70 cm long by 2 mm, lives in the connective tissues. At about 8 months female worms usually move down to the lower limbs, where the uterus containing first-stage larvae develops to fill nearly the entire adult worm.
Approximately 1 year after initial host infection, the worm is ready to emerge and to emit larvae. It migrates to the subcutaneous tissues and secretes a toxic substance that produces a painful blister. Infected people frequently try to relieve the burning sensation by immersing the affected part in water. Contact with water causes the worm's uterus to rupture and stimulates the worm to expel larvae into the water. The process is repeated intermittently over several weeks (see Figure 1).
Each female worm releases about a million microscopic first-stage larvae into the water. The larvae remain active for about 5days in pond water or step wells, where they may be ingested by cyclops exceeding a certain minimum size. When water temperature is above 21°C, the larvae inside a cyclops undergo second-stage molting and develop into the infective third stage in about 14 days. First-stage larvae swallowed directly by humans do not undergo further development and are probably killed immediately by gastric Juices. Dracunculus larvae do not reach the third stage unless they enter cyclops.
Cyclops containing third-stage larvae tend to sink to the bottom of a pond or step well, where they are more likely to be scooped up during the dry season, when water levels are low. People drawing drinking water from stagnant surface-water sources during the height of the transmission season are exposed to higher rates of infected cyclops (approximately 5 in 100 cyclops may be infected).
Although Dracunculus species are known to infect animals, the role of animal reservoir hosts in the transmission of D. medinensis to humans has not been clearly established. For example, raccoons and other wild carnivores in North America often harbor a parasite named D. insignis, which is not easily distinguished from D. medinensis. D. medinensis has also been used experimentally to infect monkeys and dogs. Even though there is no evidence to suggest that dracunculiasis is a zoonotic infection, and while the possibility of reintroduction of the parasite into unprotected human drinking water sources by possible reservoir hosts is very remote, it should not be entirely discounted.