Dracunculus: background document for the WHO guidelines for drinking-water quality

Overview

• There is strong evidence linking transmission of Dracunculus medinensis to unsafe drinking water. In the 1980s dracunculiasis, also known as guinea-worm disease, was a significant waterborne disease with an estimated 3.5 million cases. However, it is now on the verge of eradication. Ingestion of surface water containing copepods (water fleas, usually cyclops) harbouring D. medinensis is the primary source of infection. Formal drinking-water supplies have not been linked to the transmission of D. medinensis.
• Dracunculus medinensis is a parasitic, zoonotic nematode or round worm that infects people, dogs and possibly other mammalian hosts.
• Following ingestion of copepods containing Dracunculus, the larvae penetrate the stomach and intestinal wall, and mature into worms. After about 1 year the female worm migrates to reach a skin exit point. Water contact of the emerging worm leads to expulsion of thousands of immature larvae into water which can be ingested by copepods. Person-to-person transmission has not been reported.
• Emergence of the worms causes a blister or swelling of the skin with a burning sensation and intense pain. The exit site wound is prone to secondary bacterial infection and is a potential cause of permanent disability of nearby joints.
• Dracunculiasis was once endemic in 20 countries but it is now only reported from a few African countries. In 2022 this included a total of 13 cases in Chad, Ethiopia, South Sudan and the Central African Republic. People of all ages are susceptible to infection.
• The human infectivity of D. medinensis is unknown.
• Practical methods for routine analysis of D. medinensis in water are not available. Analysis is difficult because of the need to recover the aquatic copepods infected with the D. medinensis larvae and there is no culture method for the worms, except in animals such as ferrets. Molecular methods are being developed.
• Provision of a safe water supply to communities relying on surface water sources is a key component to eradicate dracunculiasis. Where a safe water supply cannot be provided, point of-use filtration is effective to remove copepods. Within a water safety plan, control measures that can be applied to prevent transmission of Dracunculus include protection of surface water sources from contamination by infected humans and dogs, eradication of copepods from surface water sources with an approved larvicide and filtration to remove copepods from surface water sources. Copepods have low resistance to chlorine while larvae released by copepods during disinfection are expected to have low to moderate resistance to chlorine.
• Since Dracunculus are non-faecal organisms and due to their complex life cycle and environmental survival, E. coli (or, alternatively thermotolerant coliforms) is not a suitable indicator for the possible presence or absence of Dracunculus in drinking-water.