Dracunculiasis is acquired when people drink water contaminated with cyclopoid copepods (or water fleas) which have ingested third-stage D. medinensis larvae.

Applying chemicals to control copepods in water sources is one of three recognized ways to interrupt transmission, including provision of safe (copepod free) water sources and the use of filters and/or boiling drinking water.

Chemical control of copepods is used when the following conditions apply:

• Unsafe sources of drinking-water are few, small or moderate in volume (500 m3 or less) and shared by many people
• Permanent sources of drinking water are not feasible
• Compliance with health education is poor
• During outbreaks of dracunculiasis
• An additional security measure is needed to prevent transmission

Chemical control has a cost–benefit ratio of 4.14 compared with that (2.61) of surveillance, health education, community participation, and availability of health care and underground water sources.

Temephos

Temephos is the chemical of use recommended by WHO because it is safe for humans and has low toxicity to mammals compared with chlorine, potassium permanganate, DDT (dichlorodiphenyltrichloroethane) and zinc carbamate.

A 50% emulsifiable concentrate is the most readily available and cost–effective formulation for copepod control.

Safety: The FAO Working Party of Experts on Pesticide Residues and the WHO Expert Committee of Pesticide Residues declared that temephos is safe for use in actual or potential sources of drinking water at a dosage not exceeding 1 part per million (WHO, 1973).

Efficacy: A concentration of 0.1 mg/l killed 100% of Cyclops vernalis.

Mode of action: Temephos inhibits production of enzyme cholinesterase after 12 hours of exposure. Depleted cholinesterase causes paralysis to copepods within 72 hours. The copepod settles to the bottom and becomes food for other organisms; some die because they cannot feed normally.

Before applying the temephos to a village water source, establish clear and courteous communication with the authorities. Explain the effects of temephos, particularly the tastes and colour of water after treatment, so that hostility, suspicion and reluctance to use the water will be prevented.

Timing the application of temephos is important. An interval between application of temephos and use of water allows the temporary discolouration, odour or taste (if water is mostly fetched in the morning then apply the temephos in the evening) to disappear.

For detailed instructions, see Applying Temephos and Evaluating Treatment (p. 14-23) and   WHO specifications and evaluation for temephos.