In the lake, however, there is the risk of bilharzia, an infection by a type of parasitic worm, so travellers who choose to swim or dive there often stock up on the treatment, Praziquantel tablets.

BBC: The planet��s pristine waters

The latest government campaign to distribute the pill has lowered bilharzia cases around Lampsar village from about 30 a month to less than 10, says Fatou Sarr Diouf, head of the regional health centre.

BBC: Using prawns to battle a killer disease in Senegal

The Aswan high dam, for example, is often cited as a cautionary example, a quixotic construction that now reduces the mighty Nile to a dribble before it trickles to the sea, leaving behind an explosion of water hyacinth, outbreaks of bilharzia, polluted irrigation channels and a build-up of sediment inland that would otherwise compensate for coastal erosion from Egypt to Lebanon.

ECONOMIST: Small projects often give better returns