The researchers warn that before large-scale projects are launched, an assessment of the health impact should be made, and action taken to prevent diseases from gaining a foothold.

However, they stress that such schemes are vital to boost agriculture in famine-hit areas.

The problem occurs because water can carry disease and provides a breeding ground for the mosquitoes that transmit it to humans.

In the rush to provide better water facilities, organisations can neglect to prepare communities for the re-emergence of disease or introduce adequate protection for the water itself.

Fighting drought

The study, published in the British Medical Journal, looked at what happened in the Tigray region of Ethiopia following the introduction of dams and irrigation systems.

The measures had been introduced to reduce local agriculture's dependence on regular rainfall following the famines - caused by drought - in 1974 and1984.

But while the scheme has had a positive impact on agriculture, the effect on health has been worrying, with an increased incidence of malaria and seven times as many children in villages near the dams getting the disease compared with those living further away.

The findings are similar to those in a study of similar projects in Sri Lanka and raise fears that, unless properly thought out, schemes to improve the environment could do as much harm as good.

'Dams are good'

Dr Peter Byass, of the School of Community Health Sciences at Nottingham University, helped run the study, which was set up by the Tigray authorities to monitor the health impact of their measures.

Malaria is carried by mosquitoes

"It's easy to jump to the conclusion that dams are bad, but that very definitely isn't the conclusion because we're talking about a very dry arid area where people and animals and farmers must have water," he said.

"We're only too well aware of the droughts that have affected this region in recent years, so the dams are a very important strategic development, but on the other hand we also have to be aware of side effects like this that may be associated with them."

However, damming projects like this were taking place around the world yet very few had included monitoring of side effects - despite the obvious impact they have.

Avoiding risks

Ray Heslop, an engineering adviser for Water Aid, a charity that helps communities gain access to clean drinking water, said the organisation was well aware of the health risks associated with new water supplies.

"We even consider the loss of water from taps, and make sure it seeps straight into the ground and doesn't lie ponding," he said.

"It is a thing we consider because the objective of our projects is to improve the quality of people's lives, and that includes their health."

However, it there was a danger that organisations working on larger-scale projects such as irrigation and dam building, would overlook such matters and would have to revise their approach.

The researchers have just completed a study of how giving villagers bed nets helps prevent the spread of malaria and compared the cost of that with the value of the benefits offered by irrigation.