DOCTORS are on the alert after high levels of a bug which can spark an infectious disease were found in a reservoir in the Lake District.

Routine sampling of water from Thirlmere Reservoir, which supplies Chorley and South Ribble, found high levels of the Cryptosporidium bug, which can cause vomiting, diarrhoea and abdominal pains.

Now health bosses and United Utilities -- the water company who performed the test -- have written to doctors in the area asking them to report any new cases.

A spokesman for United Utilities said: "There is no cause for concern and the water is safe to drink. There is no evidence of illness and the action taken in writing to GPs and health authorities is purely precautionary."

The microscopic parasite comes from the guts of animals, and most outbreaks have been associated with water, contact with animals and swimming pools.

But a spokesman for United Utilities confirmed that the Chorley and South Ribble areas could be at risk although Preston's water supply has not been affected.

He said: "If you drink a pint of water in the South Ribble area, two per cent of the water will be from the Thirlmere Reservoir. The affected areas include Chorley, South Ribble and Fylde."

Dr Steven Gee, consultant in communicable disease control at North West Area Health Authority, said: "An increased level of Cryptosporidium was detected in the water earlier this month. As far as Preston goes, it doesn't get its supply from that source. It is going to Chorley, though.

"But we have to let the public know there has been an increase. The water as we currently receive it does contain low levels which does cause a risk for people who are immuno-compromised, for example with sever cancer or aids.

"The symptoms are similar to lots of other gastrointestinal diseases. People can become infected without having any symptoms, but when you do have them they can be mild to disabling. For instance feeling off colour and vomiting to abdominal pain. They normally last a few days, but could last a few weeks. It is usually not severe."