The contamination that infected 308 people last year was probably due to sheep grazing near a North West Water reservoir in the Lake District, health authorities have said.

Steps have been taken by North West Water to remedy the situation. Obviously there is still a great deal of work to be done.

Professor Paul Hunter

During the outbreak a microscopic parasite, cryptosporidium, spread through the water supply to households in Greater Manchester and Lancashire.

An investigation led by epidemiologist Professor Paul Hunter at the Communicable Disease Surveillance Centre in Liverpool traced the source of the infection.

The water was supplied by North West Water from one of its main reservoirs at Thirlmere in the Lake District during April and May last year.

The infection, whose symptoms include diarrhoea, vomiting and headaches, can be life-threatening although most people will recover within one to four weeks said Professor Hunter.

Urgent measures

The company has been recommended to make urgent improvements at key treatment plants to prevent further outbreak.

A spokesman for North West Water said the company is spending �300m to protect the water supplies.

Professor Hunter said: "Steps have been taken by North West Water to remedy the situation and we must now wait and see if they have worked. Obviously there is still a great deal of work to be done."

Measures are under way to improve the monitoring system at Thirlmere, and change grazing procedures for sheep and cattle around the reservoir.

Professor Hunter said those with reduced immunity, such as people with HIV and AIDS, could be more seriously affected by the water infection.

The region's health authorities have advised such people to boil all drinking water.