The company warned 250,000 customers in Northampton and Daventry to boil drinking water last month after it detected the stomach bug cryptosporidium in supplies from its Pitsford water treatment works.
The situation became more serious this week as the Health Protection Agency reported that laboratory tests showed that at least 13 people had become ill. The genetic fingerprint of the cryptosporidium organism responsible was identical to the one found in the water supply.
The Drinking Water Inspectorate (DWI) is mounting an investigation into the incident. A spokesman told ENDS that the outbreak was surprising because it involved an unusual strain of cryptosporidium which had only once before been reported as infecting humans.
Cryptosporidium is a tiny protozoan parasite which is highly resistant to chlorine disinfection. Water companies normally rely on efficient filtration to protect supplies.
In this instance, water was apparently contaminated after treatment and there was no barrier to prevent the organism entering the water mains. Infections occurred despite the company issuing boil water notices and fitting an ultra-violet disinfection system once the threat had been detected.
