The boil notice was issued on 29 November to customers in Gwynedd and Anglesey.
Cryptosporidiosis is an acute gastrointestinal illness caused by a protozoan parasite, cryptosporidium. The bug is highly resistant to chlorine disinfection and is frequently spread in water supplies. To prevent its spread, water companies must maintain efficient filtration.
The National Public Health Service for Wales first warned of an increase in the number of cases of the disease on 2 November. Some 14 cases had been reported during the preceding month, compared with a norm of eight to ten. By 8 December, 174 laboratory-confirmed cases of the illness had been recorded. It is likely that many more have not been diagnosed or confirmed with faecal samples.
A health service spokesman said a case-control study had shown a "statistically significant" link with the use of unboiled tap water from Welsh Water's Cwellyn reservoir in Snowdonia.
The upland supply is treated by pressure filtration at the company's Cwellyn water treatment works. The supply was classified as "low risk" by a risk assessment conducted under regulations designed to reduce cryptosporidium contamination of water supplies.
Welsh Water's head of quality and assets, Tim Masters, would not be drawn on the likely efficiency of the process in removing cryptosporidium. However, the low-risk classification means the firm was not obliged to conduct 24-hour monitoring for the organism.
The company began sampling the water supply as a precautionary measure following the 2 November announcement. Mr Masters said the results showed "low levels, typically fewer than three oocysts in 1,500 litres, although on some occasions no oocysts are detected."
The figures are well below the "treatment limit" of one oocyst in ten litres of water specified in the regulations. However, this provides no health protection, and is merely intended to show effective filtration. If the detected organisms are viable and belong to a strain capable of infecting humans, the supply could well be the source of the outbreak.
Mr Masters said early typing of the organism suggested it was the human-specific strain, once known as "type 1" but now recognised as a distinct species and called Cryptosporidium hominis. Other strains that can affect both humans and animals, called "type 2", are now known as C parvum.
The identification of the organism has allowed the company to focus on raw sewage rather than animal sources in its search for the contamination's source. The company issued a statutory notice on the owner of a property in early December, requiring a defective septic tank to be fixed. However, it stresses that there is no proven link between the property and the contamination.
The outbreak seems to be the first in England and Wales to result in a boil notice since the cryptosporidium rules came into force in 2000. However, there have been outbreaks more recently in Scotland and Northern Ireland (ENDS Reports 332, pp 13-14 and 308, pp 29-30 ).
Some small outbreaks have been linked to water but have not resulted in boil notices. One example was an outbreak in Portsmouth in August 2005. The epidemiological evidence suggested a link with a small drinking water supply abstracted from a river. The water company responded by replacing the supply with other sources.
The Welsh outbreak came just as the DWI was finalising an information letter to water companies asking them to revise their risk assessments for cryptosporidium. The request follows new evidence on the epidemiology of the disease.
Epidemiologists have long known that cryptosporidiosis cases peak in spring and autumn. Scientists now believe spring peaks are largely caused by C parvum and the autumn peaks by C hominis.