Norwalk affects millions annually

Norwalk virus, a gut-wrenching fixture of Canadian winters whose source and seasonal nature have long been a mystery, may originate in drinking water drawn from lakes whose cool winter temperatures keep the microbe nicely preserved, suggests a new study.

University of Toronto researchers, comparing Norwalk outbreaks to river water flow and lake temperatures, theorize that a near-endless “feedback loop” sends the virus from human waste to surface waters, then to water treatment plants that are unable to eliminate the bug, and finally back into household faucets.

The theory, if proven, raises the question of whether tap water should be treated with ultraviolet exposure, virtually the only way to kill the hardy viruses, the researchers say.

The scientists stress that their hypothesis is just that, and needs to be confirmed by more study. But if it is, it would underline the risks Canadians still face from their aquatic environment, long after menaces like typhoid have been removed from the water supply, they say.

“We’re very lucky: We live in a high-income country, we have schools, buildings that don’t fall down, sewers, water treatment,” said David Fisman, the University of Toronto epidemiology professor who headed the study.

“[But] I sometimes wonder if having that infrastructure makes it feel like we are buffered against what happens to our environment…. If you have a very contaminated environment, it becomes very difficult to buffer yourself.”

The group’s “novel concept” would seem to gibe with what is already known about noroviruses, said Dr. Allison McGeer, an infectious disease specialist at Toronto’s Mount Sinai Hospital. The question — if the theory is confirmed — is whether it would be worthwhile to invest the money needed to implement ultraviolet water treatment, a costly venture, she said.

“It’s a really difficult balance,” said Dr. McGeer. “For the majority of people it’s extraordinarily unpleasant but not serious. But in people who are vulnerable for one reason or another, it can be fatal. It’s not necessarily a trivial problem.”

First identified in the early 1970s, noroviruses account for 68 to 90% of known gastroenteritis outbreaks, causing diarrhea, vomiting and stomach cramps, and affects more than 23 million North Americans annually, according to Dr. Fisman’s paper, just published in the journal EcoHealth.

The U of T group set out to explore why the virus occurs mostly in one season, an unexplained trait it shares with many other infectious diseases. They compared the timing of 253 suspected Norwalk outbreaks in the Greater Toronto Area from 2005 to 2008 with environmental data such as weather, UV levels, temperatures in Lake Ontario and flow in the Don River, which cuts a swath through the centre of Toronto.

They found that drops in lake temperature were very closely related to outbreaks, and day-to-day increases in river flow were somewhat less linked.

They theorize that norovirus is shed by people with the illness and ends up in the Don when storm conditions overwhelm the sewer system –and boost river flow. The microbe then surges into the lake, where it is protected by the refrigerator effect of cold water. The city draws its drinking water from Lake Ontario, but current treatment methods do not eliminate noroviruses, allowing the pathogens to be delivered back to residents’ homes, where the loop begins again, the paper suggests.

Once an outbreak starts, transmission occurs rapidly from person to person.

The exact hypothesis would not work for all cities, Dr. Fisman acknowledges. The idea of natural water bodies acting as a reservoir, however, especially when winter weather keeps the water cool, could be a factor in many places, he said.

Dr. McGeer suggested the theory of cold water as a source for norovirus has its limitations, though, noting that there is no evidence of Norwalk outbreaks originating in water parks.