Study suggests drinking water may be source of winter norovirus outbreaks

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TORONTO Why do nasty norovirus outbreaks seem to happen more often in winter? A new study suggests drinking water supplies may be playing a role.

The research, which looked for patterns that might explain norovirus outbreaks in Toronto, found that winter flare ups of the highly contagious condition were more likely to happen in the week after water temperatures in Lake Ontario dipped below 4 degrees Celsius or flow from the Don River into Lake Ontario was high.

The findings suggest that under certain environmental conditions, noroviruses from human sewage may proliferate in bodies of water that are used both as municipal water sources and sewage treatment outlets, eventually finding their way back into human gastrointestinal tracts through drinking water.

"It's not the time of year when people are swimming or using the beaches or anything like that," said lead author Amy Greer, a post-doctoral fellow working on the ecology and evolution of infectious diseases at the Research Institute of the Hospital for Sick Children in Toronto.

"So the question is, if we have a reservoir in the lake of environmental virus, essentially how is it that people are coming in contact with it? . . . Our findings may suggest that that (drinking water) might be something that we should look at."

The research was presented Sunday at a joint scientific conference of the American Society for Microbiology and the Infectious Diseases Society of America in Washington, D.C.

The findings are preliminary, Greer cautioned in an interview from Washington.

She and two colleagues from the Ontario Public Health Laboratory and Toronto's Mount Sinai Hospital ran statistical analyses looking for discernible patterns between environmental factors and the timing of norovirus outbreaks in the Toronto area.

They studied 253 norovirus outbreaks that were investigated by the Ontario Central Public Health Laboratory between Nov. 10, 2005 and March 6, 2008, looking to see whether precipitation levels, air temperature, and other environment conditions seemed to coincide with the outbreaks.

The only factors that seemed to be linked to outbreaks were water temperatures in Lake Ontario and the water levels of the Don River, which enters Lake Ontario just east of the city's downtown core. There was no pattern seen when they looked at water levels for other rivers that flow into the lake near Toronto.

The researchers did not take and test water samples for norovirus levels, so they do not have direct evidence with which to back up their theory. They hope, however, that scientists in Toronto and elsewhere will undertake this kind of testing to see whether water systems may be playing a role in igniting outbreaks.

The researchers also only looked at outbreaks in Toronto - leaving open questions about whether the same pattern is seen in other cities with similar sewage treatment systems, and whether municipalities that use ground water or ocean water for drinking supplies see similar outbreak patterns.

Greer noted, though, that there is some laboratory evidence of this virus "cycling" from Europe. "It's much too early for us to say whether or not that may or may not be a similar case in the GTA (greater Toronto area). But there is evidence that that may be the case in other locations."

Noroviruses trigger explosive and debilitating bouts of vomiting and diarrhea. The viruses are transmitted by what's known as the fecal-oral route. People pick up viruses on their hands - for instance in a bathroom recently used by an infected person - and then transfer the germ to their mouths by handling food or putting fingers into the mouth.

While these outbreaks can occur at any time of the year, they most commonly happen in winter month - which is why norovirus infection was formerly called "winter vomiting disease."

It has been thought that human dynamics are behind the seasonal pattern of these outbreaks.

"It's essentially for many of the same reasons that influenza tends to be seasonal," said Dr. Aron Hall, a norovirus expert with the U.S. Centers for Disease Control in Atlanta. Hall had not seen Greer's study and could not comment on her group's findings.

"People are typically indoors more and therefore have more contact with one another. They're in closer contact with one another. ... And so the more humans you pack together in a smaller area over a longer period of time the more likely you are to have infection."

According to the CDC, most norovirus outbreaks are trigger by contamination of food by an infected food handler.

The CDC website says that of 232 outbreaks of norovirus illness reported in the United States from July 1997 to June 2000, 57 per cent were due to contaminated food, 16 per cent were due to person-to-person spread, and only three per cent were linked to contaminated water. However, no source could be determined for 23 per cent of the outbreaks.

"The waterborne outbreaks of norovirus that we typically see are either due to drinking water sources in which chlorination or other disinfection systems have broken down, or in recreational situations in which either the same - disinfection or chlorination breaks down in the case of a swimming pool or something along those lines - (happened)," said Hall.



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