A long-awaited Ontario report on the prevalence of West Nile virus infection showed the ratio of people who fell severely ill with the disease was significantly higher than public health experts had predicted.

Severe disease occurred in one in 84 cases of West Nile in south Oakville in the summer of 2002, the researchers found. That rate is nearly double the rule of thumb experts normally cite when talking about West Nile: that only one in every 150 people who become infected falls seriously ill.

When researchers looked only at older people, the ratio was closer to one in 40 or 50 cases, lead author Mark Loeb noted Friday.

“What this shows is there were … a disproportionate amount of severe cases in these two postal code areas,” said Dr. Loeb, an infectious disease expert at McMaster University in Hamilton.

“What it tells is that it's something to take seriously.”

Loeb and colleagues from McMaster conducted the study earlier this year at the behest of the Ontario Health Ministry. The aim was to determine what ratio of people in south Oakville, west of Toronto, had been infected with West Nile virus in 2002  the first year human infections were known to have occurred in Canada.

Most people who become infected through a mosquito bite never know it, because they either do not become ill or have such mild symptoms they don't seek medical help.

The research was centred on two postal code areas in south Oakville, which recorded the highest concentration of cases in Ontario  and in Canada  during the summer of 2002. This past summer, Saskatchewan was Canada's West Nile hot zone, with more than 750 cases. Alberta and Manitoba were also hit hard.

Read the complete story in Friday's Globe and Mail.