Driving is still more risky than West Nile virus

Originally published in The Ottawa Citizen May 13, 2003
Original Title: It’s a skewed, skewed, risk perception world

We are in the midst of a large-scale data collection and evaluation process of West Nile virus infection and outbreak in North America. Although West Nile virus affects other areas of the world like Europe, the Middle East, Russia, Tunisia, Morocco and South Africa among others, it is difficult to extrapolate their experience to the North American arena. The North American West Nile virus is genetically distinct and seemingly more virulent than strains from other parts of the world.

Most people want to know the risk of contracting the virus this summer. Unfortunately, the three years of experience since the virus’ arrival in New York City has yet to provide a clear picture of risk. Indeed, the virus continues to spread east to west in the United States and eastward and westward from central Canada.

We do know that past infections arose at the end of August and early September in the temperate regions of North America. Mosquitoes emerge in the spring. They start the process of viral amplification in the bird-mosquito-bird cycle that peaks in the early fall. Thereafter the risk of infection decreases in humans when female mosquitoes begin semi-hibernation (diapause) and infrequently bite.

The problem is that dead crows have already turned up in Ontario. This suggests the bird either migrated from southern climes or was bitten by mosquitoes surviving the winter. We do know through reports out of New York City and Windsor, Ontario, the virus survives in diapausal mosquitoes residing in the sewer systems.

The latest 2002 Statistics Canada data indicates 307 confirmed cases and 83 probable cases of West Nile Virus in Ontario and 18 deaths. The United States Centers for Disease Control data notes 4,156 laboratory-positive cases and 284 deaths.

The death rates do not tell the entire picture. The numbers of confirmed infection are small relative to our population. However, this is a disease on the move. In other countries, the outbreaks were sporadic and did not recur every year. More Canadians and Americans will be exposed to the virus this year. How many will remain unaffected, suffer nerve and muscle damage or succumb to their illness?

To date, most people infected with the West Nile virus will not have any type of illness. Twenty per cent of people infected with West Nile virus develop West Nile fever; a mild form of the disease. The symptoms last about six days, recovery is rapid and the illness poses no long-term health risk.

The severe form of the disease, West Nile encephalitis/meningoencephalitis occurs in one in 150 infected people (especially those over 50 years old) according to past data. New reports suggest that this risk may not reflect the North American experience. Small studies suggest the risk of prolonged or permanent severe and long-lasting nervous system damage and muscle weakness is greater than one in 150. Forty per cent of people with the severe form of the disease had muscle weakness in reports out of New York.

To date, the risk of contracting the illness remains low according to the Institute for Clinical and Evaluative Sciences (www.ices.on.ca). In regions where the virus resides, few mosquitoes are infected. The risk of severe infection from one mosquito is small occurring in less than one per cent of bitten people. It is unclear why some people develop severe disease. Advanced age is the most important predictor of death and patients older than 70 years are at particularly high risk.

Person-to-person transmission does not occur with West Nile virus. Kissing or touching another person will not place you at risk of disease. Indeed, there is no evidence that you can directly contract the disease from birds, horses or other mammals.

Prevention is the only means of protecting the population at large. Liberal use of mosquito repellents on clothing or skin (depending upon the concentration of the solution), patio screens, mesh-covered tents, protective clothing and avoidance of mosquito-infested areas and draining stagnant water sources are about all we can do for now.

If we do not give a second thought about getting into our car, we should not let the West Nile “threat” alter our day-to-day lives. Death and injury from car crashes far exceeds the damage from West Nile virus. Indeed, our behaviour regarding health prevention is less than optimal. Last week, a report revealed the reticence of most Canadians to implement lifestyle changes to prevent and reduce the risk of heart disease. A woman’s lifetime risk of dying from heart disease is one in two.

Every new West Nile infection will be front-page news and mercilessly skew the perception of true risk. It will require a few seasons to determine North American risk and the behaviour of West Nile virus. As unpalatable as this may be, this is RealTV.

Send a Comment