It's tempting to conclude that the worst is over, that precautionary measures and the vicissitudes of fate and nature have somehow combined to reduce the health threat posed by the West Nile virus (WNV).
But experts say the lower incidence as of Aug. 12, 2006 (Table 1) is hardly an indicator that WNV has been contained, or even a sign that the ultimate incidence will be lower because the WNV season peaks from mid-August through September.
In fact, there's a good possibility WNV rates in Manitoba and Saskatchewan will soon soar as a species of mosquito called Culex tarsalas, a vigorous biter that loves to breed in sun-dried prairie ditches, appears more prevalent this year and “its infectivity rate is up,” says Mike Drebot, head of Health Canada's Viral Zoonoses Section.
“The dry hot summer actually can be conducive for more generations of mosquitos and hence, a greater possibility that we're going to get infected mosquitos. And, also, the virus replicates within the mosquito at a higher rate when it's hotter.”
The vectors and variables that affect WNV rates are too poorly understood to safely say the threat is dissipating, adds Drebot. One variable is the fact that birds carrying the virus are becoming “more immune as a population,” Drebot says.
But speaking on behalf of the Association of Medical Microbiology and Infectious Disease Canada, Neil Rau says it also appears there's genetic transmission of West Nile within mosquito larvae, “a sort of over-wintering mechanism,” so the spread of the virus isn't limited by annual reintroduction from migratory birds.
“It's here to stay and one of the concerns about West Nile is that it does seem to infect numerous different species of mosquitos unlike the old St. Louis Encephalitis virus [which afflicted about 2500 people in North America in the mid 1970s]. This means that WNV has a much larger environmental niche in terms of mosquito species and a much greater geographic distribution,” adds Rau, the medical director of infection, prevention and control for Halton Health Care Services in Ontario. “It also seems very hardy, in that it can tolerate continental climates that are quite north.”
The experts concede this year's lower incidence may be partially attributable to mosquito abatement programs, particularly in large urban centres, or to public education campaigns urging people to cover up and use repellants when they venture outdoors.
“Certainly, if older people take those messages seriously, the more severe forms and manifestations of the fatal form of the disease are going to be seen less frequently,” Rau says, adding that physicians are now far more familiar with the virus and more likely to diagnose and treat milder indications, so there are fewer instances of progression to meningitis and encephalitis and death.
But rising human immunity doesn't appear to be a factor, Rau insists. Studies indicate only about 3% of the population remains immune.
The reality is Canadians must now consider mosquitos as “more than a nuisance,” Rau adds. “We never had to do this before 2002. So it's kind of an end of innocence for us.”