Health Canada “nearly blindsided” by West Nile virus incidence
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* Laura Aiken

The number of confirmed human cases of infection with West Nile virus (WNV) started escalating rapidly once Health Canada began plowing through the immense backlog of probable cases at its National Laboratory in Winnipeg last year. “We were surprised by the large number of human cases … nearly blindsided,” said Harvey Artsob, Health Canada's chief of zoonotic diseases.

Dr. Colin D'Cunha, Ontario's chief medical officer, was surprised too. On Feb. 19 he reported that WNV had claimed the lives of at least 8 Ontario residents in 2002. The previous Health Canada estimate for all of Canada was only 2 deaths.

No human cases were reported in Canada until 2002. By January, Health Canada said it had identified 84 confirmed and 114 probable cases. However, Ontario and Quebec, which used their own laboratories, started reporting additional cases. In January, D'Cunha said the province had 130 confirmed cases of WNV infection and another 204 probable cases.

Artsob says the Health Canada laboratory is hard pressed to meet the demand for testing, and this explains why the number of confirmed diagnoses and deaths has continued to rise even when there are no mosquitoes around to spread WNV. Only about 20% of infected people eventually develop symptoms. The elderly and those with a weakened immune system are particularly vulnerable, but everyone is being advised to take precautions. “Most people aren't scared enough,” says Artsob.

The rate at which the virus is spreading has provincial governments and municipal health units preparing early for the 2003 mosquito season. D'Cunha thinks all provinces should follow Ontario's lead in making WNV infection reportable — he says this is the only way to track its true extent.

Municipal health units in Ontario are currently assessing high- and low-risk areas, and D'Cunha says some cities and towns will be putting larvicide in standing pools of water. Artsob says other provinces will likely adopt larvicide programs.

WNV has now spread to 5 provinces — Nova Scotia, Quebec, Ontario, Manitoba and Saskatchewan — and to all but 4 US states. As of Jan. 15, 2003, the US had reported 254 deaths and 3949 confirmed cases of West Nile fever. —* Laura Aiken*, CMAJ