The West Nile virus has appeared in at least 4 provinces this year and is likely to continue spreading, an expert at the Canadian Science Centre for Human and Animal Health says. Mike Drebot, head of Health Canada's Viral Zoonoses Section, also says that the presence of the virus in the southern US means it can now continue spreading year round. In the north it cannot spread in winter because there are no mosquitoes. As of mid-August, Health Canada (www .hc-sc.gc.ca/pphb-dgspsp/wnv-vwn /index.html) had reported no confirmed human cases in Canada, but nearly 200 birds had tested positive for the virus in Quebec, Ontario, Manitoba and Saskatchewan.
Figure. Photo by: CDC
South of the border, the US Centers for Disease Control and Prevention (www.cdc.gov/ncidod/dvbid/westnile /surv&control.htm) had reported (as of Aug. 6) 114 confirmed human cases of West Nile fever in 2002, and 5 deaths. In 2001, 50 Americans contracted the virus and 5 died; in 2000, 21 cases were reported and 2 of the patients died. The virus has now been detected in 34 states and surveillance programs have been initiated in 49 states, 5 cities and the District of Columbia.
West Nile virus (CMAJ 2000;162 [7]: 1036) is spread by mosquitoes that have fed on infected birds. The virus was first detected in Canada in 2001, when its presence was confirmed in 128 dead birds found in southern Ontario.
This year, Health Canada broadened its surveillance. As of mid-August, 1800 birds from across Canada had been tested. Monitoring birds — mostly members of the crow family — is the most effective means of surveillance, says Drebot. The birds usually die within a few days of contracting the virus.
West Nile virus is “clearly emerging and spreading,” says Drebot, but the risk to humans is low.
Less than 1% of people who become infected develop severe illnesses. Most people show no ill effects or experience only mild flu-like symptoms, such as fever and headache. However, in some immunocompromised people West Nile virus can cause potentially fatal encephalitis. — Barbara Sibbald, CMAJ