West Nile Virus (WNV)

People in North America may be experiencing the virus differently because the strain here may be more virulent than the predominant strain in Africa.

How is it spread?
The virus is normally spread by birds and the mosquitoes that bite birds - crows seem to be affected the most. Humans get WNV from a bite from an infected Culex tarsalis mosquito. Risk peaks in hot, dry weather from mid-July and September. It is not contagious, but the virus can be transmitted via blood transfusions, breast milk and organ transplants. There is no evidence that you can get infected from handling infected animals, but you probably should wear gloves to handle suspected animals. An increase in dead crows appears to predict an increase in infection rates in mosquitoes, followed by an increase in human infections.

Symptoms of WNV:
Mild cases show flu-like symptoms, including fever, headache and body aches. Some people may also develop a mild rash, or swollen lymph glands. The virus can cause the more serious West Nile neurological syndrome, which can result in weakness, paralysis, confusion, coma, and rarely death. It takes 3 to 6 days to show signs of the disease, and some people have no symptoms. There is no treatment for the virus, so prevention is important.

Children appear to have the lowest risk; older people the highest. Most people recover fully from the infection, with only 1% risk of death. In another 1% of cases, West Nile virus can cause a more serious illness called West Nile neurological syndrome, which can have serious complications, including weakness, muscle degeneration, brain damage, paralysis, coma or death. Only 25% of previously healthy people admitted to hospital with the virus (2002) were able to live independantly upon release (Canadian Medical Association Journal 2002). Some patients were gravely ill with West Nile, and others had symptoms so mild they didn't know they had it. It's possible the virus strain in North America may be more virulent than the strain in Africa.

Only 20% (1 in 5) of people who become infected actually show symptoms. These are:

Species at risk:
Humans, mosquitoes, birds (especially crows), dogs, cats, squirrels, horses, sheep, cattle, apes, wolves, reindeer and mountain goats. Dogs and cats get WNV but without extensive illness or even symptoms. Only humans and horses are at risk for encephalitis, which can be fatal.

How can it be treated?
There is no known treatment. Full recovery is the norm.

How can it be prevented?
Protect yourself from being bitten by mosquitoes! Use repellants and wear clothing that covers you. Try to eliminate standing water (5-7 days old) that mosquitoes can breed in - bird baths, wading pools, eavestrough, old tires, patio furniture, pool covers. (Information about mosquitoes)

Spraying or not?
Experts had said that spraying mosquitoes with malathion would harm more people than the virus. There is evidence this chemical is not effective in the long term and causes a variety of health problems such as difficulty breathing in humans and hormone disruption and malformed limbs in frogs. Although the links cannot be proven, more than 90% (11 million) of the adult lobsters in Long Island Sound mysteriously died not long after spraying was done. Some Larvicides such as Bti are considered safer but are very expensive. Municipalities are unwisely rushing to kill larvae or mosquitoes to try to keep voters happy.

WNV is a reportable disease in Ontario. Canadian Blood Services can test blood donations for the first time; 14 rejected donations in 2003 probably saved many lives. Some municipalities sprayed in spite of warnings about the effects of chemicals and larvicides on health. (mosquitoes, repellants, diseases spread by mosquitoes)

Time Line:

1937: West Nile virus (WNV), a flavivirus, was first isolated in Uganda. The disease is common in parts of Africa, Europe and Asia.

1999: WNV was diagnosed in North America in New York, infecting 62 (55 in New York City). 7 died - including a Canadian tourist. All the victims were over 68 years of age. The virus was found in a single dead cat.

2000: Only 21 were infected in New York, Connecticut and New Jersey; 2 died. Canada still had no cases, but was using a system of 600 "sentinel chickens" near the U.S. border, since chickens don't die of the virus, but develop antibodies that can be tested. By autumn WNV had been identified in a dead bird in southern Ontario.

2001: WNV survived the mild winter and arrived in Canada via migratory birds such as crows (dead birds near the border tested positive for the virus). 19 cases were confirmed with 2 deaths. Tests confirmed WNV in 9 mosquito pools and 128 dead birds, mostly in Ontario. In the U.S. 66 were infected, 9 died along the east coast.

2002: WNV spread with 2,000 seeking treatment, 414 cases cases and 20 deaths. Ontario had 307 confirmed (1 travel related), 85 probable cases, and 17 deaths (3 directly attributed to the virus). In Toronto, about 1,000 sought treatment, with 292 confirmed cases, 64 severely ill, and 10 deaths. Quebec had 16 confirmed cases; Alberta had 2 (both travel related). 555 dead birds had confirmed WNV, 3 in Nova Scotia, 139 in Quebec, 88 in Manitoba, 44 in Saskatchewan, the rest in Ontario (none in PEI, NB, AB, BC). 315 mosquito pools tested positive.

The U.S. had 4,161 cases, with 277 deaths in 39 states and D.C - the worst mosquito-borne epidemic in U.S.history.

2003: 1,481 cases (852 probable 462 confirmed). The prairies were hardest hit with over 1,000 cases. Saskatchewan cases were up sharply with 947 infections with 7 deaths. Manitoba and Ontario both had 2 deaths. Cases in Ontario and Quebec were down. Nova Scotia, New Brunswick, British Columbia and the Yukon reported human cases, but it was believed they were contracted during travel.

In 2004-2005 there was little West Nile virus activity. Experts disagree about the reason, but possibly weather conditions or humans becoming immune. In 2006: Manitoba: 50, Ontario: 42, Alberta: 39, Saskatchewan: 19, Quebec: 1. There were 2 deaths. (The U.S. had 4,269 cases, 177 deaths.) In 2007, Canada had the most reported cases: 2,215 with Saskatchewan 1,285 (4 deaths), Manitoba 542 (3 deaths), Alberta 93, Ontario 2.

Reported clinical cases in Canada
2002: 414   ? 20 deaths (U.S. 284 deaths)
2003: 1,481  ? 11 deaths
2004: 25
2005: 225
2006: 151   ? 2 deaths (U.S. 177 deaths)
2007: 2,215  ? 7 deaths
2008: 36
2009: 13
2010: 5
2011: 101
2012: 428   ? (U.S. 286 deaths)
Public Health Agency of Canada (PHAC)