West Nile Virus
(WNV · West Nile Encephalitis)
In this condition factsheet:
The Facts on West Nile Virus
West Nile virus is an illness that spreads from mosquitoes to humans. A mosquito becomes infected when it feeds on the blood of a bird that is carrying West Nile virus. About 2 weeks later, the mosquito is capable of spreading the virus to people and animals while biting for a blood meal. The virus is not spread from person to person, and cannot be spread directly from infected animals, such as birds, horses, or pets to people.
West Nile virus originated in the West Nile region of Uganda in 1937, and for decades it was confined to Africa, the Indian subcontinent, and parts of the Middle East and Europe. In 1999, it was detected in New York City, where 7 people died from it. Canada had its first confirmed infection in a bird in 2001. In 2002, the first confirmed human case of West Nile virus was reported in Ontario.
By 2003, West Nile virus had spread to 8 Canadian provinces and 1 territory: Nova Scotia, New Brunswick, Quebec, Ontario, Manitoba, Saskatchewan, Alberta, British Columbia, and the Yukon Territory. More than 1,400 Canadians were infected that year. Since then, there have been West Nile cases every year - the virus's continuing spread indicates that it is probably here to stay. Every year, the Public Health Agency of Canada (PHAC) publishes weekly reports during West Nile virus season on West Nile virus activity in Canada.
West Nile virus affects the central nervous system, and infection usually results in mild flu-like symptoms or no symptoms at all. However, in severe cases, infection with West Nile virus can be fatal. Affected provinces have developed aggressive strategies to tackle the problem, including surveillance programs to track the location and numbers of infected mosquitoes and birds.
Causes of West Nile Virus
Mosquitoes become infected with the virus and spread the disease by biting birds or humans. Sometimes the virus spreads from mosquitoes to horses and other animals. The virus is stored in the mosquito's salivary glands, and infected mosquitoes transmit West Nile virus to humans and animals while biting to take blood. In Canada, West Nile virus season starts as early as mid-April and lasts until October.
There is evidence that the virus can also be transmitted through blood transfusions and during organ transplants. However, the risk of transmission through these procedures is quite low. Blood donations are now tested for West Nile Virus and other infectious diseases. Evidence also suggests that West Nile can be transmitted from a pregnant woman to her unborn child as well as through breast milk.
West Nile virus belongs to a group of similar viruses, including others that are less severe, such as the viruses that cause dengue fever, yellow fever, and St. Louis encephalitis. This means that several other viruses can cause similar symptoms, and they must be ruled out before a diagnosis of West Nile virus can be confirmed.
Symptoms and Complications of West Nile Virus
Overall, most doctors believe that the risk of people becoming seriously ill with West Nile is extremely small. About 80% of those who contract the virus may not show symptoms at all. When infection does cause illness, symptoms vary from person to person and will usually appear within 2 days to 15 days.
Up to 20% of those infected may develop West Nile fever, which features mild flu-like symptoms. It is characterized by fever, headache, muscle aches, nausea, vomiting, and sometimes swollen lymph glands or a skin rash.
Health Canada and the US Centers for Disease Control and Prevention estimate that fewer than 1% of those infected – about one out of every 150 infected people – will develop severe symptoms, and fewer than that will experience life-threatening complications. People over 50, people with chronic health conditions (such as cancer, diabetes, alcoholism, or heart disease), and those with weakened immune systems are more likely to have serious health effects from West Nile virus infection.
Severe symptoms include high fever, headache, neck stiffness, vomiting, confusion, muscle weakness, tremors (shaking), and convulsions (seizures). Infection with West Nile Virus can result in encephalitis (an inflammation of the brain) or meningitis (an inflammation of the linings that cover the brain). West Nile virus can cause the brain or spinal cord to swell and block the flow of blood to the brain. This could lead to a coma, paralysis, or even death. The neurological effects may be permanent in some people.
Scientists do not know why some people recover quickly while others face long-term health problems. These problems may include:
- physical effects such as long-term muscle weakness and paralysis, fatigue, and headache
- confusion, depression, and problems with concentration and memory
- difficulties in performing daily tasks such as preparing meals and shopping
Even though the majority of people with mild flu-like symptoms may not need blood tests for West Nile, consult your doctor if you have the following symptoms:
- confusion
- convulsions or seizures
- extreme swelling or infection at the site of a mosquito bite
- fever
- muscle weakness
- severe headache
- stiff neck
- sudden sensitivity to light or an inability to perform routine tasks
