West Nile Virus

West Nile Virus

West Nile Virus, a member of the flavivirus family, is related to the viruses that cause dengue and yellow fevers. The effects of infection with West Nile virus range from no symptoms to severe illness and even death. The virus has been known for several decades, but only recently arrived in North America.

West Nile virus is spread by certain species of mosquito that obtain the virus by feeding on the blood of infected birds, major carriers of the virus in North America. The mosquitoes can then pass the virus on to humans and other animals through bites. Although people are most likely to become infected by West Nile virus through mosquito bites, the virus can also be spread through blood transfusions, organ transplants, breast milk, and from a pregnant woman to her unborn child. Infection with West Nile virus can be confirmed through blood tests that identify the presence of viral antibodies.

When infected by West Nile virus, most people do not develop symptoms, or have only mild symptoms that last for 3-6 days. Symptoms in mild cases include fever, headache, and muscular aches. Some people may also experience a mild rash or swollen lymph glands. In cases of severe illness, West Nile virus can be fatal. Symptoms in severe cases include the sudden onset of headache, high fever, stiff neck, nausea, drowsiness, confusion, loss of consciousness, lack of coordination, muscle weakness, and paralysis. People with severe cases of West Nile virus may develop meningitis (inflammation of the lining around the brain or spinal cord), encephalitis (inflammation of the brain), or acute flaccid paralysis (loss of function in one or more limbs). People with chronic diseases and weakened immune systems are at greatest risk for developing serious health effects as a result of West Nile virus infection.

There is no treatment for West Nile virus itself, but infected people receive treatment to alleviate symptoms. Some people who become seriously ill with West Nile virus make a complete recovery, while others continue to experience health problems. Scientists don't yet understand the reasons for these differences. Developmental vaccine research has been underway for several years in the United States with one candidate vaccine potentially ready for human trials in early 2005.

West Nile virus is named for the West Nile region of Uganda, where it was first identified in 1937. Since then, it has been found elsewhere in Africa, and in parts of Europe, Asia, and North America. The first record in North America was in New York City in 1999. It is unclear how the virus was transported to North America. Each year, the virus moves further west and north, with its first appearance in Canada reported in birds and mosquitoes in southern Ontario in 2001. In 2002, birds, horses, or mosquitoes in Nova Scotia, Québec, Ontario, Manitoba, and Saskatchewan tested positive for the virus. In 2003, West Nile virus activity was also reported in New Brunswick and Alberta. In 2004, there was no indication that it had spread beyond these 7 provinces.

Canada's first human cases of West Nile virus occurred in Québec and Ontario in 2002. In 2003, over 1300 people in Canada had probable or confirmed cases of the virus and 14 died. In 2004, fewer than 50 cases were reported with no deaths.