West Nile Virus

West Nile virus is transmitted through the bite of an infected mosquito.

Marjorie Friesen, BScPharm, SPI, Drug and Poison Information Pharmacist, British Columbia Drug & Poison Information Centre

West Nile virus can cause disease in humans, birds and horses. In North America, West Nile virus was first reported in New York City in 1999. Before this, it was only known in parts of Asia, Europe and Africa. By the summer of 2001, infected birds were found in southern Ontario. The first human case in Canada occurred the following year. The virus has also been detected in other provinces.

West Nile virus is transmitted through the bite of an infected mosquito. Worldwide, more than 50 species of mosquitoes are known to carry the virus. Mosquitoes become infected by feeding on the blood of birds infected with the West Nile virus. Infected mosquitoes usually prefer to take their blood meal from birds and not humans. However, when conditions are right, usually in late summer, infected mosquitoes may also bite humans and other mammals. Crows, ravens, magpies and jays are particularly susceptible to the virus and often die when they are infected. Human infection from handling West Nile virus-infected birds (live or dead) has not been reported. Monitoring programs are in place to track the spread of West Nile virus.

The overall risk to humans of becoming ill from a mosquito bite is extremely small. Even in areas where mosquitoes carry the virus, less than 1% of mosquitoes are infected.

The majority of people infected with West Nile virus experience no symptoms at all. Some infected people develop West Nile fever, a mild illness with symptoms lasting only a few days. Symptoms include fever, headache, body aches and sometimes a rash and swollen lymph glands. Less than 1% of humans bitten by an infected mosquito will develop an inflammation of the brain or spinal cord (meningitis or encephalitis). Certain people, particularly the elderly, are at increased risk of serious health effects. Potentially serious symptoms include headache, high fever, neck stiffness, weakness, stupor or tremors. Anyone who experiences these symptoms should seek immediate medical attention. The incubation period (interval between infection and onset of disease) is usually between two to 14 days.

There is no specific treatment for West Nile virus infection and no human vaccine is available. People can protect themselves against West Nile virus by avoiding mosquito infested areas, eliminating stagnant water (in which mosquitoes breed) around the home, installing window screens, wearing protective clothing, and using a personal mosquito repellent. Be sure to check with your pharmacist for advice on mosquito repellents, especially for use in children.