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. It was first detected in North America in 1999, in New York City, where 7 people died from it.
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 areas have developed aggressive strategies to tackle the problem, including surveillance programs to track the location and numbers of infected mosquitoes and birds.
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. The peak infection rates occur during July and August.
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.
West Nile virus has been found in at least 64 species of mosquitoes in the United States. Not all species are found in all parts of the country. In a given area, it is estimated that less than 1% of mosquitoes carry the West Nile virus. Scientists don't know for sure which mosquito species actually transmit West Nile to people through their bites. The uncertainty means that no one can tell health officials which mosquito larvae to kill in order to avoid a repeat of the previous summer.
West Nile virus belongs to a group of similar viruses, including others that are less severe, such as the viruses that cause dengue 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.