West Nile Virus
DefinitionWest Nile virus is transmitted by mosquitos and causes an illness that ranges from mild to severe. Mild, flu-like illness is often called West Nile fever. More severe forms of disease, which can be life-threatening, may be called West Nile encephalitis or West Nile meningitis, depending on where it spreads.
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Overview, Causes, & Risk Factors
West Nile virus is a type of organism called a flavivirus and is similar to many other mosquito-borne viruses, including Japanese encephalitis (which is found in Asia). Researchers believe the virus is spread when a mosquito bites an infected bird and then bites a person.
West Nile virus was first identified in 1937 in the West Nile region of Uganda, in eastern Africa. It was first identified in the US in the summer of 1999 in the Queens borough of New York, NY. It caused 62 cases of encephalitis and 7 deaths that summer. Since 1999 the virus has spread throughout the continental US and as of September 2002 has been identified in 42 states.
Mosquitos carry the highest amounts of virus in the early fall, thus there is a peak of disease in late August-early September. The risk of disease then decreases as the weather becomes colder and mosquitos die off.
Although many people are bitten by mosquitos that carry West Nile virus, most do not know they've been exposed. Few people develop severe disease or even notice any symptoms at all.
Data from the outbreak in Queens suggests that although 2.6% of the population was infected, only 1 in 5 infected people developed mild illness, and only 1 in 150 infected people developed brain inflammation (meningitis or encephalitis).
Risk factors for developing a worse form of the disease likely include the following:
- Conditions that suppress the immune system
- Pregnancy
- Older age
West Nile virus may also be spread through blood transfusions and organ transplantation. It is possible for an infected mother to transmit the virus to her child via breast milk.
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