West Nile virus is a mosquito-borne virus that can cause disease in humans and some animals. The virus first appeared in United States in 1999 around New York City after being imported from overseas. Since then, the virus has spread throughout the continental United States, including Vermont.

West Nile virus has been has been found in all counties of Vermont and continues to be detected in mosquitoes each year. However, it is a relatively rare cause of illness in people.

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Most people infected with West Nile virus do not develop any symptoms.

About 1 in 5 people who are infected develop a fever with other symptoms such as headache, body aches, joint pains, vomiting, diarrhea, or rash. Symptoms can last for as little as a few days or as long as several weeks.

About 1 in 150 people who are infected develop a serious illness affecting the central nervous system such as encephalitis (inflammation of the brain) or meningitis (inflammation of the membranes that surround the brain and spinal cord). Symptoms of severe illness include headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, and paralysis.


There is no specific treatment for West Nile virus infection, but the symptoms can be treated.

With severe illness, hospitalization and treatment in an intensive care unit may be required.


The vast majority of infections come from the bite of an infected mosquito. Mosquitoes are infected when they feed on an infected bird. When an infected mosquito bites a person, the virus is injected into the person and may cause illness. Birds and other animals may be infected with West Nile virus, but there is no evidence that a person can get the virus from handling live or dead animals. However, be sure to wear gloves whenever handling a dead animal, including birds.

West Nile virus can also be spread by organ transplantation, blood transfusion, from a mother to fetus, and possibly by breast milk.

Most mosquitoes in Vermont are not infected with West Nile virus. But even if an infected mosquito does bite you, your chances of getting sick are low because most people who are infected do not have any symptoms.

Fewer than 1% of people who are infected develop a severe illness, like encephalitis or meningitis. People age 50 and older have the highest risk of getting severely ill if bitten by a mosquito infected with West Nile virus.

Preventing West Nile Virus

The best way to protect yourself is to prevent mosquito bites.

You can take steps to reduce mosquito breeding areas near your home and prevent mosquito bites.

Information for Health Care Professionals

West Nile virus infection is subclinical in about 80% of people.

Approximately 20% of infections result in West Nile fever, which is characterized by non-specific symptoms such as fever, headache and fatigue. Some people will develop a skin rash on the trunk, swollen lymph nodes or eye pain.

Recovery is usually complete. In about 1% of infections, neuroinvasive disease develops, and clinical syndromes ranging from febrile headache to aseptic meningitis to encephalitis may occur. This is most common in older patients. Symptoms may include fever, gastrointestinal symptoms, ataxia and extrapyramidal signs, optic neuritis, seizures, weakness, change in mental status, myelitis, and polyradiculitis. A minority of patients with severe disease develop a maculopapular or morbilliform rash involving the neck, trunk, arms or legs. A few patients develop flaccid paralysis.

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