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What is West Nile Virus?
West Nile virus (WNV) is a mosquito-borne disease that can cause a potentially serious illness in humans. It is the leading cause of arboviral (arhropod; or carried by insects) disease in the United States.  Mosquitoes spread this virus after they feed on infected birds and then bite people, other birds, and animals.

WNV is not spread by routine person-to-person contact, and there is no evidence that people can get the virus by handling infected animals. There have been reports of WNV transmission through blood transfusions, transplanted organs, breastfeeding, through the placenta from mother to fetus, and laboratory work. 

Suspected human cases are tested in the Wyoming Department of Health's Public Health Laboratory. People with a mild infection (West Nile fever) may experience a wide range of symptoms including fever, headache, skin rash, body aches, fatigue, and swollen lymph glands. Some people with West Nile fever may also have nausea and vomiting. People with more severe infections (West Nile neuroinvasive disease, meningitis, encephalitis, or poliomyelitis) may experience high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, paralysis, and respiratory paralysis. If you have any of these symptoms, contact your health-care provider.

West Nile virus infection in humans occur primarily in the late summer or early fall, although the mosquito season is April through October. The majority of people who get infected with the virus have no illness, or at most, have an infection similar to a mild flu with fever, headache, and fatigue. The virus rarely affects the central nervous system and causes West Nile neuroinvasive disease (encephalitis, meningitis, or poliomyelitis).

However, West Nile neuroinvasive disease can occur. In fact, it is possible that people who develop encephalitis, meningitis, or poliomyelitis may have serious long-term health problems and some people never fully recover.  

What is West Nile encephalitis?
West Nile encephalitis is a brain infection caused by West Nile virus, a flavivirus previously only found in Africa, Eastern Europe, and West Asia. West Nile virus is closely related to St. Louis encephalitis virus, which is found in the United States, the Japanese Encephalitis virus from Asia, and Murray Valley encephalitis virus from Australia and New Guinea.

How can I get it?
The principal route of human WNV infection is through the bite of an infected mosquito. In 2002, additional WNV routes became apparent; however, this represented a very small proportion of cases. These routes include receiving transplanted organs and blood transfusions, trans-placental (mother to unborn child) and possibly breastfeeding transmission, and laboratory workers working with West Nile infected products.

Who is most at risk?
Anyone can get WNV. People who spend a great deal of time outside are at risk for developing WNV infection. People over 50 years of age have the highest risk of developing a severe illness because our bodies have a harder time fighting off disease as we age. People with compromised immune systems (for example, organ transplant recipients, cancer patients, individuals with HIV/AIDS or other illnesses) are also at increased risk.

What are the symptoms?
People with mild infections, West Nile fever, may experience fever, headache, body aches, skin rash, and swollen lymph glands. People with more severe infections, West Nile neuroinvasive disease, may experience high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, and convulsions. If you have any of these symptoms, contact your healthcare provider.

Are there long-term consequences?
While most infections are usually mild, symptoms of West Nile fever can last for up to 30 days. West Nile neuroinvasive disease is more serious and can result in significant long term illness, death, or serious brain damage. Symptoms of West Nile neuroinvasive disease usually last for several weeks to months. The Centers for Disease Control and Prevention (CDC) notes that neurological effects may be permanent. Some improvements may be seen over time.

Is there treatment or a vaccine?
There is no specific treatment or vaccine for West Nile virus infection. While most people fully recover from the viral infection, hospitalization may be needed in some cases.

More information about West Nile virus is available from the CDC at