West Nile virus is a potentially serious but preventable viral illness most often spread by the bite of an infected mosquito. Mosquitoes become infected when they feed on infected birds. The virus is not spread from person-to-person.

On average, about 10 cases in people per year are reported in Clark County; this includes both the neuroinvasive form of illness and the less severe form, the non-neuroinvasive West Nile fever.

No symptoms in most people: About four in five people infected with West Nile virus will not show any symptoms at all.

Mild symptoms in some people: About one in five people infected will display symptoms similar to the flu that may include:

• Fever
• Headache
• Body aches
• Nausea
• Vomiting
• Swollen lymph glands
• Skin rash on the chest, stomach and back

Symptoms may only last a few days, though even healthy people have been sick for several weeks. Fatigue and weakness can last for weeks or months.

Serious symptoms in a few people: Less than one percent of people infected with West Nile virus will develop severe illness.

• People who are over 50 years old are most at risk for becoming ill with the virus. About 74 percent of Clark County human cases since 2004 have been in people over 50.
• People who are immunocompromised due to age or other conditions are especially vulnerable to severe illness.
• About 10 percent of people who develop neurologic infection due to West Nile virus will die.

The severe symptoms may include:

• High fever
• Headache
• Neck stiffness
• Stupor
• Disorientation
• Coma
• Tremors
• Convulsions
• Muscle weakness
• Vision loss
• Numbness
• Paralysis

Incubation Period

People typically develop symptoms between two to 14 days after they are bitten by an infected mosquito.

Vaccine

Currently, there is no West Nile virus vaccine available for people.

Treatment

There are no medications to treat or prevent West Nile virus infection and there is no specific treatment. Mild symptoms such as fever and aches usually resolve on their own. In more severe cases, people usually need to go to the hospital where they can receive supportive treatment including intravenous fluids, help with breathing and nursing care.

People with mild symptoms do not necessarily need to seek medical attention for this infection. If more severe symptoms develop, such as unusually severe headaches or confusion, seek medical attention immediately. Severe West Nile virus illness usually requires hospitalization.

West Nile Virus Diagnosis

West Nile virus is diagnosed by laboratory confirmation of a blood sample.