West Nile Virus Information

West Nile Virus Information

The sources for the information in this document are the Florida Department of Health and the Centers for Disease Control and Prevention. For additional information about West Nile virus, please visit http://www.cdc.gov/westnile/faq/genquestions.html.

What is West Nile virus?

West Nile virus is an arthropod-borne virus (arbovirus) most commonly spread by infected mosquitoes.  West Nile virus can cause febrile illness, encephalitis (inflammation of the brain) or meningitis (inflammation of the lining of the brain and spinal cord).

West Nile virus transmission has been documented in Europe and the Middle East, Africa, India, parts of Asia, and Australia.  It was first detected in North America in 1999, and has since spread across the continental United States and Canada.

How is West Nile virus transmitted?

Most people get infected with West Nile virus by the bite of an infected mosquito.  Mosquitoes become infected when they feed on infected birds.  Infected mosquitoes can then spread the virus to humans and other animals.

In a very small number of cases, West Nile virus has been spread through blood transfusions, organ transplants, and from mother to baby during pregnancy, delivery, or breastfeeding.

What are the incubation period and symptoms of West Nile virus?

The incubation period is usually 2 to 6 days, but ranges from 2 to 14 days.  This period can be longer in people with certain medical conditions that affect the immune system.

Most people (70-80%) who become infected with West Nile virus do not develop any symptoms.

About 1 in 5 people who are infected will develop a fever with other symptoms such as headache, body aches, joint pains, vomiting, diarrhea, or rash.  Most people with this type of West Nile virus disease recover completely, but fatigue and weakness can last for weeks or months.

Less than 1% of people who are infected will develop a serious neurologic illness such as encephalitis or meningitis (inflammation of the brain or surrounding tissues).  The symptoms of neurologic illness can include headache, high fever, neck stiffness, disorientation, coma, tremors, seizures, or paralysis.

Serious illness can occur in people of any age. However, people over 60 years of age are at the greatest risk for severe disease.  People with certain medical conditions, such as cancer, diabetes, hypertension, kidney disease, and people who have received organ transplants, are also at greater risk for serious illness.

Recovery from severe disease may take several weeks or months.  Some of the neurologic effects may be permanent.  About 10% of people who develop neurologic infection due to West Nile virus will die.

What is the incidence for West Nile virus?

West Nile virus disease cases have been reported from all 48 lower states.  The only states that have not reported cases are Alaska and Hawaii. 

Seasonal outbreaks often occur in local areas that can vary from year to year.  The weather, numbers of birds that maintain the virus, numbers of mosquitoes that spread the virus, and human behavior are all factors that can influence when and where outbreaks occur.  Most people are infected from June through September.

What is the diagnosis and treatment for West Nile virus?

Diagnosis is based on a combination of clinical signs and symptoms and specialized laboratory tests of blood or spinal fluid.  These tests typically detect antibodies that the immune system makes against the viral infection.

There are no medications to treat or vaccines to prevent West Nile virus infection.  Over-the-counter pain relievers can be used to reduce fever and relieve some symptoms.

People with milder symptoms typically recover on their own, although some symptoms may last for several weeks.  In more severe cases, patients often need to be hospitalized to receive supportive treatment, such as intravenous fluids, pain medication, and nursing care.

How can people reduce the chance of getting infected?

The most effective way to avoid West Nile virus disease is to prevent mosquito bites:

  • Use insect repellents when you go outdoors.  Repellents containing DEET, picaridin, IR3535, and some oil of lemon eucalyptus and para-menthane-diol products provide longer-lasting protection.
  • Wear long sleeves and pants from dusk through dawn when many mosquitoes are most active.
  • Install or repair screens on windows and doors.  If you have it, use your air conditioning.
  • Help reduce the number of mosquitoes around your home.  Empty standing water from containers such as flowerpots, gutters, buckets, pool covers, pet water dishes, discarded tires, and birdbaths.