What is west nile virus?

West Nile virus is a mosquito-borne virus that can cause a range of symptoms of varying severity in humans. The virus is closely related to viruses that cause St. Louis encephalitis, the most common mosquito-borne encephalitis in the United States, and Japanese encephalitis.

Most people with the virus either don't have symptoms or have only a mild illness. However, the illnesses caused by West Nile virus may be serious and may include encephalitis — inflammation of the brain — or meningitis — inflammation of the membranes and fluid surrounding the brain and spinal cord. Less than 1 percent of people infected with the virus develop serious illness.

West Nile virus is relatively new to the United States, making its first known appearance in August 1999 during an outbreak in New York City. That outbreak first resulted in the death of a number of birds at the Bronx zoo. Later that year dozens of people in the New York City area became seriously ill, and seven people died.

Since 1999 the virus has spread widely in the United States. Health officials have detected it in more than 40 states, including all of the Eastern and Central states and in several Western states, including California.

The virus has spread in the United States along the migratory patterns of birds. Wild and domestic birds, mainly crows, are the primary reservoirs for the West Nile virus. Once birds bring the virus to new areas, mosquitoes bite the birds, and then the mosquitoes can spread the virus to humans.

West Nile virus was first detected in Uganda in 1937. Besides its recent appearance in the United States, it has since been found throughout Asia and the Middle East and in parts of Europe, Russia, India and Indonesia.

Health officials aren't sure how the virus got to the United States. It may have arrived through an infected bird that was imported to the United States.

Encephalitis Meningitis

Signs and symptoms

Common signs and symptoms of West Nile virus infection include:

• Fever
  
  
• Headache
  
  
• Muscle aches
  
  
• Backache
  
  
• Skin rash
  
  
• Swollen lymph glands

The infection usually doesn't involve the brain. However, if it does, signs and symptoms may also include:

• Severe headache
  
  
• Stiff neck
  
  
• Stupor
  
  
• Disorientation
  
  
• Coma
  
  
• Tremors
  
  
• Convulsions
  
  
• Muscle weakness
  
  
• Paralysis

Causes

When a mosquito bites a bird infected with the West Nile virus, the virus enters the mosquito's salivary glands and incubates there for 10 to 14 days. When the infected mosquito bites an animal or human, the virus is passed into the host's bloodstream, where it may cause illness.

After the West Nile virus enters the bloodstream, it multiplies and proceeds to the brain, crossing the blood-brain barrier — a barrier system that separates the blood from the central nervous system. Once the virus crosses that barrier and infects the brain's meningeal tissues, an inflammatory response occurs and symptoms arise.

West Nile virus transmission occurs mostly during warm weather, when mosquito populations are active. It takes from 5 to 15 days' incubation after you've been bitten by an infected mosquito before signs and symptoms of the illness appear.

Risk factors

The risk of West Nile virus is seasonal and begins in spring, with the peak time for infection in mid- to late August.

The risk of developing a serious West Nile virus-related illness after being bitten by an infected mosquito is less than 1 percent. In most cases people who become infected with the virus recover fully.

Anyone bitten by an infected mosquito is at risk of acquiring the virus, although the risk of severe infection is greatest for people who are over age 70 or who have a weakened immune system.

When to seek medical advice

See your doctor as soon as possible after you experience signs and symptoms that might suggest West Nile encephalitis. Although most people infected with the virus recover fully, the virus can result in serious, life-threatening illness.

Screening and diagnosis

Your doctor can confirm the presence of West Nile virus in your body by drawing a sample of your blood to be analyzed in the laboratory. If you have West Nile virus, an analysis of your blood sample may show:

• A rising level of an antibody to the West Nile virus
  
  
• A positive DNA test for the West Nile virus
  
  
• A positive culture of the West Nile virus

If your doctor suspects that West Nile virus has caused you to have meningitis or encephalitis, he or she may conduct the following diagnostic tests:

• Spinal tap (lumbar puncture). The most common way to diagnose meningitis is to analyze the cerebrospinal fluid surrounding your brain and spinal cord. A needle inserted between the lower vertebrae of your spine extracts a sample of fluid for laboratory analysis. Analysis of the fluid may reveal presence of an infection or an increased white blood cell count, a signal that your immune system is fighting an infection. Sensitive DNA methods may allow detection of the virus in the spinal fluid.
  
  
• Electroencephalography (EEG). This procedure, which takes about a half-hour, measures the waves of electrical activity produced by your brain. It's often used to diagnose and manage seizure disorders. A number of small electrodes may be attached to your scalp with paste or an elastic cap as you recline. You remain still during the test, but at times you may be asked to breathe deeply and steadily for several minutes or to stare at a patterned board. At times a light may be flashed in your eyes. These actions are meant to stimulate your brain. The electrodes pick up the electrical impulses from your brain and send them to the EEG machine, which records your brain waves on a moving sheet of paper.
  
  
• Brain imaging. A computerized tomography (CT) or magnetic resonance imaging (MRI) scan may reveal inflammation and swelling of your brain, which may occur in encephalitis.

Spinal tap (lumbar puncture) Computerized tomography MRI

Treatment

Most people who become infected with West Nile virus don't become seriously ill and do recover fully.

There's no specific antiviral treatment against West Nile virus. If you develop a serious infection, you may need to be hospitalized. There, your doctor may have you receive therapy that includes intravenous fluids and pain relievers. For more severe or life-threatening infection, you may need additional treatment in an intensive care unit. These additional treatments may include airway management with respiratory support (a ventilator). Doctors can also treat your specific symptoms and help prevent you from developing secondary infections such as pneumonia.

Prevention

The Centers for Disease Control and Prevention has been working with the Department of Agriculture, the Association of Public Health Laboratories as well as other federal and state agencies to prevent future outbreaks of West Nile virus. Even in states where the virus hasn't yet been observed, state public health departments are taking measures to detect the earliest cases of an outbreak.

Efforts to detect and contain the virus include:

• Sampling mosquito and bird populations for West Nile virus
  
  
• Increasing surveillance of animals and humans for infection
  
  
• Eliminating mosquito-breeding areas
  
  
• Increasing physician awareness and reporting of the virus so that its spread may be tracked
  
  
• Conducting public awareness campaigns to alert people as to how to reduce their risk of exposure to the virus

Eliminating mosquito-breeding sites is an effective form of early-stage control that prevents widespread pesticide use later on and is also a control measure everyone can use.

If West Nile virus activity is detected, chemicals called larvicides, approved by the Environmental Protection Agency (EPA), may be used to kill mosquito larvae before they develop into adult mosquitoes. In addition, EPA-approved adulticides can be used to kill adult mosquitoes in a more focused and limited manner.

Your best bet to prevent acquiring West Nile virus and other mosquito-borne illnesses is to take precautions to avoid exposure to mosquitoes. Steps you can take to help control West Nile virus include the following:

• Eliminate standing water in your yard. Mosquitoes breed and multiply in standing water.
  
  
• Unclog roof gutters.
  
  
• Empty unused swimming pools.
  
  
• Change water in birdbaths at least weekly.
  
  
• Remove old tires or any unused containers that might hold water and serve as a breeding ground for mosquitoes.
  
  
• Keep an eye out for sick or dying birds and report them to your local health department.

To reduce your exposure to mosquitoes:

• Avoid unnecessary outdoor activity when mosquitoes are most prevalent, such as dawn, dusk and early evening.
  
  
• Wear long-sleeved shirts and long pants when going into mosquito-infested areas.
  
  
• Apply an insect repellent that contains 20 percent to 30 percent DEET. To minimize the risk of adverse reactions, apply it sparingly and avoid concentrations of more than 30 percent. Don't apply this type of insect repellent to children less than 3 years old. Be sure to follow the manufacturer's directions for use.
  
  
• Spray clothing with insect repellent.

A vaccine is available to protect horses from West Nile Virus. No vaccine is available for humans, but several companies are working to develop a human vaccine.

— September 10, 2002 —

Brain & Nervous System Center