West Nile Virus


West Nile virus is a virus that is typically transmitted by mosquitoes. It first appeared in the US in 1999, but has commonly been found in Africa, Asia, the Middle East, and Europe. Most infections with this virus cause no illness at all. However, about 1/5 of people infected with the virus suffer flu -like symptoms such as fever, nausea, vomiting, headache, and fatigue. About 1/150 people infected with the virus develop neurologic symptoms. These symptoms include the following:

  • Encephalitis—inflammation of the brain
  • Meningitis—inflammation of the membrane covering the brain and spinal cord
  • Poliomyelitis—paralysis combined with fever and meningitis


Swollen brain

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West Nile infection is caused by a virus. Most cases occur after a bite from an infected mosquito. The mosquito picks up the disease from biting an infected bird. It then passes the virus on when it bites a person, horse, dog, or some other animal. An increase in dead birds may signal an increased risk for the transmission of this virus.

Case reports indicate that West Nile virus can also be transmitted through blood transfusions or organ transplants. Because infected donors may not have any symptoms, tests to screen blood for this virus may be used.

Another case showed that a woman passed West Nile virus to her baby through breast milk. Experts are studying this possible route of transmission. However, due to the many well-established health benefits of breastfeeding, the Centers for Disease Control (CDC) has not changed their breastfeeding recommendations. Breastfeeding women who feel ill or suspect that they have West Nile should talk with their doctors.

Risk Factors

A risk factor is something that increases your chance of getting a disease or condition.

Being bitten by an infected mosquito poses the greatest threat. A small risk is associated with the following:

  • Receiving a blood transfusion
  • Receiving an organ transplant

Risk factors for a more severe case of the disease include:

Mosquito Bite—Greatest Risk Factor for West Nile Virus

Mosquito bite

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Most people who become infected with West Nile virus experience no symptoms. About 20% develop a mild condition called West Nile fever. It lasts about 3-6 days. One in 150 people develop a serious neurologic disease. It may last for weeks. Some effects, such as fatigue, memory loss, difficulty walking, or muscle weakness may be permanent. About 12% of hospitalized patients do not survive.

The majority of cases develop in late summer and early fall.

Seek medical care immediately if you develop any symptoms.

Symptoms of West Nile fever include the following:

  • Fever
  • Generally not feeling well
  • Lack of appetite
  • Nausea
  • Vomiting
  • Headache
  • Body aches
  • Eye pain
  • Muscle pain
  • Swollen lymph nodes
  • Rash

Symptoms of serious neurologic disease include the following:

  • High fever
  • Stiff neck
  • A change in mental status
  • Confusion or disorientation
  • Stupor
  • Severe muscle weakness
  • Paralysis


The doctor will ask about your symptoms and medical history, and perform a physical exam. He or she may inquire about:

  • Recent mosquito bites
  • Outdoor activities
  • Use of insect repellent
  • Travel to areas where West Nile is present

Some symptoms of this disease may be due to other conditions. Tests help determine the cause of the symptoms. Tests may include those listed here:

  • Blood tests—for antibodies to the virus and to check for abnormalities associated with West Nile infection
  • Lumbar puncture (spinal tap)—removal of a small amount of fluid from the spinal column to check for signs of infection
  • MRI scan of the head—a test that uses magnetic waves to make pictures of the inside of the head; in this case to look for abnormal areas in the brain
  • Electroencephalogram (EEG)—a test that records the brain's activity by measuring electrical currents through the brain
  • Electromyography and nerve conduction studies—a test in which electrical current is measured in a muscle or passed through a nerve to determine the condition of that nerve and to determine the reason for muscle weakness


No definitive treatment exists for West Nile infection. The treatment given is supportive. For severe cases, it may include a machine to help with breathing. Care includes intravenous fluids and preventing other infections. Two drugs are being studied to see if they can shorten the length of symptoms or decrease the disease’s severity. These drugs are alpha-interferon and ribavirin.


The best preventive measure is to avoid mosquito bites. Tips to do so include the following:

  • Avoid going outdoors at dawn or dusk.
  • Wear long pants and long-sleeve shirts when outdoors.
  • Use an insect repellent with the chemical DEET (N,N-diethyl-3-methylbenzamide).
  • Repair screens to prevent mosquitoes from entering the house.
  • Remove standing water, such as birdbaths, to prevent mosquito breeding.
  • Clean clogged gutters to remove pooled water.

Other prevention tips include the following:

  • Donate your own blood before elective surgery.
  • Do not donate blood if you are feeling ill or have a fever.
  • Do not touch dead birds unless you are wearing disposable gloves.
  • Notify the public health department if you find a dead bird.

Researchers are developing a vaccine for people at high risk.


Centers for Disease Control and Prevention

National Institute of Allergy and Infectious Diseases

US Food and Drug Administration


BC Health Guide

Health Canada


Centers for Disease Control and Prevention website. Available at: http://www.cdc.gov .

Glass JD, Samuels O, Rich MM. Poliomyelitis due to West Nile virus [letter]. N Engl J Med . 2002;347:1280-1281.

Petersen LR, Marfin AA. West Nile virus: a primer for the clinician. Ann Intern Med . 2002;137:173-179.

Petersen LR, Roehrig JT, Hughes JM. West Nile virus encephalitis. N Engl J Med . 2002;347:1225-1226.

Last reviewed March 2008 by David Horn, MD, FACP

Please be aware that this information is provided to supplement the care provided by your physician. It is neither intended nor implied to be a substitute for professional medical advice. CALL YOUR HEALTHCARE PROVIDER IMMEDIATELY IF YOU THINK YOU MAY HAVE A MEDICAL EMERGENCY. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with any questions you may have regarding a medical condition.

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