West Nile virus is typically transmitted by mosquitoes. It first appeared in the US in 1999. It has been found most often 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 including:

  • Fever
  • Nausea
  • Vomiting
  • Headache
  • Fatigue

About 1/150 people infected with the virus develop neurologic symptoms including:

  • 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
© 2009 Nucleus Medical Art, Inc.



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.

West Nile virus may also be passed through blood transfusions]]> or organ transplants. Infected donors may not have any symptoms. Tests to screen blood for this virus may be used.

In one case, West Nile virus was passed through breast milk. Experts are studying this possible route. Breastfeeding women who feel ill or suspect that they have West Nile should talk with their doctors.


Risk Factors

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:

  • Age: over 50
  • Having a condition that weakens the immune system, such as diabetes]]> or ]]>HIV infection]]>

Mosquito Bite—Greatest Risk Factor for West Nile Virus

Mosquito bite
© 2009 Nucleus Medical Art, Inc.



Most people who become infected with West Nile virus have 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. A physical exam will be done. 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—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. Severe cases it may need a machine to help with breathing. Care includes IV 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:


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.