West Nile Virus

West Nile virus (WNV) is a virus carried by mosquitoes. Most people infected with the virus do not have symptoms or have only a mild illness. Less than 1 in 100 people who are infected with the virus develop serious illness. Serious forms of illness caused by WNV include encephalitis (inflammation of the brain) or meningitis (inflammation of the membranes that cover the brain and spinal cord).

Wild and domestic birds, mainly crows, carry the West Nile virus. Mosquitoes become carriers of the virus when they bite infected birds. Humans can get the virus when they are bitten by infected mosquitoes. There are no known cases of a human getting WNV from an infected bird, only from the bite of an infected mosquito.

The risk of West Nile virus is seasonal in the northern states of the US and begins in the spring. The peak time for infection is mid to late August. In milder southern climates, the infection may occur year-round. The risk of severe infection is greatest for people who are over 70 years old or who have a weakened immune system.

West Nile virus may be spread from person to person through blood transfusions and organ transplants. The virus might also be transmitted through breast milk. However, the risk of transmission of the virus to the baby is believed to be very low. If you are breast-feeding and you have a WNV infection, the American Academy of Pediatrics and the CDC recommend that you continue breast-feeding.

The infection is not spread by normal person-to-person contact like touching, kissing, or caring for someone who is infected.

About 4 out of 5 infected people have no symptoms. Children are more likely to have symptoms. When symptoms do occur, they are usually mild and last a few days. Symptoms of WNV infection may include:

• fever
• headache
• muscle aches
• backache
• skin rash
• swollen lymph glands.

A WNV infection usually does not involve the brain. However, a few infected people (and even fewer children) develop encephalitis or meningitis. Symptoms of these less common but severe illnesses include:

• tremors
• stiff neck
• severe headache
• paralysis
• muscle weakness
• confusion
• convulsions
• coma.

Some people develop a poliolike syndrome with sudden weakness and paralysis.

Symptoms usually appear 2 to 15 days after you were bitten by an infected mosquito.

Your health care provider will ask about your symptoms and examine you. Tests you may have are:

• blood tests
• spinal tap (lumbar puncture), a procedure in which a needle is inserted between two bones of the spine into the spinal canal to take a sample of spinal fluid to test for meningitis
• electroencephalogram (EEG), a test that measures the electrical activity of the brain (brain waves)
• computerized tomography (CT) or magnetic resonance imaging (MRI) scans.

There is no specific medicine to treat West Nile virus. If you have a serious infection, you may need to stay at the hospital. You may be given intravenous (IV) fluids and pain relievers. For severe or life-threatening infection, you may need treatment in an intensive care unit.

Most people infected with WNV, including nearly all children, do not get seriously ill, and they recover fully.

If you have a serious infection, you may be ill for weeks. You may have some injury to the nervous system and brain. The injury is sometimes permanent.

If you get West Nile virus, you will probably be immune to future infection by the virus, but your immunity might decrease over time.

• Rest.
• Take acetaminophen (Tylenol) or an anti-inflammatory medicine such as ibuprofen if you are having fever, headache, or muscle aches.
• Contact your health care provider right away if you develop:
  • a stiff neck
  • a headache that is getting worse
  • a fever of 102°F (39°C) or higher that does not go down with medicine
  • seizures (convulsions)
  • slurred speech
  • confusion
  • paralysis (inability to use an arm or leg).

If you are older or live alone, you may need someone to be checking on you often to make sure your symptoms are not getting worse in ways you may not realize, such as confusion and coma.

Yes, WNV can be prevented. Take precautions to avoid exposure to mosquitoes:

• Stay indoors at dawn, dusk, and in the early evening, when mosquitoes are most likely to be around.
• Wear long-sleeved shirts and long pants when outdoors.
• Use an insect repellent on skin that is not covered by clothing whenever you are outdoors. Don't use more repellent than recommended in the package directions. Don't put repellent on open wounds or rashes. Do not apply it to your eyes or mouth. When using sprays, do not spray it directly on your face--spray the repellent on your hands first and then put it on your face. Wash the spray off your hands. Be careful with children because repellents can make them ill.

  Repellent products containing either DEET or picaridin as active ingredients have been proven to provide longer-lasting protection than others. Oil of lemon eucalyptus, a plant-based repellent, hasn't been as well tested. But in some studies it provided as much protection as repellents with low concentrations of DEET. Oil of lemon eucalyptus should not be used on children under age 3.

  • Adults should use products with no more than 35% DEET. DEET should be washed off your body when you go back indoors.
  • Picaridin is a repellent just recently made available in the US. It can be less irritating to the skin than DEET.
• Spray clothes with repellents because mosquitoes may bite through thin clothing. Do not spray repellent on the skin under clothing. You may spray clothing with DEET or permethrin. Some products containing permethrin are recommended for use on clothing, shoes, bed nets, and camping gear. Permethrin is highly effective as an insecticide and as a repellent. Permethrin-treated clothing continues to work after repeated laundering. The permethrin insecticide should be reapplied according to the label instructions. Some commercial products are available pretreated with permethrin. Do not put permethrin on your skin.
• Install or repair window and door screens so it is harder for mosquitoes to get indoors.
• Mosquitoes lay eggs in water. To reduce mosquito breeding, drain standing water. Routinely empty water from flowerpots, pet bowls, clogged rain gutters, swimming pool covers, buckets, barrels, cans, and other items that collect water.

Note: Vitamin B and ultrasonic devices DO NOT help prevent mosquito bites.

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