West Nile virus (WNV) is an infectious disease that first appeared in the United States in 1999. Infected mosquitoes spread the virus that causes it. People who get WNV usually have no symptoms or mild symptoms. The symptoms include a fever, headache, body aches, skin rash, and swollen lymph glands. They can last a few days to several weeks, and usually go away on their own.

If West Nile virus enters the brain, however, it can be life-threatening. It may cause inflammation of the brain, called encephalitis, or inflammation of the tissue that surrounds the brain and spinal cord, called meningitis. A physical exam, health history and laboratory tests can diagnose it.

Older people and those with weakened immune systems are most at risk. There are no specific vaccines or treatments for human WNV disease. The best way to avoid WNV is to prevent mosquito bites:

• Use insect repellent
• Get rid of mosquito breeding sites by emptying standing water from flower pots, buckets or barrels
• Stay indoors between dusk and dawn, when mosquitoes are most active
• Use screens on windows to keep mosquitoes out

West Nile virus (WNV) is a member of the flavivirus family, which includes other mosquito-borne viruses such dengue and Zika. WNV emerged for the first time in the Western Hemisphere in New York City in 1999 and has since spread across the United States. It is now the most common arthropod-borne virus found in the U.S.

Most people (about 8 out of 10) who are infected with WNV develop no symptoms at all, while a smaller proportion develop mild symptoms of fever, body aches, skin rash, and swollen lymph nodes. If the virus crosses the blood-brain barrier, however, it can cause life-threatening conditions that include inflammation of the brain and spinal cord. There are no specific therapies for serious WNV infection and there is currently no vaccine available to prevent infection.

Key points

• West Nile virus disease (West Nile) is caused by a virus primarily spread to people through the bite of infected mosquitoes.
• West Nile virus is maintained in the environment between mosquitoes and birds.
• People do not spread the infection other than rarely through blood transfusions, organ transplantation, and mother to baby.

Primary cause

West Nile virus disease is caused by a virus that is found in the United States and many other places in the world.

The virus belongs to a group of viruses called flaviviruses. St. Louis encephalitis virus and Powassan virus are two other flaviviruses that also circulate in the United States.

How it spreads

• West Nile virus circulates in the environment between mosquitoes (primarily Culex species) and birds.
• People become infected with the virus when mosquitoes feed on infected birds and then bite people.
• People are considered dead-end hosts because unlike birds, they do not develop high enough levels of virus in their bloodstream and cannot pass the virus on to other biting mosquitoes.
• The virus is rarely transmitted from person-to-person by blood transfusion, organ transplantation, and mother to baby, during pregnancy, delivery, or breastfeeding.
• Because the virus can be transmitted through blood and organ transplantation, persons who were recently diagnosed with West Nile virus infection should not donate blood or bone marrow for 120 days following infection.
• Prevent getting sick with West Nile by preventing mosquito bites.

Key points

• West Nile virus disease is caused by virus in the genus Flavivirus.
• West Nile virus is maintained in the environment by Culex species mosquitoes and avian hosts.
• Humans are dead-end hosts but can rarely transmit the virus through blood transfusion, organ transplantation, perinatally, and breastfeeding.

Virus

West Nile virus is a member of the family Flaviviridae, genus Flavivirus. West Nile virus has a single-stranded, positive-sense RNA genome. The virus particles are spherical and have a diameter of 40 nm.

Other medically important flaviviruses found in the United States include St. Louis encephalitis virus and Powassan virus.

Transmission

West Nile virus is most commonly spread to people by the bite of an infected mosquito.

Mosquitoes become infected when they feed on infected birds. Infected mosquitoes then spread West Nile virus to people and other animals by biting them.

In nature, West Nile virus cycles between mosquitoes (especially Culex species) and birds. Some infected birds can develop high levels of the virus in their bloodstream and mosquitoes can become infected by biting these infected birds. After about a week, infected mosquiotes can pass the virus to more birds when they bite.

Mosquitoes with West Nile virus also bite and infect people, horses, and other mammals. However, humans, horses, and other mammals are 'dead end' hosts. This means that they do not develop high levels of virus in their bloodtsream, and cannot pass the virus on to other biting mosquitoes.

In a very small number of cases, West Nile virus has been spread through:

• Exposure in a laboratory setting
• Blood transfusion and organ transplant
• Mother to baby, during pregnancy, delivery, or breast feeding

West Nile virus is not spread:

• Through coughing, sneezing, or touching
• By touching live animals
• From handling live or dead infected birds.
  • Avoid bare-handed contact when handling any dead animal.
  • If you are disposing of a dead bird, use gloves or double plastic bags to place the carcass in a garbage can.
• Through eating infected animals, including birds. Always follow instructions for fully cooking meat.

What to know

• West Nile virus rarely can be spread through blood transfusion.
• The blood supply in the United States is screened for West Nile virus.

Can I get infected with West Nile virus by donating blood?

No. You cannot get infected with West Nile virus by donating blood.

Can I get infected with West Nile virus by receiving a blood transfusion?

Yes, but it is unlikely. A small number of West Nile virus infections have been reported from blood transfusions. However, blood collection agencies have been screening all donated blood for West Nile virus since 2003 to minimize this risk.

Is donated blood tested for West Nile virus?

Yes. All donated blood is tested for West Nile virus. Any blood product found to be infected is removed from the blood supply.

Can I donate blood if I was diagnosed with West Nile virus infection?

If you have been diagnosed with West Nile virus infection or disease (West Nile) by your doctor, you should not donate blood for 120 days. If you are diagnosed shortly after giving blood, you should tell your blood center. Donation centers try to ensure that donors who recently had West Nile virus infections do not give blood for 120 days. Let the center know if you have had a West Nile virus infection. They will help you decide if it is safe for you to donate.

I recently had a transfusion. Should I be concerned about getting infected with West Nile virus?

If you recently had a transfusion, you should be aware of the very small risk for West Nile virus infection. This risk may be higher during the summer when the virus is most likely to infect blood donors. You should contact your doctor if you think you have symptoms caused by West Nile virus or other concerns.

Can I be treated if I develop West Nile after receiving blood?

There are no specific medicines that can treat West Nile. You should contact your doctor if you think you have symptoms of West Nile. They may be able to give you medicine to help lessen some of your symptoms.

What to know

• West Nile virus rarely can be spread through organ transplantation.
• Living organ donors are typically screened for West Nile virus infection prior to donation.
• Deceased organ donors are not routinely screened for West Nile virus infections.

Can I be infected with West Nile virus from a transplanted organ?

Yes, it is possible with roughly one organ donor transmitting West Nile virus to the organ recipients each year in the United States. The risk of being infected with West Nile virus from an organ is not known. The number of West Nile virus disease (West Nile) cases in a year, the time of year, and where the organ donor lives can impact your chance of being infected with West Nile virus from a transplanted organ.

Are organ donors tested for West Nile virus?

Unlike blood donors, not all deceased organ donors are tested for West Nile virus. However, most centers screen living donors and some centers screen deceased donors for West Nile virus. Organ donors might not be tested because of the time it may take to get results. In addition, the current tests for West Nile virus may not work well for all organ donors.

Can I be an organ or tissue donor if I was previously infected with West Nile virus?

You can still become an organ or tissue donor if you have had West Nile virus infection in the past. A small number of patients have been infected with West Nile virus from a transplanted organ. However, those organs came from donors who were infected with West Nile virus days to weeks (not months or years) before their donation. Currently, most deceased organ donors are not screened for West Nile virus.

Will I get very ill if I am infected with West Nile virus from a mosquito after having a transplant?

The risk of a transplant patient getting very ill from West Nile virus is not known. One study found that 1 in 40 persons infected with West Nile virus after a transplant got severe disease. The chance of getting West Nile may depend on your age, other health conditions, medications you are taking, how much West Nile virus is circulating where you live, and what steps you take to prevent mosquito bites.

Can I be treated if I get West Nile after having a transplant?

There are no specific medicines that can treat West Nile. You should contact your doctor if you think you have symptoms of West Nile. They may be able to give you medicine to help lessen some of your symptoms.

What to know

Women infected with West Nile virus during pregnancy have a low risk of transmitting the virus to their babies.

I am pregnant. Am I at higher risk for getting infected with West Nile virus?

No. Pregnant women are not at higher risk for West Nile virus infection.

I am pregnant and was just diagnosed with West Nile virus infection. Is my baby at risk of infection?

A woman who is infected with West Nile virus during pregnancy can possibly transmit the virus to her baby, but the risk is low. Only a few cases of West Nile virus in newborns have been reported. Pregnant women should take precautions to reduce their risk for West Nile virus infection by avoiding mosquitoes, wearing protective clothing, and using insect repellent.

If I am pregnant, should I use insect repellents?

Yes. Protecting yourself from mosquito bites is the only way to prevent infection with West Nile virus. In addition to wearing protective clothing such as long-sleeve shirts and long pants, use insect repellents. Repellents containing active ingredients which have been registered with the EPA are considered safe for pregnant and breastfeeding women.

What to know

• Breastfeeding women infected with West Nile virus have a very low risk of transmitting the virus to their babies.
• The benefits of breastfeeding outweigh the unknown risk of passing West Nile virus through breast milk.

Can West Nile virus be transmitted through breast milk?

Possibly. It appears that West Nile virus may be transmitted through breast milk, although this is likely a rare occurrence.

In 2002, a woman developed encephalitis due to West Nile virus from a blood transfusion she received shortly after giving birth. Laboratory analysis showed evidence of West Nile virus in breast milk collected from the mother soon after she became ill. She had been breastfeeding her baby and about 3 weeks after birth the infant tested positive for West Nile virus. Because of the infant's minimal outdoor exposure, it is unlikely that infection was from a mosquito. The infant had no symptoms of West Nile virus infection and remained healthy.

Should I continue breastfeeding if I have West Nile?

Yes. The risk for West Nile virus transmission through breastfeeding is unknown. However, the health benefits of breastfeeding are well established. Therefore, there are no recommendations for a woman to stop breastfeeding because of West Nile virus disease (West Nile).

If I am breastfeeding, should I use insect repellents?

Yes. Protecting yourself from mosquito bites is the only way to prevent infection with West Nile virus. In addition to wearing protective clothing such as long-sleeve shirts and long pants, use insect repellents. Repellents containing active ingredients which have been registered with the EPA are considered safe for breastfeeding women.

Key points

• West Nile virus has been detected in variety of bird species.
• Some infected birds, especially crows and jays, are known to get sick and die from the infection.
• Reporting and testing of dead birds is one way to check for the presence of West Nile virus in the environment.
• Some surveillance programs rely on citizens to report dead bird sightings to local authorities.

How do birds get infected with West Nile virus?

West Nile virus is transmitted to birds through the bite of an infected mosquito. Mosquitoes become infected by biting infected birds. Some birds that are predators (such as hawks and owls) or scavengers (such as crows) may become infected after eating sick or dead birds that were already infected with West Nile virus. Some birds that eat infected mosquitoes may also become infected.

Do birds infected with West Nile virus die or become ill?

Although some infected birds, especially crows and jays, frequently die of infection, most birds survive. Since West Nile virus was discovered in the United States in 1999, the virus has been detected in over 300 species of dead birds.

Can I get West Nile virus directly from birds?

There is no evidence that a person can get infected from handling live or dead infected birds. However, you should avoid bare-handed contact when handling any dead animal. If you must pick up a dead bird, use gloves or an inverted plastic bag to place the bird in a garbage bag.

What should I do if I find a dead bird?

State and local agencies have different policies for collecting and testing birds, so check with your state health department or state wildlife agency for information about reporting dead birds in your area. Wildlife agencies routinely investigate sick or dead bird events if large numbers are impacted. This type of reporting could help with the early detection of illnesses like West Nile virus or avian influenza (bird flu), known to cause death in birds. If local authorities tell you to simply dispose of the bird, don’t handle it with your bare hands. Use gloves or an inverted plastic bag to place the bird in a garbage bag, which can then be disposed of in your regular trash.

Why did some states and local areas stop collecting dead birds to test for West Nile virus?

West Nile virus is found in all 48 contiguous states (not in Alaska or Hawaii) and the virus circulates in mosquitoes and birds every year. Because West Nile virus is well established, some states and local jurisdictions are no longer collecting dead birds for testing. Instead, they have chosen to shift staff and funding resources away from testing of dead birds to other areas of West Nile virus surveillance and control.

West Nile virus can result in febrile illness or neurologic disease, including meningitis or encephalitis.

No symptoms in most people. Most people (8 out of 10) infected with West Nile virus do not develop any symptoms.

Febrile illness (fever) in some people. About 1 in 5 people who are infected develop a fever with other symptoms such as headache, body aches, joint pains, vomiting, diarrhea, or rash. Most people with febrile illness due to West Nile virus recover completely, but fatigue and weakness can last for weeks or months.

Serious symptoms in a few people. About 1 in 150 people who are infected develop a severe illness affecting the central nervous system such as encephalitis (inflammation of the brain) or meningitis (inflammation of the membranes that surround the brain and spinal cord).

• Symptoms of severe illness include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis.
• Severe illness can occur in people of any age. However, people over 60 years of age are at greater risk for severe illness if they are infected (1 in 50 people). People with certain medical conditions, such as cancer, diabetes, hypertension, kidney disease, and people who have received organ transplants, are also at greater risk.
• Recovery from severe illness might take several weeks or months. Some effects might be permanent.
• About 1 out of 10 people who develop severe illness affecting the central nervous system die.

If you think you or a family member might have West Nile, talk with your health care provider.

• Healthcare providers diagnose West Nile virus infection based on:
  • Signs and symptoms
  • History of possible exposure to mosquitoes that can carry West Nile virus
  • Laboratory testing of blood or spinal fluid
• Your healthcare provider can order tests to look for West Nile virus infection or other infections that can cause similar symptoms.

There is no specific treatment for West Nile. Rest, fluids, and pain medications may relieve symptoms.

• No specific medicines available to treat West Nile. Antibiotics do not treat viruses.
• Rest, fluids, and over-the-counter pain medications may relieve some symptoms.
• In severe cases, patients often need to be hospitalized to receive supportive treatment, such as intravenous fluids, pain medication, and nursing care.

Most people infected with West Nile virus are believed to have lifelong immunity or protection from getting the disease again. Some people who have weakened immune systems from certain conditions or medications might not have a strong immune response to the initial infection or their immunity may wane over time. However, most people are protected from getting West Nile again once they have had it.

Key points

• There are no vaccines or medicines to prevent West Nile virus disease (West Nile).
• The best way to prevent West Nile is to protect yourself from mosquito bites.

Prevention tips

West Nile virus is spread to people through the bite of an infected mosquito. Mosquitoes bite during the day and night.

There are no vaccines or medicines to prevent West Nile.

The best way to prevent West Nile is to protect yourself from mosquito bites.

• Use insect repellent
• Wear long-sleeved shirts and pants
• Treat clothing and gear
• Take steps to control mosquitoes indoors and outdoors

What is West Nile virus?

West Nile virus is mainly spread to people through the bite of an infected mosquito. However, there have been a very small number of cases where the virus was spread by blood transfusion, organ donation, or from mother to baby during pregnancy, birth, or when breastfeeding.

Most people who get West Nile virus infection do not feel sick. For people who do feel sick, symptoms can include fever, headache, tiredness, nausea, vomiting, muscle aches, and a rash. Symptoms usually last from a few days to a few weeks.

Some people can have more serious disease. Adults over 60 years old or people with certain medical conditions are more likely to get seriously ill. Symptoms of serious West Nile virus disease include high fever, headache, neck stiffness, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness, and paralysis.

Who can get West Nile virus?

West Nile virus is found in countries throughout the world. Travelers going to Africa, Europe, the Middle East, west and central Asia, or North America can get West Nile virus.

The following activities can increase a traveler’s chance of getting infected:

• Spending a lot of time outdoors
• Traveling during times of the year when mosquitoes are more common, such as during the summer.

What can travelers do to prevent West Nile virus?

There are no vaccines or medicines that prevent West Nile virus. Travelers can protect themselves against West Nile virus infection by preventing mosquito bites.

Use an EPA-registered insect repellent

• Use Environmental Protection Agency (EPA)-registered insect repellents with one of the active ingredients below. When used as directed, EPA-registered insect repellents are proven safe and effective, even for pregnant and breastfeeding women. If also using sunscreen, always apply insect repellent after sunscreen.
  • DEET
  • Picaridin (known as KBR 3023 and icaridin outside the US)
  • IR3535
  • Oil of lemon eucalyptus (OLE)
  • Para-menthane-diol (PMD)
  • 2-undecanone

Insect Repellent Tips for Babies and Children

• Dress your child in clothing that covers arms and legs.
• Cover strollers and baby carriers with mosquito netting.
• When using insect repellent on your child:
• Always follow label instructions.
  • Do not use products containing oil of lemon eucalyptus (OLE) or para-menthane-diol (PMD) on children under 3 years old.
  • Do not apply insect repellent to a child’s hands, eyes, mouth, cuts, or irritated skin.
  • Adults: Spray insect repellent onto your hands and then apply to a child’s face.
  • If also using sunscreen, always apply insect repellent after sunscreen.

Wear long-sleeved shirts and long pants

Treat clothing and gear with permethrin

• Use 0.5% permethrin to treat clothing and gear (such as boots, pants, socks, and tents) or buy permethrin-treated clothing and gear.
  • Permethrin is an insecticide that kills or repels insects like mosquitoes and sand flies.
  • Permethrin-treated clothing provides protection after multiple washings.
  • Read product information to find out how long the protection will last.
• If treating items yourself, follow the product instructions.
• Do not use permethrin products directly on skin.

Keep mosquitoes out of your hotel room or lodging

• Choose a hotel or lodging with air conditioning or window and door screens.
• Use a mosquito net if you are unable to stay in a place with air conditioning or window and door screens or if you are sleeping outside.

Sleep under a mosquito net

• Sleep under a mosquito net if you are outside or when screened rooms are not available. Mosquitoes can live indoors and bite during the day and night.
• Buy a mosquito net at your local outdoor store or online before traveling overseas.
• Choose a mosquito net that is compact, white, rectangular, with 156 holes per square inch, and long enough to tuck under the mattress.
• Permethrin-treated mosquito nets provide more protection than untreated nets.
  • Permethrin is an insecticide that kills mosquitoes and other insects.
  • To determine if you can wash a treated mosquito net, follow the label instructions.

If you are bitten by mosquitoes, avoid scratching the bites and apply over-the-counter anti-itch or antihistamine cream to relieve itching.

After Travel

If you traveled and feel sick, particularly if you have a fever, talk to a healthcare provider and tell them about any areas you recently traveled to.