Key Messages

• About 80% of people infected with WNV do not develop symptoms and may never know they were infected.
• About 20% of people infected with WNV develop mild illness with flu-like symptoms like fever, headache, and body aches.
  • Most people with mild illness recover completely.
  • Most symptoms of mild illness typically resolve within 14 days for most people. However, fatigue and weakness may last for weeks or months.
• Less than 1% of people infected with WNV develop severe illness that affects the central nervous system, requires hospitalization, or results in death.
  • Central nervous system infection (neurologic illness) can cause inflammation of the brain (encephalitis) and the membranes around the brain and spinal cord (meningitis). Rarely, paralysis can also occur.
  • Symptoms include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness, and paralysis.
  • Recovery from neurologic illness can take several weeks or months. Some effects might be permanent.
  • About 10% of people with neurologic illness die.

• Anyone who lives in or travels to an area where WNV circulates in the contiguous United States is at risk of infection.
  • All states in the contiguous United States have reported cases.
  • People who spend more time outdoors or cannot keep mosquitoes out of their residence between dusk and dawn are at higher risk of infection.
• If infected, anyone can develop mild or severe WNV illness.
• The risk of severe illness—illness that affects the central nervous system, requires hospitalization, or results in death—increases with age or if you have a chronic medical condition or weakened immune system due to an underlying health condition or medication.
  • The risk of severe illness increases with age. For example, people 65 years and older are three times as likely to develop neurologic illness than people younger than 65 years.
  • People with certain chronic medical conditions like cancer, diabetes, high blood pressure (hypertension), or kidney disease are at higher risk of severe illness.
  • People with a weakened immune system, including those who take medications that weaken the immune system, are at higher risk of severe illness. For example, rituximab, ocrelizumab, or similar medications, used to treat cancers, autoimmune diseases, and inflammatory conditions or prevent rejection of transplanted organs, weaken the immune system.

• If you think you or a family member might be sick with WNV, talk with your healthcare provider about ordering a test to look for an infection.
• There are no medicines to treat WNV disease.
• For mild illness, you can often manage your symptoms by:
  • Taking over-the-counter medication, like acetaminophen, for fever, pain, and headaches. Avoid ibuprofen or other non-steroidal anti-inflammatory drugs (NSAIDs) if you live in an area with dengue virus, which is also transmitted by mosquitoes.
  • Staying hydrated by drinking lots of fluids
  • Resting
• Seek immediate medical attention if you experience high fever, neck stiffness, muscle weakness, confusion, or tremors.
• People with severe illness often need to be hospitalized to receive supportive treatment, such as intravenous fluids, pain medication, and nursing care.

Note: Prevention messaging should be tailored to the local area since some recommended actions may not be relevant for all parts of the United States.

• The best way to protect against WNV infection is to prevent mosquito bites.
  • Use Environmental Protection Agency (EPA)-registered insect repellent.
  • Wear long, loose-fitting shirts and pants.
  • Avoid being outside between dusk and dawn.
  • Use screens on windows and doors or air conditioning, if available.
  • Use larvicides like mosquito dunks in smaller areas with permanent standing water like ponds.
  • Contact a local mosquito control district or program or licensed professional to treat vegetation near your home or larger areas with permanent standing water.
• There are no licensed vaccines or medications to prevent WNV disease.
  

EPA-registered insect repellents

• Use EPA-registered insect repellents with one of the following active ingredients: DEET, picaridin, IR3535, oil of lemon eucalyptus, para-menthane-diol, or 2-undercanone.
• When used as directed, EPA-registered insect repellents are proven safe and effective, including for pregnant and breastfeeding women.
  • Do not use products containing oil of lemon eucalyptus or para-menthane-diol on children under 3 years old.
  • Do not apply repellent to a child's hands, eyes, mouth, cuts, or irritated skin.
• We do not know the effectiveness of non-EPA-registered insect repellents, including some natural repellents.
• Use the Insect Repellent Chatbot to help you choose a repellent, how to apply repellent, and other ways to prevent mosquito bites.
• Use 0.5% permethrin to treat clothing and gear (such as boots, pants, socks, and tents) or buy permethrin-treated clothing and gear.
  • Permethrin-treated clothing provides protection after multiple washings.
  • Read product information to find out how long the protection will last.
  • Do not use permethrin products directly on skin.

• WNV is transmitted primarily through the bite of an infected Culex species mosquito.
• West Nile virus circulates in the environment between mosquitoes and birds. People become infected with the virus when mosquitoes feed on infected birds and then bite people.
• WNV is introduced by inoculation of viral particles into the skin at the mosquito feeding site. From there, WNV spreads to regional lymph nodes where it replicates and then enters the bloodstream causing viremia. This may be followed by invasion of the central nervous system in some cases.
• Humans, horses, and other mammals are "dead end" hosts, meaning that viremia is not high enough for an infected person to transmit the virus to uninfected mosquitoes.
• WNV transmission can also occur through blood transfusion and organ transplantation.
  • Since universal WNV screening of the U.S. blood supply started in 2003, transmission by this route is rare.
  • Living tissue donors are routinely screened for WNV.
  • WNV screening of living and deceased solid organ donors is currently not required. However, most centers do screen living donors and some centers screen deceased donors for WNV.
• WNV has rarely been transmitted through intrauterine, peripartum, breastmilk, percutaneous (laboratory), and conjunctival exposure.
• Standard precautions are recommended when treating patients suspected or confirmed as having WNV disease in healthcare settings.

• The incubation period from mosquito bite to onset of acute systemic illness is usually 2–6 days but ranges between 2–14 days and may be longer in immunocompromised patients.
• About 80% of human WNV infections are asymptomatic.
• If symptomatic, most people develop febrile illness referred to as non-neuroinvasive disease—or West Nile fever—often involving fever, fatigue, headache, myalgia, rash, vomiting, and/or diarrhea.
• Less than 1% of infected people develop neuroinvasive WNV disease.
  • Neuroinvasive WNV disease typically manifests as aseptic meningitis, encephalitis/meningoencephalitis, or acute flaccid myelitis.
  • Symptoms of meningitis can include headache, photophobia, and meningismus. Meningitis typically develops within 24-48 hours of illness onset.
  • Patients with encephalitis can develop confusion, movement disorders such as tremors, myoclonus, parkinsonism, and cerebellar ataxia, and progress to stupor or coma. Encephalitis typically develops within 24-48 hours of illness onset.
  • Acute flaccid myelitis (AFM) can develop concurrently with meningitis or encephalitis and cause asymmetric limb weakness, areflexia, and cranial neuropathies, and can lead to neuromuscular respiratory failure. AFM typically develops within 24-48 hours of illness onset.
  • WNV-associated Guillain-Barré syndrome can occur 1-8 weeks following acute WNV infection and is characterized by ascending, symmetric weakness and sensory and autonomic dysfunction.
• Many patients have long-term physical, cognitive, and functional sequelae after being hospitalized for WNV disease, with 30–40% of patients being discharged to long-term care or rehabilitation facilities, and >50% having ongoing symptoms for over a year after illness.
• About 10% of people with neuroinvasive disease will die from their illness.

• Anyone can develop neuroinvasive disease following WNV infection, but the risk increases with age.
  • Based on blood donor studies, about 2% of people aged ≥65 years develop neuroinvasive disease compared with <0.5% of people <65 years.
  • Patients aged 60-69 years are twice as likely and patients aged ≥70 years are more than 6 times as likely to be hospitalized with WNV disease compared to younger adults.
  • Mortality in patients with neuroinvasive disease is highest for people aged ≥70 years (~20%) compared with younger age groups (~2% for people <50 years).
• Risk of neuroinvasive disease is also increased for patients with cancer, diabetes, hypertension, renal disease, or a weakened immune system.
  • Patients taking B-cell depleting therapies or similar medications to treat cancers, autoimmune diseases, and inflammatory conditions or prevent rejection of transplanted organs are at higher risk of neuroinvasive disease.
  • Mortality from neuroinvasive disease is higher for people with severe immunosuppression (30–40%).
• Healthcare providers should emphasize mosquito bite prevention, especially for people who are at higher risk of developing neuroinvasive disease if infected with WNV.

• Consider WNV disease in the differential diagnosis of patients with flu-like or unexplained neurological symptoms, especially during months when mosquitoes are active.
• For non-immunocompromised patients, testing for WNV-specific IgM antibodies in serum and/or cerebrospinal fluid (CSF) is recommended.
  • WNV IgM antibodies usually become detectable 3–8 days after onset of illness; if initial testing is negative, it should be repeated on or after the eighth day of illness.
  • In some cases, positive presumptive IgM results should be confirmed by neutralizing antibody testing at CDC or state public health laboratories. Confirmatory testing should be considered for patients with unusual clinical presentations, fatal cases, or to ensure WNV is the infecting virus (e.g., when possible exposure to another cross-reacting flavivirus like dengue or Powassan virus occurred).
• For immunocompromised patients, reverse transcription (RT)-PCR of serum or CSF might be more sensitive than antibody testing for diagnosis of acute WNV disease, especially for patients on B cell-depleting monoclonal antibodies like rituximab.
• Patients with immunocompromising conditions may have prolonged viremia and delayed or absent antibody response.
  • Symptomatic presentation in immunocompromised patients can occur outside of the usual arboviral season (typically June to October or longer in warmer regions).
  • Molecular testing may be preferred for some patients.
• Correct diagnosis is important to stop unnecessary therapies (e.g. antibiotics), limit further diagnostic evaluation, and help inform patient outcomes or direct public health prevention measures. Correct diagnosis can also help to identify rare cases exposure to infected blood or organs.
• Promptly report cases to state or local health departments to allow for appropriate control measures. Timely testing and diagnosis are critical to effective management of WNV disease, enabling early medical intervention, prompt reporting, and public health response.

• There are no approved or recommended therapies to treat WNV infections or disease. Clinical management is supportive.
  • Various drugs have been evaluated or empirically used for WNV disease. However, none has shown conclusive benefit to date.
  • Mild illness can be managed with over-the-counter medication for fever, pain and headaches; staying hydrated; and rest.
• Patients with severe meningeal symptoms often require pain control for headaches and antiemetic therapy and rehydration for associated nausea and vomiting.
• Patients with encephalitis require close monitoring for the development of elevated intracranial pressure, seizures, or inability to protect their airway.
• Patients with acute flaccid myelitis should be monitored closely for acute neuromuscular respiratory failure that can develop rapidly and require prolonged ventilatory support.

• Culex species mosquitoes spread WNV and other viruses like St. Louis encephalitis virus. These mosquitoes are different from the species that spread some other mosquito-borne diseases like dengue, yellow fever, and Zika.
• Only female mosquitoes bite people and animals to get a blood meal. Female mosquitoes need a blood meal to produce eggs.
• Culex species mosquitoes are most active from dusk to dawn. When they are not active, they usually rest in grass, shrubs, and other vegetation.
• Culex mosquitoes prefer to lay eggs on the surface of permanent standing water like ponds, edges of lakes, swamps, unmaintained swimming pools, and wastewater treatment plants. They also use floodwater areas, like storm drains, ditches, low-lying pastures, and agricultural areas (citrus groves or rice fields). Sometimes they use small containers around the house like buckets, unused tires, planters, toys, birdbaths, or flowerpots.

House-hold level control

• Keep mosquitoes outside:
  • Use screens on windows and doors or use air conditioning, if available.
  • Repair holes in screens to keep mosquitoes outdoors.
• Emptying small containers that could hold standing water outside the house is important for general mosquito control.
• For the mosquitoes that spread WNV:
  • Use mosquito dunks or other larvicides in smaller areas with permanent standing water—like ponds or unmaintained swimming pools—to kill mosquito larvae before they grow into adult mosquitoes.
  • Contact a local mosquito control district or program or licensed professional about using an adulticide on your property to treat cool, dark, and humid areas where adult Culex mosquitoes rest, especially thick vegetation.
  • Contact a local mosquito control district or program about treating larger areas with permanent standing water on or near your property, such as agricultural areas, low-lying pastures, and wastewater treatment plants.

Best practices when using insecticides

• Contact a local mosquito control district or program or licensed professional to apply adulticides for you.
• Always follow the label instructions if you are applying an insecticide yourself, such as larvicides or adulticides to kill larvae, pupae, or adult mosquitoes.
  • Use larvicides to treat standing water that will not be used for drinking or cannot be covered, dumped, or removed (for example, ponds or unmaintained pools).
  • Do not apply more product than recommended or reapply more often than stated on product label instructions.
  • Do not apply adulticides directly on outdoor fruits and vegetables or inside near food.
  • Do not apply adulticides to plants in bloom or to plants that bees, butterflies, and other pollinators visit.
  • Keep people and pets out of the area you treat until the adulticide product has dried, usually about 1 hour.

Community-level control

• Local mosquito control professionals monitor the number of mosquitoes in the community and whether they are infected with viruses that can make people sick.
• If the number of mosquitoes in an area becomes too high or if they are found to be infected with WNV, mosquito control professionals may take steps to kill mosquito larvae and adult mosquitoes. These steps can include:
  • Removing places where mosquitoes can lay eggs or water source reduction
  • Applying insecticides, such as larvicides to decrease the number of mosquito larvae and pupae, or adulticides to decrease the number of adult mosquitoes
  • Increasing outreach to the public.
• Only adult female mosquitoes spread viruses to people. During an outbreak, the use of adulticides is an important method used to decrease the overall number of adult mosquitoes.
• Aerial and truck spraying help to control and reduce the number of mosquitoes that can spread viruses. This can reduce the chances of people and animals in the community getting sick.
  • When applied by a trained professional and according to the product label, truck and aerial spraying pose minimal risk to people, pets, animals, and the environment.
  • Aerial and truck spraying of adulticides occurs when adult mosquitoes are most active and when pollinating insects are not active—between dusk and dawn.
  • Larvicides can be applied during the day to prevent larvae from growing into adult mosquitoes. Depending on the size of the body of water with larvae, trained professionals might spray larvicides using backpack sprayers, trucks, or airplanes.
• While proper use and application of insecticides pose little to no risk to human health, people can avoid exposure by keeping their family and pets inside with the windows closed while insecticides are applied in their area.