West Nile Virus

West Nile virus is a common virus spread to humans through the bite of an infected mosquito. Most people who get West Nile virus do not develop any symptoms. However, some people can develop a mild, flu-like illness. A small number of people can develop a serious, life-threatening illness that affects the brain and spinal cord.

What is West Nile Virus?

West Nile virus is a common virus that spreads to humans primarily through the bite of an infected mosquito. It is most active during mosquito season, which typically runs from summer through fall. While many people infected with West Nile virus experience no symptoms, some may develop a mild illness, and a small percentage can develop severe neurological disease affecting the brain and spinal cord.

West Nile virus (WNV) is a type of virus called a flavivirus. Birds are the natural hosts for the virus. Mosquitoes become infected when they bite infected birds. These infected mosquitoes can then spread the virus to humans and other animals, such as horses, when they bite them. The virus is not spread from person to person through casual contact, such as touching or kissing. It also does not spread through coughing or sneezing. The first human cases of West Nile virus in the United States were identified in New York City in 1999. Since then, the virus has spread across the continental United States. Cases usually occur during warmer months when mosquitoes are most active, typically from June through September.

Symptoms

Most people (about 8 out of 10, or 80%) infected with West Nile virus do not develop any symptoms at all. For those who do, symptoms usually appear 3 to 14 days after a mosquito bite. Symptoms can range from a mild, flu-like illness to a severe disease affecting the brain and spinal cord.

About 1 in 5 people (20%) who get West Nile virus will develop West Nile fever. These symptoms are usually mild and can include: * Fever * Headache * Body aches * Joint pain * Vomiting * Diarrhea * Rash Most people with West Nile fever recover completely, but fatigue and weakness can sometimes last for weeks or months. Less than 1 in 100 people (less than 1%) infected with West Nile virus develop a severe illness that affects the brain and spinal cord. This is called neuroinvasive disease. These severe forms include: * **Encephalitis:** Inflammation of the brain. * **Meningitis:** Inflammation of the membranes surrounding the brain and spinal cord. * **Acute flaccid paralysis:** Sudden weakness in the limbs. Symptoms of severe illness can include: high fever, severe headache, stiff neck, stupor (a state of near-unconsciousness or insensibility), disorientation, coma, tremors or convulsions (seizures), muscle weakness, and paralysis. These severe illnesses can be life-threatening and may lead to long-term neurological problems or death.

Causes & risk factors

West Nile virus is caused by a virus transmitted to humans through the bite of an infected mosquito. Mosquitoes become infected after feeding on birds that carry the virus. Anyone can get West Nile virus, but certain factors increase your risk of developing severe illness, particularly age and certain medical conditions.

The primary cause of West Nile virus infection in humans is the bite of an infected mosquito. Mosquitoes, especially those in the *Culex* species, pick up the virus from infected birds. When these mosquitoes then bite a person, they can transmit the virus. While rare, other ways the virus can spread include blood transfusions, organ transplants, or from mother to baby during pregnancy, delivery, or breastfeeding. West Nile virus is not spread through casual contact, such as touching, kissing, or sharing food. Most people who get West Nile virus do not develop severe illness. However, some factors increase your risk of developing a more serious form of the disease: * **Age:** People over 60 years old have a higher risk of developing severe neuroinvasive disease. Their immune systems may not respond as effectively to the virus. * **Weakened Immune System:** People with conditions that weaken their immune system, such as cancer, kidney disease, diabetes, or those who have received an organ transplant, are at higher risk. * **Other Medical Conditions:** Certain underlying medical conditions, like high blood pressure (hypertension) or kidney disease, may also increase the risk of severe illness. Living in an area where West Nile virus is active and being outdoors during peak mosquito activity (dawn and dusk) also increases your risk of exposure to infected mosquitoes.

How it's diagnosed

Diagnosing West Nile virus usually involves a physical exam, a review of your symptoms, and laboratory tests to detect the virus or antibodies your body makes to fight it. These tests are especially important for people with severe symptoms affecting the brain or spinal cord.

If you have symptoms that suggest West Nile virus, your doctor will likely start with a physical examination and ask about your recent activities, such as mosquito bites or travel. To confirm a diagnosis, laboratory tests are used. These tests typically look for: * **Antibodies:** Your immune system produces proteins called antibodies to fight off infections. Blood tests can detect specific West Nile virus antibodies, indicating a recent or past infection. * **Viral RNA:** In some cases, especially with severe illness, tests can directly detect the genetic material (RNA) of the virus in samples of blood or cerebrospinal fluid (CSF). Cerebrospinal fluid is the fluid that surrounds your brain and spinal cord. A sample of cerebrospinal fluid is usually collected through a procedure called a lumbar puncture (spinal tap). This test is often done if your doctor suspects a severe form of the disease, such as meningitis or encephalitis. Imaging tests, like magnetic resonance imaging (MRI) of the brain, may also be used to look for brain inflammation in severe cases.

Treatment options

There is no specific antiviral medication or vaccine available to treat or prevent West Nile virus infection in humans. Treatment focuses on managing symptoms and providing supportive care, especially for those who develop severe illness.

For most people with mild West Nile fever, treatment involves rest, fluids, and over-the-counter pain relievers to help reduce fever and relieve aches. Your doctor may recommend acetaminophen (Tylenol, others) or ibuprofen (Advil, Motrin IB, others) for comfort. For people with severe West Nile virus infection (neuroinvasive disease), treatment requires hospitalization. Supportive care in a hospital setting may include: * **Intravenous (IV) fluids:** To prevent dehydration. * **Pain medication:** To manage severe headaches and body aches. * **Medications to reduce fever:** To control high body temperature. * **Breathing support:** If the virus affects your ability to breathe on your own. * **Nursing care:** To prevent secondary infections and other complications. * **Physical therapy:** To help with muscle weakness and coordination problems that may result from the illness. These treatments do not cure the virus but help the body recover and manage the serious symptoms.

Recovery & outlook

The recovery and outlook for West Nile virus vary greatly depending on the severity of the illness. Most people with mild symptoms recover completely within a few days or weeks. However, those who develop severe neurological disease may face a longer recovery and could experience long-term health problems.

For the majority of people who experience mild West Nile fever, recovery is usually complete. Symptoms typically resolve within days to weeks. Some individuals may experience lingering fatigue or weakness for several weeks or even months after the initial illness. For the small percentage of people who develop severe neuroinvasive disease, the outlook can be more challenging. Recovery can take many weeks or months, and some individuals may experience permanent neurological damage. Potential long-term problems can include: persistent fatigue, memory problems, difficulty concentrating, muscle weakness, tremors, paralysis, and depression. About 1 in 10 people (10%) who develop severe neuroinvasive disease die from the infection. Older adults and those with weakened immune systems are at higher risk for severe outcomes and death. Rehabilitation, including physical and occupational therapy, can help manage long-term effects and improve quality of life for those with lasting complications.

When to see a doctor

You should see a doctor if you develop symptoms of West Nile virus, especially if you have been bitten by mosquitoes or live in an area where the virus is active. Seek immediate medical attention if you experience severe symptoms like a high fever, severe headache, stiff neck, confusion, or muscle weakness.

It is important to contact your doctor if you experience mild symptoms such as fever, headache, body aches, or a rash, particularly if these symptoms appear after mosquito exposure. Your doctor can assess your condition and determine if testing for West Nile virus is appropriate. **Seek immediate medical care (call 911 or go to the nearest emergency room) if you or someone you know develops any of these severe symptoms:** * High fever (103°F or higher) * Severe headache that comes on suddenly * Stiff neck * Disorientation or confusion * Stupor or coma * Tremors or convulsions (seizures) * Muscle weakness or paralysis These symptoms could indicate a serious and life-threatening infection of the brain or spinal cord, requiring urgent medical evaluation and treatment. Early medical attention can be crucial for managing severe West Nile virus disease.

Frequently asked questions

How can I prevent West Nile virus infection?

You can prevent West Nile virus by avoiding mosquito bites. Use insect repellent containing DEET, picaridin, IR3535, or oil of lemon eucalyptus. Wear long sleeves and pants when outdoors, especially at dawn and dusk when mosquitoes are most active. Eliminate standing water around your home, such as in bird baths, clogged gutters, and old tires, as these are breeding grounds for mosquitoes.

Can West Nile virus spread from person to person?

No, West Nile virus does not spread from person to person through casual contact. You cannot get the virus by touching, kissing, or sharing food with an infected person. In very rare cases, it can spread through blood transfusions, organ transplants, or from a pregnant or breastfeeding mother to her baby.

Is there a vaccine for West Nile virus?

Currently, there is no vaccine available for humans to prevent West Nile virus infection. However, vaccines are available for horses, which are also susceptible to the virus. Research is ongoing to develop a human vaccine.

How long do West Nile virus symptoms last?

For most people with mild West Nile fever, symptoms typically last a few days to a few weeks. However, some individuals may experience lingering fatigue and weakness for several weeks or even months. For those with severe neuroinvasive disease, recovery can take many months, and some effects may be permanent.

What should I do if I think I have West Nile virus?

If you suspect you have West Nile virus, especially after mosquito exposure and if you develop symptoms like fever, headache, or body aches, you should contact your doctor. If you experience severe symptoms such as a high fever, stiff neck, confusion, or muscle weakness, seek immediate medical attention.

Where is West Nile virus most common?

West Nile virus has been found in all continental United States and is most common during the summer and early fall months. It is also present in parts of Canada, Mexico, Central and South America, Europe, Africa, and Asia. The specific areas with active virus can change each year, so it's important to check local public health advisories.

Sources

• MedlinePlus — West Nile Virus
• Mayo Clinic — West Nile Virus
• Cochrane Library — West Nile Virus