Symptoms of West Nile Virus

West Nile fever is a mosquito-borne viral infection of which 75 percent of cases will have little to no recognizable symptoms. The remaining 25 percent may develop a fever, headache, vomiting, or rash. While the West Nile virus rarely causes major illness in healthy adults or children, those with compromised immune systems (such as the elderly and people living with HIV) are at an increased risk of severe complications, including meningitis and encephalitis.

Frequent Symptoms

People infected with the West Nile virus will typically develop symptoms within two to 14 days of exposure. The most common symptoms include:

• Headache
• Fever
• Muscle aches (myalgia)
• Joint pain (arthralgia)
• Excessive sweating
• Nausea
• Vomiting
• Diarrhea
• Swollen lymph glands (lymphadenopathy)
• A maculopapular rash (characterized by small, red bumps)

The symptoms tend to be mild and may last for a few days or weeks. In absence of a rash, people will often describe the infection as being similar to a mild flu or a bad summer cold. More often than not, the symptoms will resolve on their own without treatment.

Complications

The West Nile virus is a neurotropic virus, meaning that it preferentially attacks the nervous system. In the vast majority of cases, the body's immune defenses can control and eventually neutralize the virus on their own.

However, the same may not hold true for people whose immune systems are compromised. This places certain groups—such as the elderly, organ transplant recipients, people with advanced HIV, and those undergoing cancer chemotherapy—at an increased risk of severe and potentially life-threatening complications.

The types and severity of symptoms are largely dependent on the parts of the nervous system affected. Collectively, the complications are referred to as West Nile neuroinvasive disease (WNND) and include encephalitis, meningitis, meningoencephalitis, and poliomyelitis. Overall, WNND is associated with a 9 percent risk of death. The rate is believed to be higher in the elderly.

West Nile Encephalitis

West Nile encephalitis is a condition in which the virus causes inflammation of the brain. It does so by crossing the blood-brain barrier that surrounds the brain and filters out harmful agents. The West Nile virus is one of the few insect-borne viruses readily able to do this.

West Nile encephalitis is the most common manifestation of WNND. It typically causes fever, headaches, confusion, forgetfulness, extreme lethargy, and changes in personality or behavior.

Between 30 percent and 50 percent of people with West Nile encephalitis will experience unilateral muscle weakness (meaning on one side of the body). Of these, some may progress to flaccid paralysis, a type of paralysis in which muscles are unable to contract.

West Nile Meningitis

West Nile meningitis is a condition in which the virus causes the inflammation of the meninges, the three membranes that surround and encase the brain and spinal cord. While meningitis can cause many of the same physical symptoms of West Nile encephalitis, it doesn't typically alter a person's behavior or personality, or cause confusion. Nausea, vomiting, neck pain or stiffness, light sensitivity (photophobia), and the fear of loud sounds (phonophobia) are also common.

West Nile Meningoencephalitis

West Nile meningoencephalitis is a complication affecting both the brain and meninges. Men between ages 60 and 89 are 20 times more likely to develop West Nile meningoencephalitis than the general population, while people with compromised immune systems are at a 40-fold increased risk.

While meningoencephalitis shares many of the same neurologic symptoms of meningitis and encephalitis, they tend to be more severe and longer lasting (and may, in some cases, become permanent) with this particular complication. The risk of death is also higher, hovering between 12 percent and 15 percent. The mortality risk among the elderly may be as high as 35 percent.

West Nile Poliomyelitis

West Nile poliomyelitis, like other forms of polio, is characterized by the acute and often incapacitating loss of motor control. Unlike other neurologic complications of West Nile fever, poliomyelitis may not be accompanied by fever, headache, or other common symptoms of infection.

The condition is characterized by the sudden onset of flaccid paralysis on side of the body, usually without the loss of sensation. The paralysis is often preceded by pain and can strike quickly, usually within two to eight days of the first appearance of symptoms.

Less commonly, West Nile poliomyelitis can affect the respiratory system and require mechanical ventilation to help the person breath. It can also cause the loss of sphincter control, resulting in urinary or fecal incontinence.

While the paralysis may lead to permanent impairment, milder cases can often improve significantly as the affected nerve cells gradually recover and reestablish connections. People with fewer affected limbs tend to show better improvement overall. With that being said, most of the strength recovery will occur during the first six to eight months following the onset of symptoms, eventually plateauing with less noticeable improvement.

West Nile Reversible Paralysis

West Nile reversible paralysis is a less severe, temporary form of paralysis, again affecting only one side of the body. Although the condition is not well understood, it is believed to be caused by inflammation of the same part of the spinal cord (called the anterior horn) that triggers poliomyelitis and Lou Gehrig's disease.

What differentiates West Nile reversible paralysis from West Nile poliomyelitis is that the reflex responses remain intact even when muscle weakness occurs. While the initial paralysis can be profound, it will eventually reverse with little visible impairment of motor function.

When to See a Doctor

Getting a mosquito bite does not mean you will get West Nile fever. Most people infected with the West Nile virus will either never even know it or simply mistake it for a mild flu. Even if you discover you've been infected, the chances are good that you will get better without any problems or treatment.

West Nile Virus Doctor Discussion Guide

Get our printable guide for your next doctor's appointment to help you ask the right questions.

Download PDF

Sign up for our Health Tip of the Day newsletter, and receive daily tips that will help you live your healthiest life.

Sign Up

You're in!

Thank you, {{form.email}}, for signing up.

There was an error. Please try again.

With that being said, if you are elderly or immune-compromised, you need to seek immediate care if you experience a severe headache, high fever, neck stiffness, confusion, light sensitivity, or sudden muscle weakness. These may be signs encephalitis or meningitis, both of which require emergency treatment.

The West Nile virus is no longer a disease associated with foreign travel. You can get it in the United States as readily as Africa and the Middle East. Fatalities, which have mainly occurred among the elderly, have generally been low, the annual rate ranging from as few as 2 to as many as 286 in the United States.