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Frequently Asked Questions (FAQ) About West Nile Virus


Q: Could I be infected with West Nile virus and not know it? Back Top

A: The short answer is yes. Based on research done in New York City, experts now suspect that a larger percentage of people who are exposed to the virus get infected. But fewer than 20% of them show any symptoms at all; and less than 1% go on to develop severe illness. 
West Nile virus is a new virus that was first detected in the United States in New York City in 1999. You can get the virus if you are bitten by an infected mosquito. The mosquito becomes infected by feeding on an infected bird or animal; the virus then hides out in the mosquito's salivary glands. When the mosquito feeds on you (or a bird or animal), the virus can be passed on. In certain instances, the virus multiplies in the bloodstream.

The overwhelming majority of people who are infected with the virus do not have any symptoms and don't feel sick. Most of the rest have minor flu-like symptoms, such as fever, headache and body aches. In some cases, especially in the elderly, the virus may lead to a serious condition called encephalitis, in which the brain becomes inflamed and swollen. Symptoms of encephalitis include high fever, severe headache, stiff neck, confusion, loss of consciousness, seizures, muscle weakness and paralysis.

Most people do not need to worry about getting sick from West Nile virus. Since the virus was first detected in the U.S., there have been 149 confirmed cases of illness due to the virus. Just 18 people -- mostly elderly -- have died, according to the Centers for Disease Control and Prevention.

But if you are concerned, or live in an area with many mosquitoes, it is worth taking steps to prevent mosquito bites.

Q: What is the best treatment?

A: Right now, there is no specific treatment for West Nile virus, but researchers are testing a Hepatitis C drug -- alpha interferon -- to see if helps prevent brain damage in people who develop encephalitis.

People over 50 who get the virus in their spinal fluid can enter the trial. So can younger adults if they have a West Nile virus brain infection -- encephalitis. If the trial shows that alpha interferon helps, it will quickly be used more widely. The form of interferon A now being tested is Intron A, made by Schering Plough Corp. The company is helping pay for the study.

The treatment will not reverse any brain damage caused by encephalitis. However, researchers hope that treating patients early in the course of infection will prevent such damage. The drug does have serious side effects. It can cause severe flu-like symptoms and rarely other serious problems, such as bone marrow suppression.

Cornell University researcher James Rahal, MD, leads the study of Intron A to treat West Nile virus. Last year, he tested the treatment on 15 Louisiana patients with St. Louis encephalitis virus, a close cousin of West Nile virus. Treated patients improved, but there were side effects. The results were good enough for the FDA to approve the new study. "I am hopeful it is effective, but I am not deceiving myself," Rahal tells WebMD. "The results were sufficiently encouraging, but we just don't know yet whether it will work."

The new study will enroll 40 patients from all over the U.S. To enter the trial, patients must begin treatment within four days of entering the hospital. Patients age 50 and older must have evidence of West Nile infection in their spinal fluid. Patients age 18-49 must have clinical evidence of West Nile encephalitis.

Half of the people enrolled in the three-week study will get a placebo -- a fake drug -- instead of Intron A. Neither the people in the trial nor their doctors will know whether they are getting the real drug or the placebo. This kind of study is the only sure way to know whether the drug really works.

Until those results are known, people sickened by the virus receive intensive supportive care to control symptoms while they recover. In severe cases, they may be admitted to the hospital, and receive intravenous fluids, breathing support, and antibiotics to prevent other infections.

If you suspect you have been infected by West Nile virus, contact your healthcare provider. If anyone in your family develops symptoms such as high fever, confusion, muscle weakness, a stiff neck, and severe headache, you should contact your doctor immediately.

Q: Is there a test to see if I have it?

A: Yes, there is a special blood test your doctor can perform to determine if you have been infected with the West Nile virus. But before drawing blood, your doctor will probably ask you questions to determine if you could have been exposed to mosquitoes carrying the virus in the first place.

Q: Does everyone who gets infected become severely ill?

A: No. Experts estimate that less than 1% of those infected with West Nile virus will develop severe illness, and only 3-15% of severely ill people will die as a result. 
In people who develop severe illness, West Nile virus multiplies in that individual's blood system following transmission by an infected mosquito; the virus then travels to the brain. It interferes with normal brain functioning and causes inflammation of brain tissue (called encephalitis).

Q: Who is at risk?

A: Technically, anyone living in an area where West Nile virus has been detected can become infected. West Nile virus has been commonly found in humans and birds and other animals in Africa, Eastern Europe, West Asia, and the Middle East. In 1999, the virus was discovered in the New York City area and has since spread along the Eastern seaboard, through the south, and into the Midwest.
In the temperate zone of the world, cases of illness caused by the West Nile virus occur primarily in the late summer and early fall. In warmer climates, West Nile virus can be transmitted year-round.

Though anyone can be infected, certain populations are at higher risk of developing severe illness:

  • People over age 50
  • Anyone will an illness that suppresses the immune system (such as HIV)
  • Anyone on medications that suppress the immune system (such as certain drugs for rheumatoid arthritis or organ transplants)

Q: Could I get the virus from a blood transfusion or an organ transplant?

A: Four people who got organs from the same donor came down with confirmed or suspected West Nile disease in the summer of 2002. The CDC says "organ transplantation appears to have been the source" of these infections because the donor had West Nile virus. But it's a complicated situation. All of these people lived in areas of Florida and Georgia where mosquitoes carry the virus.

The organ donor died from injuries in a car wreck. In the unsuccessful effort to save her life, doctors gave her several blood transfusions. These transfusions came from more than 60 different people. It's possible that she got West Nile virus infection from these blood transfusions. It's also possible she got the virus from a mosquito bite before her accident. CDC and state health authorities are investigating each of the 60-odd blood donors and some dozen recipients of their blood.

In areas where West Nile virus has been around for many years, there is very little evidence that blood transfusions spread the infection. The FDA's Center for Biologics, Evaluation, and Research (CBER) is charged with ensuring the safety of the U.S. blood supply.

"If I personally needed a blood transfusion, the last thing I'd be worried about is West Nile virus infection," CBER deputy director Jesse Goodman said in a Sept. 4, 2002, press conference. "If I needed an organ transplant because I was chronically ill, the last thing I would be worried about is West Nile virus infection."

Q: What are the best ways to protect my young children, my pets, and myself?

A: The best way to protect yourself and your family is to avoid getting bitten by mosquitoes. That means:

  • Stay indoors at dawn, dusk, and the early evening (mosquitoes are more active then)
  • Wear long-sleeved shirts and long pants when outdoors
  • Spray clothing with bug repellents containing permethrin or DEET since mosquitoes may bite through thin clothing
  • Apply insect repellent SPARINGLY to exposed skin. Be especially careful when using DEET on young children (consult with your pediatrician)

Note: Vitamin B and ultrasonic devices are not effective in repelling mosquitoes. 
See our Take Action Tip Sheet for more detailed information.

Dogs and cats can become infected with the West Nile virus in the same way humans can. Like humans, they can, on rare occasion, become severely sick. But more often than not, your family pet will recover fully on its own. There is no evidence that an infected pet can transmit the virus to a human.

Q: Are pregnant women in special danger if infected?

A: There is no documented evidence that a pregnancy is at risk due to infection with West Nile virus. However, if you are pregnant and suspect that you may have symptoms of West Nile virus infection, check with your doctor. As always, check with your doctor before taking any medications when you are pregnant.

Q: If a mosquito bites me, what symptoms should I watch for, and how soon will they appear?

A: It is important to remember that less than 1% of mosquitoes carry the West Nile virus. In rare cases where a bug bite leads to infection, that person may experience mild symptoms such as fever, headache and body aches, occasionally coupled with a skin rash and/or swollen lymph glands. More severe infections may be marked by headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, paralysis and rarely, death. 
The incubation period in humans (the time it takes between getting infected and developing symptoms) is usually between three and 15 days.

Right now, there is no specific treatment for West Nile virus. In the more severe cases, intensive supportive treatment (including being admitted to the hospital, intravenous fluids, breathing support, and prevention of other infections) is used to control the symptoms while the person recovers.

Q: Can I get the virus from birds, my pets, or other people?

A: No. There is no evidence that West Nile virus is transmitted directly from birds, pets, horses, other animals or humans. However, if a mosquito were to bite an infected bird, animal, or person and then bite you, you could be infected. At this point, there have been no reports suggesting that being around pets or other people puts you at higher risk.

Q: Are scientists close to a vaccine?

A: Though there is no vaccine yet, scientists are reportedly moving steadily toward that goal. Several companies have been studying and testing potential vaccines. Additionally, any potential West Nile vaccine has been "fast-tracked" for development and approval -- meaning the FDA must act to approve any candidate vaccine within six months of submission. Despite all the effort, it is impossible to predict when a vaccine might eventually come to market.

Q: Are there other forms of mosquito-borne encephalitis I should know about?

A: There are two other important ones: St. Louis encephalitis and Japanese encephalitis.

Outbreaks of St. Louis encephalitis, caused by a virus called the St. Louis virus and spread via mosquito bites, can occur throughout most of the United States. The last major epidemic of St. Louis encephalitis occurred in the Midwest from 1974 to 1977. During that outbreak, more than 2,500 cases in 35 states were reported to the CDC. Currently, outbreaks of St. Louis encephalitis have been limited in size (usually less than 30 cases), although the potential still exists for an epidemic. The most recent outbreak of St. Louis encephalitis occurred in New Orleans in 1999, with 20 reported cases.

Japanese encephalitis is the leading cause of encephalitis due to a virus in Asia, with 30,000-50,000 cases reported annually. (Less than 1 case per year is reported in U.S. civilians and military personnel traveling to and living in Asia. Rare outbreaks in U.S. territories in Western Pacific have occurred.) It is transmitted by mosquitoes breeding in rice fields that are infected with the Japanese encephalitis virus.

All three viruses -- West Nile, Japanese, and St. Louis -- belong to a class of viruses called "flaviviruses" and are genetically related.