West Nile Virus

The West Nile virus is an arbovirus (arthropod-borne virus). It is transmitted between susceptible hosts via blood-feeding insect (arthropod) vectors such as ticks and mosquitoes. Like all viruses, the West Nile virus requires a living host in order to replicate. The primary hosts of the West Nile virus are wild birds.

This figure shows a surface-shaded image of the West Nile virus particle produced by Purdue University biologists using cryoelectron microscopy. The surface is composed of proteins that enable the virus to bind with and invade a host cell. The particle is approximately 50 nanometers in diameter, or about 1/1000th of the width of a human hair. (Purdue Department of Biological Sciences image)

The virus, first identified in 1937 in the West Nile region of Uganda, was detected in the United States two years ago in New York State.

West Nile virus is a mosquito-borne disease that can cause encephalitis, or a brain infection. Mosquitoes acquire the virus from birds and pass it on to other birds, animals and people. Mosquitoes spread this virus after they feed on infected birds and then bite people, other birds and animals. It is not spread by person-to-person contact and there is no evidence that people can get the virus by handling infected animals. West Nile virus cases occur primarily in the late summer or early Fall, although the mosquito season is April through October.

"Encephalitis" means an inflammation of the brain and can be caused by viruses and bacteria, including viruses transmitted by mosquitoes. West Nile encephalitis is an infection of the brain caused by West Nile virus, a flavivirus commonly found in Africa, West Asia, and the Middle East. It is closely related to St. Louis encephalitis virus found in the United States. West Nile virus has been commonly found in humans and birds and other vertebrates in Africa, Eastern Europe, West Asia, and the Middle East, but until 1999 had not previously been documented in the Western Hemisphere. It is not known from where the U.S. virus originated, but it is most closely related genetically to strains found in the Middle East.

How is it transmitted?

The West Nile virus is an arborvirus (arthropod-borne virus), transmitted between hosts via a blood-feeding insect (called the virus vector). The primary hosts of the West Nile virus are wild birds, and the primary vectors are bird-feeding mosquitoes, mainly of the Culex species. These organisms are vital for the virus’ transmission cycle: birds provide an environment in which the virus can replicate at high levels – high enough that blood picked up by mosquitoes contains virus particles. The mosquitoes are needed for the spread of the virus particles to new hosts.

In some cases, animals other than the primary hosts may become infected with the virus. The West Nile virus, for example, can also infect humans. These alternative hosts are often referred to as “dead end” hosts, as they are not part of the virus’ normal transmission cycle. These organisms do not usually produce enough virus particles to allow them to become a reservoir for the virus. The infection thus reaches a dead end in this host.

Mosquitoes are usually the primary vectors for transmission of the West Nile virus. In Africa and Asia, however, ticks have been found carrying the virus.

West Nile virus is only spread via its insect vector; it is NOT transmitted person-to-person or bird-to-person.

In warmer climates, the virus can be found year-round. In temperate climates, the virus usually occurs in late summer or early fall (ie – in peak mosquito season). It is not known how the virus returns at the end of the cold season, although there are a few theories:

The virus may:

survive in hibernating Culex mosquitoes
be transmitted to offspring by infected female Culex mosquitoes
establish a chronic infection in its hosts
get reintroduced each spring by chronically infected migratory birds.

CDC Image

Symptoms

Symptoms generally occur five to 15 days following the bite of an infected mosquito. Most people who are infected have no symptoms or may experience a fever and headache before fully recovering. People with mild infections may experience fever, headache, body aches, skin rash and swollen lymph glands. More severe infections result in high fever, headache, neck stiffness, stupor, disorientation, tremors, occasional convulsions and paralysis. At its most serious, the infection can result in coma, permanent neurological damage, and death. There is no vaccine and treatment is directed at relieving symptoms. Because they are viral infections, antibiotics are not effective and there is no specific treatment available.

What Is West Nile Virus?
West Nile virus (WNV) is a potentially serious illness. Experts believe WNV is established as a seasonal epidemic in North America that flares up in the summer and continues into the fall. This fact sheet contains important information that can help you recognize and prevent West Nile virus.

What Are the Symptoms of WNV?
WNV affects the central nervous system. Symptoms vary.

No Symptoms in Most People. Approximately 80 percent of people who are infected with WNV will not show any symptoms at all.
Mild Symptoms in Some People. Up to 20 percent of the people who become infected will display mild symptoms, including fever, headache, and body aches, nausea, vomiting, and sometimes swollen lymph glands or a skin rash on the chest, stomach and back. Symptoms typically last a few days.
Serious Symptoms in a Few People. About one in 150 people infected with WNV will develop severe illness. The severe symptoms can include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis. These symptoms may last several weeks, and neurological effects may be permanent.

How Does It Spread?

Infected Mosquitoes. Generally, WNV is spread by the bite of an infected mosquito. Mosquitoes are WNV carriers that become infected when they feed on infected birds. Infected mosquitoes can then spread WNV to humans and other animals when they bite.
Transfusions, Transplants, and Mother-to-Child. In a very small number of cases, WNV also has spread through blood transfusions, organ transplants, breastfeeding and even during pregnancy from mother to baby.
Not through touching. WNV is not spread through casual contact such as touching or kissing a person with the virus.

How Soon Do Infected People Get Sick?
People typically develop symptoms between 3 and 14 days after they are bitten by the infected mosquito.

How Is WNV Infection Treated?
There is no specific treatment for WNV infection. In cases with mild symptoms, people experience symptoms such as fever and aches that pass on their own. In more severe cases, people usually need to go to the hospital where they can receive supportive treatment including intravenous fluids, help with breathing and nursing care.

What Should I Do if I Think I Have WNV?
Mild WNV illness improves on its own, and people do not necessarily need to seek medical attention for this infection. If you develop symptoms of severe WNV illness, such as unusually severe headaches or confusion, seek medical attention immediately. Severe WNV illness usually requires hospitalization. Pregnant women and nursing mothers are encouraged to talk to their doctor if they develop symptoms that could be WNV.

What Is the Risk of Catching WNV?
For most, risk is low. Less than 1 percent of people who are bitten by mosquitoes develop any symptoms of the disease and relatively few mosquitoes actually carry WNV.

Greater risk for those outdoors a lot. People who spend a lot of time outdoors are more likely to be bitten by an infected mosquito. They should take special care to avoid mosquito bites.

People over 50 can get sicker. People over the age of 50 are more likely to develop serious symptoms of WNV if they do get sick and should take special care to avoid mosquito bites.

Risk through medical procedures is low. The risk of getting WNV through blood transfusions and organ transplants is very small, and should not prevent people who need surgery from having it. If you have concerns, talk to your doctor before surgery.

Pregnancy and nursing do not increase risk of becoming infected with WNV.

What Can I Do to Prevent WNV?
The easiest and best way to avoid WNV is to prevent mosquito bites.

When you are outdoors, use insect repellents containing DEET (N, N-diethyl-meta-toluamide). Follow the directions on the package.
Many mosquitoes are most active at dusk and dawn. Consider staying indoors during these times or use insect repellent and wear long sleeves and pants. Light-colored clothing can help you see mosquitoes that land on you.
Make sure you have good screens on your windows and doors to keep mosquitoes out.
Get rid of mosquito breeding sites by emptying standing water from flower pots, buckets and barrels. Change the water in pet dishes and replace the water in bird baths weekly. Drill drainage holes in tire swings so water drains out. Keep children's wading pools empty and on their sides when they aren't being used.

What Is the CDC Doing About WNV?
CDC is working with state and local health departments, the Food and Drug Administration and other government agencies, as well as private industry, to prepare for and prevent new cases of WNV.

Some things CDC is doing include:

Coordinating a nation-wide electronic database where states share information about WNV
Helping states develop and carry out improved mosquito prevention and control programs
Developing better, faster tests to detect and diagnose WNV
Creating new education tools and programs for the media, the public, and health professionals
Opening new testing laboratories for WNV

What Else Should I Know?

If you find a dead bird: Don't handle the body with your bare hands. Contact your local health department for instructions on reporting and disposing of the body.

Credit: CDC, Microbiology Department, Mount Sinai Hospital, Toronto, ON, Canada, Purdue University