West Nile virus is transmitted by mosquitoes. If you become infected with West Nile virus, you may not experience any signs or symptoms or you may only experience minor ones such as a skin rash and headache. However, some people who become infected with West Nile virus develop a life-threatening illness that includes inflammation of the brain.

West Nile virus is common in areas such as Africa, West Asia and the Middle East. It first appeared in the United States in the summer of 1999 and since then has been found in all 48 contiguous states.

Exposure to mosquitoes where West Nile virus exists increases your risk of getting West Nile virus. Protecting yourself from such exposure, such as by using mosquito repellent and wearing clothing that covers your skin, can reduce your risk.

Most have no symptoms
Most people infected with the West Nile virus have no signs or symptoms.

Mild infection signs and symptoms
About 20 percent of people develop a mild infection called West Nile fever. Common signs and symptoms of West Nile fever include:

• Skin rash
• Headache
• Fever
• Diarrhea
• Nausea
• Vomiting
• Backache
• Muscle aches
• Lack of appetite
• Swollen lymph glands

Serious infection signs and symptoms
In less than 1 percent of infected people, the virus causes a serious neurological infection. Such infection may include inflammation of the brain (encephalitis) or of the brain and surrounding membranes (meningoencephalitis). Serious infection may also include infection and inflammation of the membranes surrounding the brain and spinal cord (meningitis), inflammation of the spinal cord (West Nile poliomyelitis) and acute flaccid paralysis — a sudden weakness in your arms, legs or breathing muscles. Signs and symptoms of these diseases include:

• High fever
• Severe headache
• Stiff neck
• Disorientation or confusion
• Stupor or coma
• Tremors or muscle jerking
• Signs and symptoms similar to Parkinson's disease
• Lack of coordination
• Convulsions
• Pain
• Partial paralysis or sudden weakness

Symptoms of West Nile fever usually last three to six days, but symptoms of encephalitis or meningitis can linger for weeks, and certain neurological effects, such as muscle weakness, may be permanent.

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West Nile virus transmission cycle

Infection transmitted by mosquitoes
In the United States, wild birds, especially crows and jays, are the main reservoir of West Nile virus, but the virus is actually spread by certain species of mosquitoes.

Here's how the transmission cycle works:

When a mosquito bites a bird infected with the West Nile virus, the virus enters the mosquito's bloodstream and circulates for a few days before settling in the salivary glands. When the infected mosquito bites an animal or a human, the virus then enters the host's bloodstream, where it may cause serious illness.

Although the exact mechanism of illness is unknown, West Nile virus probably enters the host's bloodstream, multiplies and moves on to the brain, crossing the blood-brain barrier — a barrier that separates the blood from the central nervous system. Once the virus crosses that barrier and infects the brain or its linings, the brain tissue becomes inflamed and symptoms arise.

West Nile virus transmission occurs mostly during warm weather, when mosquito populations are active. The incubation period — the period between when you're bitten by an infected mosquito and the appearance of signs and symptoms of the illness — ranges from two to 14 days.

Other possible routes of transmission
In rare cases, it's possible for West Nile virus to spread through other routes, including:

• Organ transplantation and blood transfusion. Donated organs are not yet screened for West Nile virus. However, blood donors are screened for the virus, substantially reducing the risk of infection from blood transfusions.
• Mother to unborn child. Although it appears that a mother may pass the virus to her unborn baby, it's not known whether this can result in abnormalities in the child.
• Breast-feeding. In a few instances, mothers have transmitted West Nile viruses to their infants through their breast milk. But experts at the Centers for Disease Control and Prevention (CDC) say that such cases are extremely rare and should not affect a woman's decision to breast-feed her baby.
• Laboratory acquisition. Some laboratory workers involved in West Nile surveillance and research have contracted the disease from infected animals.

Your overall risk of getting West Nile virus depends on these factors:

• Time of year. In temperate regions, West Nile virus follows a seasonal pattern starting in early spring, with the peak time for infection occurring in late summer and early fall. If you live in southern climates, you may run the risk of infection year-round.
• Geographic region. Visiting or living in areas of the country where mosquito-borne viruses are common increases your risk of exposure to West Nile virus. But West Nile virus has been reported in all 48 contiguous states, including desert regions such as Arizona.
• Time spent outside. If you work or spend time outdoors, you have a greater chance of being bitten by an infected mosquito.

Even if you are infected, your risk of developing a serious West Nile virus-related illness is extremely small — less than 1 percent of people who are bitten become severely ill. And most people who do become sick recover fully. You're more likely to develop a severe or fatal infection if you:

• Are an adult over the age of 50
• Have a weakened immune system, such as from receiving an organ transplant

See your doctor as soon as possible if you experience signs and symptoms that might suggest West Nile infection — especially if you're pregnant or breast-feeding. Although most people infected with the virus recover fully, the virus can result in serious, life-threatening illness or permanent muscle problems.

Your doctor can confirm the presence of West Nile virus in your body by analyzing a sample of your blood or the fluid surrounding your spinal cord (cerebrospinal fluid). Signs of the disease include:

• A rising level of antibodies to the West Nile virus. Antibodies are immune system proteins that attack foreign substances, such as viruses.
• A positive ribonucleic acid (RNA) test for the West Nile virus.

If your doctor suspects a serious, West Nile virus-related illness such as meningitis or encephalitis, you may have one or more of the following tests:

• Lumbar puncture (spinal tap). The most common way to diagnose meningitis is to analyze the cerebrospinal fluid surrounding your brain and spinal cord. A needle inserted between the lower vertebrae of your spine is used to extract a sample of fluid for laboratory analysis. The fluid sample may show an elevated white cell count — a signal that your immune system is fighting an infection — and antibodies to the West Nile virus.
• Brain imaging. In some, but not all, cases, a CT or MRI scan can reveal brain inflammation and swelling.

Supportive therapy
Most people recover from West Nile virus without treatment. Even those who develop encephalitis or meningitis may only need supportive therapy with intravenous fluids and pain relievers.

Interferon therapy
Scientists are investigating interferon therapy — a type of immune cell therapy — as a treatment for encephalitis caused by West Nile virus. Some research shows that people who receive interferon may recover better than those who don't receive the drug, but more study is needed.

Your best bet for preventing West Nile virus and other mosquito-borne illnesses is to avoid exposure to mosquitoes and eliminate mosquito-breeding sites. To help control West Nile virus:

• Eliminate standing water in your yard. Mosquitoes breed in pools of standing water.
• Unclog roof gutters.
• Empty unused swimming pools.
• Change water in birdbaths at least weekly.
• Remove old tires or any unused containers that might hold water and serve as a breeding place for mosquitoes.
• Watch for sick or dying birds and report them to your local health department.

To reduce your own exposure to mosquitoes:

• Avoid unnecessary outdoor activity when mosquitoes are most prevalent, such as at dawn, dusk and early evening.
• Wear long-sleeved shirts and long pants when you go into mosquito-infested areas.
• Apply mosquito repellent with DEET to your skin and clothing. Choose the concentration based on the hours of protection you need — the higher the percentage (concentration) of the active ingredient, the longer the repellent will work. Keep in mind that chemical repellants can be toxic, and use only the amount needed for the time you'll be outdoors. Don't use DEET on the hands of young children or on infants under 2 months of age. Instead, cover your infant's stroller or playpen with mosquito netting when outside. According to the Centers for Disease Control and Prevention, oil of lemon eucalyptus, a more natural product, offers the same protection as DEET when used in similar concentrations.

A vaccine is available to protect horses from West Nile virus. No vaccine is available for humans, but work to develop a human vaccine is under way.