Most people bitten by a mosquito carrying West Nile virus (WNV) do not get sick.
The majority of people who are bitten by an infected mosquito do not notice any symptoms. It is thought that those who have been exposed to WNV develop lifelong immunity.
West Nile fever is a minor, self-limiting illness that resembles the flu Table 01.
Only about 20 - 30% of those infected with the West Nile virus will notice symptoms of illness. If noticeable symptoms of illness occur, they will develop 3 to 14 days after being infected. Those exhibiting mild symptoms are said to have West Nile fever Table 01. Those with West Nile fever usually do not require medical treatment, and will recover fully.
Table 1. Symptoms of West Nile fever
Slight fever Headache Body aches in the muscles and/or the joints Fatigue Occasionally, a skin rash and swollen lymph glands may also occur
West Nile encephalitis is a serious illness that occurs in less than 1% of those infected with the virus Table 02.
Once the West Nile virus is transmitted to a person through a mosquito bite, the virus multiplies in the bloodstream. The blood-brain barrier exists to protect the brain from infection. However, the West Nile virus is capable of crossing the blood-brain barrier, which may cause encephalitis.
Encephalitis is when there is inflammation (swelling) of the brain. Encephalitis can be caused by bacteria, fungi, or viruses, including the West Nile virus. Encephalitis happens when brain tissue becomes irritated due to exposure to these germs. When the tissues are irritated, they swell. See Table 02 for symptoms of West Nile encephalitis. These symptoms may last several weeks.
Meningitis may sometimes occur with encephalitis. Meningitis is the swelling of the membranes (meninges) that surround the brain and spinal cord. A small number of people with West Nile Virus affecting the nervous system develop paralysis, usually in one part of the body. They may or may not also have meningitis or encephalitis.
West Nile encephalitis is fatal in 10-15% of cases. Those that survive may suffer permanent disability. Lasting effects on the nervous system may include problems walking, talking, and thinking. The permanent effects may be mild or severe.
Table 2. Symptoms of West Nile encephalitis
High fever Change in behavior, personality, or consciousness (alertness). This may include confusion, disorientation, and even memory loss or hallucinations Intense headache Severe muscle weakness that may progress to paralysis (inability to move the muscles) Fatigue/lethargy Photophobia (eyes sensitive to light) Tremors Seizures Coma, which may lead to death If accompanied with meningitis, a stiff neck may also occur
The risk of being bitten by an infected mosquito is very small. People who live in or travel to areas where West Nile virus (WNV) is active have a higher risk of being exposed to the virus.
Only about 2% of mosquitoes in an affected region carry the virus during the peak of a WNV outbreak. If an infected mosquito bites you, your risk of developing serious illness from the virus is very low. Only 20-30% of those infected with the virus develop West Nile fever (minor, flu-like symptoms). Less than 1% of those that get the virus become seriously ill with West Nile encephalitis.
Advanced age or a weakened immune system can increase your risk of becoming ill after exposure to the virus.
Persons older than 50 years of age have the greatest chance of becoming seriously ill with WNV. People with weakened immune systems, such as the chronically ill, also have a greater risk of becoming ill with WNV. Individuals in the higher-risk groups should be especially careful to avoid mosquito bites (see Prevention).
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