West Nile Virus Symptoms & Causes

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What causes West Nile Virus?

West Nile virus is a mosquito borne illness, which means that it is caused by the bite of an infected mosquito. There is no evidence that suggests that the disease is spread by any other insect besides mosquitoes—and it can’t spread between people. While animals can become infected, they can’t spread the infection either.

In a few cases, West Nile virus has been spread through blood transfusions. However, the risk of developing the infection this way is very low due to routine screening of donated blood. There have also been a few reports of pregnant women passing on West Nile virus to their fetuses.

Risk factors

The overall risk of becoming infected with West Nile virus can depend on:

  • Time spent outdoors: People who spend a lot of time working or playing outdoors are more at risk of getting West Nile virus because they have a greater chance of being bitten by an infected mosquito. 
  • Time of year and day: In North America, cases of West Nile virus are more frequent between from July through early September, when the mosquito population is at its peak. Mosquitoes are also most active during dusk and dawn, especially in damp or heavily wooded areas.
  • Geographic region: Cases of West Nile virus have been reported in most parts of the United States, but the highest rates are seen in Western and Midwestern states.


What are the symptoms of West Nile Virus?

Most children with West Nile virus do not show any signs of illness. In 20 percent of cases, the symptoms of West Nile virus in children are “flu-like” and include:

  • Fever
  • Headache
  • Body aches
  • Tiredness
  • Joint pain
  • Swollen lymph glands
  • Skin rash 

Since West Nile-related encephalitis is caused by a virus, symptoms may appear along with signs of an upper respiratory infection (cold, sore throat), or gastrointestinal problems (diarrhea, nausea, vomiting or rash).

In less than 1 percent of cases, children develop a severe infection that causes swelling of the brain (encephalitis), which can cause brain damage. The symptoms of encephalitis depend on the part of the brain that is inflamed, the degree of inflammation and the person’s age and overall health (adults age 50 and older and people with a weakened immune system due to cancer treatments or organ transplantation are greater at risk).

Some of the most common symptoms of encephalitis include: 

  • High fever
  • Severe headache
  • Neck stiffness
  • Convulsions
  • Nausea/vomiting
  • Confusion (disorientation)
  • Tremors

When to see a doctor

Symptoms of West Nile virus usually occur anywhere from three days to two weeks after getting bitten by an infected mosquito. Although mild cases of West Nile virus can be treated at home, encephalitis needs to be treated by a doctor. See your doctor if you notice any concerning changes in your child’s behavior. Diagnosing West Nile virus early can help speed up recovery and prevent any serious complications from occurring. 

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