West Nile Virus: Symptoms And Causes

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The West Nile virus infection is mostly transmitted by mosquitoes. Most people who are infected with the West Nile virus either do not develop signs or symptoms or they have only minor ones, such as headache and fever. But, in some cases, people develop life – threatening illnesses which include inflammation of the spinal cord or brain. The mild signs and symptoms of West Nile virus infection generally go away on their own.

But, the severe signs and symptoms, such as sudden weakness, disorientation, fever and severe headache, require immediate attention. The exposure to mosquitoes where the West Nile virus exists can increase your risk of getting infected. [1] You need to protect yourself from mosquitoes by using mosquitoes repellent and wearing and clothing that covers your skin to reduce your risk. You need to avoid unnecessary outdoor activity, when mosquitoes are most prevalent, such as in the early morning, at dusk and at dawn.

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West Nile virus symptoms

Most people who are infected with West Nile virus do not have signs or symptoms.

Mild infection signs and symptoms: It is noticed that about 20% of people develop a mild infection that is known as West Nile virus [2]. Here are the most common sign and symptoms:

  • Skin rash
  • Fever
  • Fatigue
  • Headache
  • Diarrhea
  • Body aches
  • Vomiting

Serious infection signs and symptoms: It is noticed that in less than 1% of infected people, this virus can cause a serious neurological infection, including an inflammation of the membranes that are surrounding the brain and spinal cord (meningitis) and inflammation of the brain (encephalitis). [3] Here are the signs and symptoms of neurological infections:

  • Partial paralysis or muscle weakness
  • High fever
  • Seizures
  • Severe headache
  • Tremors or muscle jerking
  • Stiff neck
  • Stupor or coma
  • Disorientation or confusion

Usually, the signs and symptoms of West Nile virus last a few days, but the signs and symptoms of meningitis or encephalitis can linger for weeks or months. Some neurological effects, like the muscle weakness, can be permanent.

West Nile virus causes

Usually, this virus spreads to humans and animals via infected mosquitoes. It is known that mosquitoes become infected when they feed on infected birds. You cannot get infected from casual contact with an infected animal or person [1]. The most West Nile virus infections happen during the warm weather, when mosquitoes are active. The incubation period (this is the period between when you are bitten by an infected mosquito and the appearance of signs and symptoms of the illness) ranges from 2 – 14 days.

This virus happened in Europe, Asia, Africa and the Middle East. In the United States, it appeared in the summer of 1999, and since then it has been reported in every state, except Alaska and Hawaii, as well as in Canada [2]. There are some cases in which the West Nile virus might have spread through other routes, including transplantation and blood transfusion. Also, the blood donors are screened for the virus, substantially reducing the risk of infection from blood transfusions. Also, there have been some reports of possible transmission of this virus from mother to child during the pregnancy or breast – feeding or exposure to the virus in the lab, but these are rare and not conclusively confirmed.

Risk factors

The most cases of this virus in the United States happen June through September [4]. Cases of West Nile virus have been reported in all 48 lower states. Even if you are infected with West Nile virus, your risk of developing a serious West Nile virus – related illness is extremely small. It is estimated than less than 1% of people who are infected become severely ill. Also, most people who become sick will recover fully. Here are some risk factors for developing a severe or fatal infection:

  • Age: If you are older, then you have increased risk of getting this virus. [5]
  • Certain medical conditions: Some diseases, like kidney disease, hypertension, diabetes and cancer can increase your risk of getting West Nile virus. Also, the organ transplant can increase your risk for West Nile virus.


[1] Colpitts TM, Conway MJ, Montgomery RR, Fikrig E. West Nile virus: Biology, transmission, and human infection. Clinical Microbiology Reviews. 2012;25(4):635–48. doi:10.1128/CMR.00045-12

[2] World Health Organization. West Nile virus. 2017. Retrieved from www.who.int/news-room/fact-sheets/detail/west-nile-virus

[3] Illinois Department of Public Health. West Nile virus. Retrieved from www.idph.state.il.us/public/hb/hbwestnile.htm

[4] Petersen LR. Epidemiology of West Nile virus in the United States: Implications for arbovirology and public health. Journal of Medical Entomology. 2019;56(6):1456-62.

[5] Yao Y, Montgomery RR. Role of immune aging in susceptibility to West Nile virus.Methods in Molecular Biology. 2016;1435:235–47.


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