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U.S. Experiencing Rise in West Nile Virus

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September 6, 2012

The United States is currently experiencing the biggest rise in West Nile virus since 2004, according to the Centers for Disease Control and Prevention (CDC). As of September 4, the number of cases was nearly 2,000 with 87 deaths. The total for 2011 was 712 cases, the lowest since 2001.

Reported in 48 states, the ‘seasonal epidemic’ is highest in Texas with roughly 45% of cases. The mayor of Dallas, Mike Rawlings, declared a state of emergency as the number of identified cases continued to climb. The total reached 888 confirmed cases and 35 deaths as of September 4, making 2012 the worst year in the state's history. The CDC reports that when West Nile virus reaches such proportions, it is considered a ‘seasonal epidemic.’

For the latest numbers, see the CDC's West Nile Virus web page.

Named for the West Nile region in Uganda where it was first identified, the virus is transmitted to humans through the bite of an infected mosquito that has generally fed on an infected bird. Mosquito-borne infections occur between June and September, reaching their peak in mid-August.

"It is not clear why we are seeing more activity this year," said Marc Fischer, medical epidemiologist with the CDC, although a hotter summer with increased rainfall has been suggested as a major cause. Such conditions can result in increased numbers of mosquitos feeding on infected birds and spreading the disease. "Regardless of the reasons for the increase, there is no medical treatment and people should be aware of the West Nile virus activity in their area and take action to protect themselves and their family," added Fischer.

WNV is not contagious from animals to people or from person to person through casual contact. Moreover, all blood donors are screened for the virus, so the risk of transmission via blood transfusion is minimal.

Eighty percent of infected people have no symptoms or mild ones and generally recover on their own. One in five may develop a mix of high fever, joint pain, vomiting and diarrhea with variation in severity. At its worst, in about 1% of cases, West Nile virus can develop into serious neurological illnesses, such as infection and swelling of the brain (encephalitis) and/or the tissues surrounding the brain and spinal cord (meningitis) with an estimated death rate of about 10%.

Testing for West Nile virus is done only when signs and symptoms are present, thus the majority of cases, which are asymptomatic, are not detected and not reported. With symptoms, a doctor may test an individual's blood for antibodies to the virus or may perform a spinal tap (lumbar puncture) to look for antibodies in the fluid surrounding the brain and spinal cord when encephalitis or meningitis is suspected.

Currently, there are no medications to treat West Nile virus and no vaccine available to prevent it, although researchers are working on one. Individuals need to follow their own course of prevention based on the following tips from the CDC and others:

  • Avoid being outdoors at dawn and dusk when mosquitoes are most active.
  • Wear a long-sleeve shirt or long pants when outdoors at these times.
  • Apply DEET or other approved insect repellent to exposed skin or clothing.
  • Eliminate where possible all standing water, which serves as mosquito breeding areas, such as in buckets, flower pots, patio furniture covers, bird baths, etc. Other protective actions include tightening of door and window screening and cleaning gutters. 
  • Contact your local health department for information on the current situation and recommended actions to reduce risk of mosquito bites.

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used. To access online sources, copy and paste the URL into your browser.

(September 4, 2012) Center for Disease Control and Prevention. 2012 West Nile Virus Update. Available online at through Accessed September 6, 2012.

(2012, August 4) West Nile virus on the rise, health official say. CNN news. Available online at through Accessed August 20, 2012.

(August 1, 2012) Gever, John. CDC Reports Surge in West Nile Cases. Medpage Today. Available online at through Accessed August 20, 2012.

(August 12, 2010) Thompson, E. West Nile Causes, Symptoms and Treatment. Everyday Health. Available online at through Accessed August 20, 2012.

CDC. Press Release. West Nile virus disease cases up this year. August 1, 2012. Available online at through Accessed August 20, 2012.

(August 15, 2012) Climate Change and Health: Dramatic Rise in West Nile Virus. Daily Kos. Available online through Accessed August 20, 2012.

(August 22, 2012) Gardener, Amanda. West Nile Outbreak Could Be Biggest Ever: CDC. Healthday. Available online at through Accessed August 27, 2012.