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West Nile Virus (WNV)

Human, mosquito, wildlife and equine surveillance for West Nile Virus and other arboviral infections of public health significance began on July 1, 2012. On 8-10-12 DHMH sent out a press release with the current data on cases: West Nile Virus Detected in Maryland Resident and Mosquitoes

General History of Disease
Arthropod-borne viruses (arboviruses) are pathogens affecting human and animal health throughout much of the world. All arboviruses are maintained in nature in a complex cycle involving nonhuman vertebrate hosts (birds and small mammals primarily) and arthropod vectors (mosquitoes, other flies and ticks).

Much has been learned about West Nile virus (WNV) since its first appearance in the Western hemisphere ten years ago. West Nile virus, though known since 1937, had never been detected in the United States until late August and September 1999, when 62 people in New York City and the surrounding areas became ill. Seven of the confirmed cases died.

Observations during the 2000 outbreak indicated a far broader geographic distribution of the virus and a decrease in the number of fatalities compared to 1999. This decrease may have been due to an aggressive nationally integrated detection and response system. In 2001, WNV became firmly established along the east coast, gulf coast and mid-western states, and affected more than 66 people in 10 states, resulting in 9 fatalities (13.6% case-fatality rate). In the years that followed, West Nile virus activity has expanded rapidly across the country and subsequently receded in the East Coast areas.

Illnesses Caused by WNV
Generally, WNV does not cause symptoms in most people who have been exposed to it. However, in some individuals, WNV can cause a mild to moderate infection including fever, muscle aches, rash, swollen lymph nodes, and a "sick" feeling. This illness, West Nile Fever, typically begins approximately 3-15 days after the mosquito bite, lasts a few days, and then subsides. In rare instances, some febrile patients may have long-lasting neurological effects.

A very small percentage (<1%) of people exposed to WNV or other arboviruses can experience an inflammation of the brain (encephalitis) or of the tissues that surround the brain (meningitis). Encephalitis and aseptic meningitis due to arboviruses can cause death. Since the 1999 New York City outbreak, deaths from WNV infection have occurred primarily among persons over 50 years of age. Researchers are currently investigating why some people seem to get seriously ill from WNV, while others do not. Clinical researchers are continuing to work on development of human vaccines for WNV.

About the Disease in Maryland
In Maryland, the arboviruses known to seriously affect human health include Eastern equine encephalitis virus (EEE), St. Louis encephalitis virus (SLE) and West Nile virus (WNV). A fourth arbovirus, LaCrosse encephalitis, may occur in Maryland. EEE is known to be endemic in Maryland and affects equines primarily, with irregular outbreaks affecting people. EEE is a rural disease associated with swamps and marshes. An epidemic (occurrence of disease in humans) of SLE occurred in Central and Eastern Maryland in the mid-1970's, with no evidence of recurrence since 1975.

Maryland's experience with WNV and other arboviruses has been similar to that of many other regions of the country. In the 2001 season, Maryland reported 6 human cases with 3 fatalities which quickly jumped to 36 human cases with 7 fatalities in 2002 and then rose to 73 human cases with 8 fatalities during the peak year of 2003. In 2004, 16 human cases (no fatalities), 31 mosquito pools, and one equine were confirmed with West Nile infection in Maryland. The number of human cases in the State declined once more in 2005, with 5 cases (no fatalities) reported.

In 2006, this number jumped to 11 cases (including one fatal case). In 2007, Maryland officials documented relatively low levels of WNV activity in both human and non-human populations. A total of 10 human cases were documented in six Maryland jurisdictions. There were also 6 WNV-positive mosquito pools and one EEE-positive mosquito pool confirmed in Maryland that year.

Similar arboviral results were reported in 2008, with 14 human cases, 23 WNV-positive mosquito pools, two WNV-positive horses, four EEE-positive mosquito pools and four mosquito pools positive for Cache Valley Virus. Decreased arboviral activity was observed in 2009 with a slight rise in 2010.

In 2009, Maryland reported one human WNV case, nine mosquito pools, and one horse positive for WNV, and an additional horse confirmed with EEE infection.

In 2010, there were 23 human WNV cases, eight mosquito pools, one WNV-positive horse, and two human Lacrosse encephalitis cases.

Last year in 2011, 19 human WNV cases were reported, including one fatality. In addition, 15 mosquito pools, 8 birds, and two horses were reported with WNV infection. A single human case of St. Louis encephalitis was also reported and six mosquito pools were confirmed with Cache Valley Virus.

Things You Should Know

Key points:

  • The general public is at low risk of being infected with WNV or other arboviruses.
  • WNV and other arbovirus are spread by the bite of a mosquito.
  • Reduce the risk of getting bitten by a mosquito (use repellents, wear clothing on extremities, stay indoors at dawn and dusk, routine screen maintenance).
  • Eliminate sites around residential areas, commercial establishments, and recreational areas where mosquitoes can breed and develop.
  • Contact the Maryland Department of Agriculture at 410-841-5870 for further information about mosquito control.
  • Reduce you and your family's exposure to any insecticide being applied and if you feel you or they are ill contact your primary care physician. Information regarding possible side effects or symptoms of exposure will be available on the DHMH Office of Environmental Health Coordination website at
  • The latest arboviral surveillance findings will be made available and updated regularly on the Maryland West Nile Virus Information page at
  • Surveillance for West Nile virus activity in dead birds in Maryland was discontinued as of 2003. Residents reporting sick or injured birds may call the Wildlife Nuisance Animal Hotline at 877-463-6497 for information about wildlife rehabilitators in their area.

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