West Nile Virus Basics
West Nile Virus is one of several mosquito-transmitted
diseases that has been seen in Minnesota.
The typical incubation period for West Nile is 2-6
days, although it can be as long as 15 days.
Most people infected with West
Nile virus will be asymptomatic or experience a flu-like illness
20% of those bitten by an infected
mosquito will develop the symptoms of West Nile fever
1 out of 150 symptomatic people
will develop the more severe form of West Nile encephalitis
West Nile Fever
West Nile fever symptoms include:
Sudden onset of high fever
Prominent muscle aches and
Rash (more commonly associated
with West Nile fever than encephalitis)
Swollen lymph nodes
West Nile encephalitis
Less than 1% of those infected with West Nile will
develop severe neurological symptoms consistent with encephalitis or
West Nile encephalitis symptoms
The death rate among those showing symptoms of severe
West Nile infection (encephalitis or meningitis) is around 10% and
most severe cases are elderly people.
West Nile Encephalitis is a viral
disease transmitted to people and horses through the bite of an
West Nile virus is maintained in a
transmission cycle involving one or more species of mosquitoes and
birds. Current research is focusing on which mosquitoes and birds
are most important in this cycle.
West Nile Virus is usually found
in Africa and southern Europe. The virus was first reported in North
America during a 1999 outbreak of encephalitis in New York City.
Most people infected with West
Nile virus will have either no symptoms or a very mild illness. A
small percentage of people, especially elderly patients, may develop
encephalitis (inflammation of the brain). Approximately 10% of these
encephalitis cases are fatal.
Most of the severe human cases of
West Nile encephalitis begin with sudden onset of fever, headache,
stiff neck, and vomiting. The illness progresses quickly to include
confusion and other mental status changes, altered reflexes,
convulsions, and coma. There is no treatment for West Nile
encephalitis other than supportive care.
Approximately 33% of symptomatic
horses are put down or die from West Nile virus infections.
Since 1999, West Nile virus has moved rapidly to 47
states, the District of Columbia, 75 Canadian Provinces, 24 Mexican
States, Dominican Republic, El Salvador, Jamaica, and the Cayman
Islands. West Nile virus was detected in Minnesota (July 23rd, 2002).
9,858 human West Nile cases were reported in the United
States in 2003.
With our abundant mosquito and bird populations, we
expect that West Nile virus will become established in Minnesota.
Similar to other mosquito-transmitted diseases already established in
this area (LaCrosse encephalitis, Western equine encephalitis, and
Eastern equine encephalitis), West Nile virus will likely cause
sporadic illness in humans (especially elderly people) and horses.
Personal protection measures such as use of mosquito
repellents, avoiding outdoor exposures at dusk and dawn (peak feeding
time for many mosquitoes), and wearing long-sleeved shirts and long
pants can reduce the risk of West Nile encephalitis.
Removal of water-holding containers (mosquito breeding
sites) from residential areas will reduce numbers of several mosquito
The first indication of West Nile virus in an area is
usually an outbreak of the disease in wild bird populations. Over 255
species of birds have been found to be infected, but dead American
crows and Blue Jays are especially apparent in outbreak areas. If you
find dead birds during the summer months, please report them to the
Minnesota Department of Health at 612-676-5055 or