West Nile Virus (WNV) is an arbovirus
that can cause illness in humans, including encephalitis
(inflammation of the brain). Mosquitos get the virus by
feeding on infected birds and can then pass it on to other
birds, and occasionally to other animals and people. The
virus is not spread person-to-person.
An arboviral encephalitis is a viral
infectious disease affecting the brain that is transmitted
by a mosquito or other insect vector. The virus attacks the
brain, causing inflammation and swelling. Arboviruses belong
to several groups of viruses that usually infect birds and
are transmitted from bird-to-bird by mosquitos. The name "ar-bo-virus"
comes from the fact that they are transmitted by arthropods
(insects and other "bugs"). WNV is caused by a specific type
of arbovirus known as a flavivirus, which was found
previously only in Africa, eastern Europe and west Asia. WNV
is closely related to other arboviruses such as St. Louis
Encephalities (SLE), found in the U.S., Japanese
Encephalities, found in southeast Asia, and Murray Valley
Fever, found in Australia and New Guinea.
In the U.S., WNV cases are most
prevalent in late summer and early autumn; in Nevada,
mosquito season is typically April through October.
Prior to 1999, no WNV cases had been
reported in the United States, although the virus had a wide
distribution in Africa, Asia, the middle east and Europe and
caused occasional epidemics. In 1999, the virus appeared in
New York City, and was responsible for an encephalitis
outbreak causing 62 human cases and seven deaths.
Between 1999 and 2003, WNV expanded
from the northeastern U.S. to the south central states and
westward, appearing on the west coast for the first time in
2002. Across the U.S., the number of human WNV cases in 2002
increased dramatically and greatly expanded in geographic
range (4,161 reported cases with 277 deaths). The 2002
epidemic was the largest WNV epidemic ever reported and the
largest of any reported arboviral encephalitis epidemic in
the Western Hemisphere.
In 2003, WNV activity continued to
increase, and was reported in 48 states and the District of
Columbia. Today, WNV is firmly established throughout most
of the US.
WNV was first detected in Nevada in
2004, and has been reported in all counties.