| The alphavirus western equine
encephalitis (WEE) was first isolated in California in 1930
from the brain of a horse with encephalitis, and remains an
important cause of encephalitis in horses and humans in North
America, mainly in western parts of the USA and Canada. In the
western United States, the enzootic cycle of WEE involves passerine
birds, in which the infection is inapparent, and culicine mosquitoes,
principally Cx. tarsalis, a species that is associated with
irrigated agriculture and stream drainages. The virus has also
been isolated from a variety of mammal species. Other important
mosquito vector species include Aedes melanimon in California,
Ae. dorsalis in Utah and New Mexico and Ae. campestris in New
Mexico. WEE virus was isolated from field collected larvae of
Ae. dorsalis, providing evidence that vertical transmission
may play an important role in the maintenance cycle of an alphavirus.
Expansion of irrigated agriculture in the North Platte River
Valley during the past several decades has created habitats
and conditions favorable for increases in populations of granivorous
birds such as the house sparrow, Passer domesticus, and mosquitoes
such as Cx. tarsalis, Aedes dorsalis and Aedes melanimon. All
of these species may play a role in WEE virus transmission in
irrigated areas. In addition to Cx. tarsalis, Ae. dorsalis and
Ae. melanimon, WEE virus also has been isolated occasionally
from some other mosquito species present in the area. Two confirmed
and several suspect cases of WEE were reported from Wyoming
in 1994. In 1995, two strains of WEE virus were isolated from
Culex tarsalis and neutralizing antibody to WEE virus was demonstrated
in sera from pheasants and house sparrows. During 1997, 35 strains
of WEE virus were isolated from mosquitoes collected in Scotts
Bluff County, Nebraska.
Human WEE cases are usually first seen in June or July. Most
WEE infections are asymptomatic or present as mild, nonspecific
illness. Patients with clinically apparent illness usually have
a sudden onset with fever, headache, nausea, vomiting, anorexia
and malaise, followed by altered mental status, weakness and
signs of meningeal irritation. Children, especially those under
1 year old, are affected more severely than adults and may be
left with permanent sequelae, which is seen in 5 to 30% of young
patients. The mortality rate is about 3%.
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