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                                        West Nile Virus

Alaska Epidemiology Bulletin No. 9 - May 6, 2003
USGS West Nile Virus Disease Surveillance Maps - U.S.A.
Equine West Nile Virus Disease - Alberta, Canada
Center for Disease Control - West Nile Virus

WNV CycleBasic Facts about West Nile Virus in Alaska

The virus must be transmitted from an infected bird to horse (or man) by the same mosquito.

The virus must first be brought into Alaska by a migrating bird.  An infected horse (or most other species) can not generally transmit the disease.

 The State of Alaska, Department of Health and Social Services, Division of Public Health, released the following statement in their May 6, 2003 Epidemiology Bulletin:                                                                        

 Alaska is unlikely to become a WNV endemic or enzootic zone for several reasons. Birds that serve as WNV reservoirs must be viremic at the time of the blood meal for a mosquito to become infected. Viremia is transient (estimated at 2-4 days); therefore, most birds migrating to Alaska from an enzootic zone are likely to have cleared the virus before arrival. As well, the mosquito species that are the most efficient vectors of WNV are not present in Alaska. Finally, mosquitoes require at least 10 days at 30°C (or 86°F) to amplify the virus. Locally-acquired WNV could occur only if viremic migratory birds arrive in Alaska when the appropriate species of mosquitoes are active and when temperatures would permit adequate amplification of virus. With all those factors in place, virus could potentially spill over into non-migratory birds, humans, horses, or other Alaska animals.”

The State of Alaska has publicly stated that the West Nile Virus will unlikely occur in Alaska.  Temperature extremes and lack of diseased bird carriers make the potential of horse infection unlikely.

For owners that intend to travel outside with their horses, vaccine is important.  Generally, The West Nile Vaccine is a 2 shot series three to four weeks apart.   After the two shot series, an annual booster is required, similar to most other vaccines.  The initial vaccine series must be administered at least 4 weeks prior to the mosquito season.  

A large number of mosquito species can transmit the disease.  The following is a list of known carriers of the West Nile Virus.  Alaska has only a small number of species able to transmit West Nile Virus.

 Mosquito Species that can transmit West Nile Virus  

Aedes species

Aedes albopictus

Aedes aegypti

Aedes vexans

Aedes cinereus

Anopheles species

Anopheles barberi

Anopheles atropos

Anopheles crucians/bradleyi

Anopheles punctipennis

Anopheles quadrimaculatus

Anopheles walkeri

Coquillettidia species

Coquillettidia perturbans

Culiseta species

Culiseta inornata

Culiseta melanura

Culex species

Culex erraticus

Culex nigripalpus

Culex pipiens

Culex quinquefasciatus

Culex restuans

Culex salinarius

Culex tarsalis

Culex territans

Deinocerites species

Deinocerites cancer

Ochlerotatus species

Ochlerotatus atropalpus

Ochlerotatus atlanticus/tormentor

Ochlerotatus canadensis

Ochlerotatus cantator

Ochlerotatus japonicus

Ochlerotatus sollicitans

Ochlerotatus taeniorhynchus

Ochlerotatus triseriatus

Ochlerotatus trivittatus

Orthopodomyia species

Orthopodomyia signifera

Psorophora species

Psorophora ciliata

Psorophora columbiae

Psorophora ferox

Uranotaenia species

Uranotaenia sapphirina

*Mosquito data was derived from the Center for Disease Control web site as of 1999

Only four of the above listed species of mosquito are native to Alaska. The following information about Alaska mosquitoes was graciously provided to me by:

 James J. Kruse, Ph.D.
Adjunct Faculty and Curator of Entomology
University of Alaska Museum
907 Yukon Drive
Fairbanks, AK, USA 99775-6960
http://www.uaf.edu/museum/collections/ento/

 Aedes vexans
Aedes vexans is a summer mosquito, and probably wouldn't be a problem unless
WNV became established in the local bird population. In that case, it would
probably serve to amplify the problem. It is only single brooded in
Alaska,
which may serve to temper any WNV outbreak potential. It ranges in an area
roughly bounded by the
Alaska range, but also lives in the Tanana River
valley at least as far north as
Fairbanks, and east and south throughout the
area. It overwinters in the egg stage. Eggs are laid on soil in areas likely
to be flooded by spring melt water. Preferred oviposition sites also contain
detritus and good herbaceous cover. The larvae feed on bacteria, so stagnant
pools/vernal ponds that contain very little dissolved oxygen are best for
them. Multiple re-flooding of areas may bring on a surge in population,
since not all eggs hatch with an initial flooding.

Aedes cinereus
    Aedes cinereus is a late spring mosquito, is easily identified, and is
single brooded. It has been taken in tundra zones near
Nome and Kotzebue and
ranges east and south throughout the state from those cities. Larvae are
found in early, cold woodland pools, partially shaded, shallow, and semi
permanent. Carex and Calamagrostis marshes are favored habitats. Late stage
larvae have been collected in the
Fairbanks area between May 26 - July 15,
peak June 3. Late stage larvae can be found in
Anchorage after June 20, and
sometimes persist into October. It has never been considered a pest in
Alaska.

Culex territans
 Culex territans: Not much exists in the literature on this species as it is
not implicated in any other human disease. I am surprised that it has been
implicated with WNV at all since it typically prefers cold-blooded
vertebrates, especially frogs. Interestingly enough, it is also implicated
in canine heartworm, which would seem equally unlikely. Makes one wonder
about cryptic species... It is not common anywhere in its range, which (in
Alaska) is forested lowlands from Valdez and the Cook Inlet area north to
the
Yukon river. Larvae prefer semi permanent or permanent pools, ponds,
rivers and lake shores with Equisetum, Carex rostrata, or Scirpus validus,
Ranunculus, and Lemna cover. It appears to be a summer mosquito. I don't
assess this one as a threat to humans at all.

Ochlerotatus canadensis
Ochlerotatus canadensis is a spring/fall mosquito, and is easily identified.
Mated adult females over winter. This one could be a problem when viremic
migrating birds are present in the spring. It ranges in an area roughly
bounded by
Bethel, Galena, south slope of the Brooks Range and east and
south throughout the area. It is a rather aggressive species that can be a
problem locally. It is documented as being the dominant pest mosquito
starting in late July near Eielson AFB (near North Pole, AK, in the
Interior). However, it is very particular in its larval habitat, normally
present in SHADED temporary to semi-permanent pools CONTAINING FALLEN
LEAVES, and thus relatively easily controlled around urban areas using
environmental control methods.

The Center for Disease Control web site lists the known bird carriers of the West Nile Virus.  A large number of the birds on the list will migrate to Alaska.

Native North American Bird Species Known to Carry WNV

Bittern, Least - Ixobrychus exilis
Blackbird, Brewer's - Euphagus cyanocephalus
Blackbird, Red-winged - Agelaius phoeniceus
Blackbird, Rusty - Euphagus carolinus
Bluebird, Eastern - Sialia sialis
Bufflehead - Bucephala albeola
Bobwhite, Northern - Colinus virginianus
Canvasback - Aythya Valisinera
Cardinal, Northern - Cardinalis cardinalis
Catbird, Gray - Dumetella carolinensis
Chickadee, Black-capped - Poecile atricapillus
Chickadee, Carolina - Poecile carolinensis
Cormorant, Double-crested - Phalacrocorax auritus
Cowbird, Brown-headed - Molothrus ater
Crane, Mississippi Sandhill - Grus canadensis pulla
Crane, Sandhill - Grus canadensis
Crane, Whooping - Grus americana
Crow, American - Corvus brachyrhynchos
Crow, Fish - Corvus ossifragus
Cuckoo, Yellow-billed - Coccyzus americanus
Dickcissel - Spiza americana
Dove, Mourning - Zenaida macroura
Duck, Wood - Aix sponsa
Eagle, Bald - Haliaeetus leucocephalus
Eagle, Golden - Aquila chrysaetos
Egret, Great - Ardea alba
Falcon, Prairie - Falco mexanicus
Finch, House - Carpodacus mexicanus
Finch, Purple - Carpodacus purpureus
Flamingo, American - Phoenicopterus ruber
Flicker, Northern - Colaptes auratus
Flycatcher, Scissor-tailed - Tyrannus forficatus
Flycatcher, Traill's - Empidonax traillii/alnorum
Goldeneye, Common - Bucephala clangula
Goldfinch, American - Carduelis tristis
Goose, Canada - Branta canadensis
Goose, Emperor - Chen canagica
Goose, Snow - Chen caerulescens
Goshawk, Northern - Accipiter gentilis
Grackle, Boat-tailed - Quiscalus major
Grackle, Common - Quiscalus quiscula
Grackle, Great-tailed - Quiscalus mexicanus
Grebe, Pied-billed - Podilymbus podiceps
Ground-Dove, Common - Columbina passerina
Grouse, Ruffed - Bonasa umbellus
Gull, Great Black-backed - Larus marinus
Gull, Herring - Larus argentatus
Gull, Laughing - Larus atricilla
Gull, Ring-billed - Larus delawarensis
Hawk, Broad-winged - Buteo platypterus
Hawk, Cooper's - Accipiter cooperii
Hawk, Harris' - Parabuteo unicinctus
Hawk, Red-shouldered - Buteo lineatus
Hawk, Red-tailed - Buteo jamaicensis
Hawk, Rough-legged - Buteo lagopus
Hawk, Sharp-shinned - Accipter striatus
Hawk, Swainson's - Buteo swainsoni
Heron, Great Blue - Ardea herodias
Heron, Green - Butorides virescens
Hummingbird, Ruby-throated - Archilochus colubris
Jay, Blue - Cyanocitta cristata
Jay, Steller's - Cyanocitta stelleri
Kestrel, American - Falco sparverius
Killdeer - Charadrius vociferus
Kingbird, Eastern - Tyrannus tyrannus
Kingfisher, Belted - Ceryle alcyon
Kite, Mississippi - Ictinia mississippiensis

Kite, Swallow-tailed - Elanoides forficatus
Magpie, Black-billed - Pica pica
Mallard - Anas platyrhynchos
Martin, Purple - Progne subis
Merganser - Mergus merganser
Merlin - Falco columbarius
Mockingbird, Northern - Mimus polyglottos
Nighthawk, Common - Chordeiles minor
Night-Heron, Black-crowned - Nycticorax nycticorax
Night-Heron, Yellow-crowned - Nyctanassa violacea
Nuthatch, White-breasted - Sitta carolinensis
Oriole, Baltimore - Icterus galbula
Osprey - Pandion haliaetus
Ovenbird - Seiurus aurocapillus
Owl, Barn - Tyto alba
Owl, Barred - Strix varia
Owl, Eastern Screech - Otus asio
Owl, Great Horned - Bubo virginianus
Owl, Northern Saw-whet - Aegolius acadicus
Owl, Short-eared - Asio flammeus
Owl, Snowy - Nyctea scandiaca
Parula, Northern - Parula americana
Pelican, American White - Pelecanus erythrorhynchos
Phoebe, Eastern - Sayornis phoebe
Pigeon, White-crowned - Columba leucocephala
Rail, Virginia - Rallus limicola
Raven, Common - Corvus corax
Robin, American - Turdus migratorius
Sanderling - Calidris alba
Sapsucker, Yellow-bellied - Sphyrapicus varius
Scrub-Jay, Western - Aphelocoma californica
Shrike, Loggerhead - Lanius ludovicianus
Skimmer, Black - Rynchops niger
Sparrow, Fox - Passerella iliaca
Sparrow, Savanah - Passerculus sandwichensis
Sparrow, Song - Melospiza melodia
Swallow, Barn - Hirundo rustica
Swan, Tundra - Cygnus columbianus
Swift, Chimney - Chaetura pelagica
Teal, Cinnamon - Anas cyanoptera
Thrasher, Brown - Toxostoma rufum
Thrush, Hermit - Catharus guttatus
Thrush, Wood - Hylocichla mustelina
Titmouse, Tufted - Baeolophus bicolor
Towhee, Eastern - Pipilo erythrophthalmus
Turkey, Wild - Meleagris gallopavo
Turnstone, Ruddy - Arenaria interpres
Veery - Catharus fuscescens
Vireo, Black-whiskered - Vireo altiloquus
Vireo, Warbling - Vireo gilvus
Vulture, Black - Coragyps atratus
Vulture, Turkey - Cathartes aura
Warbler, Blackpoll - Dendroica caerulescens
Warbler, Black-throated Blue - Dendroica striata
Warbler, Canada - Wilsonia canadensis
Warbler, Hooded - Wilsonia citrina
Warbler, Kentucky - Oporornis formosus
Warbler, Nashville - Vermivora ruficapilla
Warbler, Yellow - Dendroica coronata
Warbler, Yellow-rumped - Dendroica aestiva
Waterthrush, Northern - Seiurus noveboracensis
Waxwing, Cedar - Bombycilla cedrorum
Wigeon, Eurasian - Anas penelope
Woodpecker, Downy - Picoides pubescens
Woodpecker, Red-headed - Melanerpes erythrocephalus
Wren, Carolina - Thryothorus ludovicianus
Wren, Winter - Troglodytes troglodytes
Yellowthroat, Common - Geothlypis trichas

***The starling and the house sparrow which are not native to North America can also carry the disease

 

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