Basic 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  

*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

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