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West Nile Virus (WNV)

By: Dr. Branson Ritchie

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Some research has suggested a clear link between increased activity of WNV in free-ranging birds and epidemics of WN fever in humans, while other work has questioned the role, if any, that birds play in virus transmission to mammals. In the US outbreak, the WNV recovered from birds, mosquitoes and humans was identical, suggesting that infected free-ranging birds may have served as a reservoir for the virus that was transmitted by mosquitoes to humans and horses.

The most important factor for WN virus maintenance and transmission is a high concentration of an effective mosquito vector. In Euroasia, WNV has been recovered from more than 40 different species of mosquitoes. The majority of virus isolates are from mosquitoes, and occasionally ticks, that preferentially feed on birds like those in the genera Culex and Aedes. Mosquitoes in both of these genera have also been shown to transmit St. Louis encephalitis in North America. The close relationship between WNV and St. Louis encephalitis virus suggests that WNV could easily become established at least in the eastern United States where St. Louis encephalitis virus is endemic.
There have been a number of methods proposed for how WNV, and other arthropod-transmitted viruses, can overwinter. These include virus survival in hibernating mosquitoes, winter survival of mosquitoes in expansive heated complexes (like subways), transovarially transfer of virus to the progeny of infected mosquitoes and reintroduction of virus to an area via migratory birds.

Most birds are considered susceptible to WNV infections. Some birds develop an infection that results in a large quantity of virus circulating in the blood (particularly crows and house sparrows), while others clear the virus rapidly. Free-ranging birds that have the highest concentration of virus in their blood are most important in the continued transmission of the virus to vector mosquitoes. Most domestic fowl are considered unimportant as reservoirs for WNV. Companion birds maintained indoors would be at a reduced risk of infection and would also be considered unimportant as a virus reservoir.

Most mammals infected with WN virus have only small quantities of virus in their blood for a brief period. Thus, infected mammals are not considered important in the maintenance or dissemination of this virus. Some horses may play a minor role in maintaining the virus in their local habitats. Most infected horses develop a transient fever while infected pigs and the majority of dogs remain unaffected. In the summer of 2002, there was a confirmed case of a dog in Illinois dying from West Nile virus. So far, this has been the only confirmed canine fatality. Despite this, dogs are not considered at significant risk of developing illness from WN virus.

Affected Birds

Some birds from geographically diverse regions that have been documented with WNV infections. Birds involved in the US outbreak are bulleted.

  • Bald eagle                        
  • Black drongo
  • Blackbird
  • Blue jay
  • Cattle egret
  • Chicken
  • Chilean flamingos
  • Common wood shrike
  • Cooper's hawk
  • Coots (multiple species)
  • Cormorants
  • Corn-buntings
  • Crows (multiple species)
  • Doves (multiple species)
  • Ducks (multiple species)
  • Egyptian vulture
  • European nuthatch
  • Geese (multiple species)
  • Golden oriole
  • Goldenbacked woodpeckers
  • Great spotted cuckoo        
  • Gulls (multiple species)
  • Hawks (multiple species)
  • Herons (multiple species)
  • Hoopee
  • Indian mynah
  • Jackdaw
  • Jungle babbler
  • Kestrel
  • Kingfishers (multiple species)
  • Kukrichare thrush
  • Lapwing                 
  • Magpie (multiple species)
  • Olive thrasher
  • Owls (multiple species)
  • Paradise flycatcher
  • Pheasants (multiple species)
  • Pied bushchat
  • Pigeon
  • Red bishop
  • Red-billed teal
  • Ringed-neck parakeet        
  • Robins (multiple species)
  • Rock-partridge
  • Rook
  • Sandhill crane
  • Small green bee-eater
  • Spur-winged plover
  • Starling
  • Stone-curlow
  • Tawny eagle
  • Vasa parrot         
  • Warbler         
  • Yellow-billed cuckoo

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