West Nile Virus in Birds


Epidemiology

  • Birds is the natural reservoir where the virus is maintained in an enzootic ornitophilic-mosquito-bird cycle
  • Infected birds are generally asymptomatic
  • Outbreaks can occur if the WNV is introduced into humanized habitats where there are mosquitoes with wider host range present.
  • Migratory birds are considered important for the local and long distance transmission of the virus
  • Most susceptible species are those that belong to the family Corvidae (ex. American crow, blue jay, black-billed magpie, fish crow).
  • American robin and house sparrow are also considered to be a main reservoir of WNV in urban areas of North America and Europe
  • Transmission of WNV to birds can be due to:
  • In endemic areas, wild bird infections usually starts in spring/early summer → bird mortality peaks from midsummer/early fall → human/horse cases takes place a few weeks after the onset of bird mortalities.
  • In wild birds, infection is greater than disease (ex. encephalitis or mortality)
  • Although asymptomatic in most birds, considerable avian mortality occured in Israel and North America.


    Pathology of natural WNV infection

  • Due to wide cellular and tissue tropism, there are no pathognomonic macroscopic lesions
  • Chickens: less susceptible and may not have observable macroscopic lesions
  • Crows/jays: highly susceptible, will die rapidly after short incubation period and have few acute or no observable macroscopic lesions
  • Birds that survive longer with disease will have more pronounced macroscopic lesions
  • Raptors: observed cerebral atrophy and malacia, which can be maintained chronically
  • In most birds, microscopic lesions are predominantely found in CNS, heart, kidney, spleen and liver
  • Differences in pathology of WNV infected bird species likely due to combination of host/enviornmental factors, intrinsic virulence and pathogenicity of viral strain.

    Citation:
    Gamino, V. and Hofle, U. (2013) Pathology and tissue tropism of natural West nile virus infection in birds: a review. Vet Res. 44(38):doi:10.1186/1297-9716-44-39.

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