West Nile Virus in Birds
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.
- infected prey
- consumption of contaminated water
- horizontal contact transmission
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.
- Most characteristic macroscopic changes: emaciation, dehydration, multiorgan hemorrhages, petechiae and congestion
- Other macroscopic changes observed: splenomegaly, hepatomegaly, myocardial pallor and pale mottling in liver/spleen/kidney
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.