West Nile Virus in Birds

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American Crows (Corvus brachyrhynchos), some of the most susceptible birds to West Nile Virus infection. They are the most typically studied species for WNV infection, and are used for national surveillance systems for avian deaths and WNV spread. Photo by Ingrid Taylar [xxi]

West Nile Virus, of the family Flaviviridae, is a zoonotic disease that infects many species, including humans, throughout the world. Birds are the virus’s primary natural reservoir, where they act as the amplifier host. but their host competency (their ability to perpetuate the virus' development and spread) and response to infection varies between species; they can have diverging viral loads, viremia, viral shedding, clinical signs, and morbidity. Many factors may affect a particular organism and/or species’ susceptibility and response to infection, based on their life histories, immune-response capabilities, previous immunities, breeding systems, genetics, and stress levels. Birds vary widely in how they are affected by West Nile Virus.

Most mammals, including horses and humans, are “dead-end” hosts for West Nile Virus, as, after infection, the virus does not become in high enough concentration in the blood to then be spread to insect vectors. Many bird species, however, are important “amplification-hosts” for the spread of West Nile Virus, as the virus multiplies to exorbitant numbers within them. Some species can generate the highest levels of virus particles in their blood presently known for any viral infection; they can have millions of times more viral particles in their blood than similarly infected mammals. [2 for now]

Initial Infection

West Nile Virus transmission cycle. From CDC.

Birds may become infected with West Nile Virus in multiple ways. The most common occurrence is through a mosquito vector. However, studies have shown that they may contract the infection through ingestion of infected organisms or direct contact with infected materials.

Insect Vectors

Over 40 species of mosquitos have been found to carry West Nile Virus. The most important for spread is the genus Culex, which mainly feed on birds. If a mosquito feeds off an infected host with high enough viral load (approximately one million virus particles per milliliter of blood), it will take up the virus into its gut. The virus will then multiply and spread to the rest of the mosquito’s body, including its salivary glands. The vector may then feed off another animal and transmit the virus to a new host.
Other blood sucking insects, such as ticks, have been found to be able to transmit the virus, though they are not as important for spread.

Oral Transmission

By ingesting infected mice or other birds, a carnivorous bird may present viremia.

Contact Transmission

Studies have shown transmission of infection between animals. feces.

Species Susceptibilities and Competence

Chance of a mosquito picking up West Nile Virus from an infected host. Birds are the primary competent host for viral spread through mosquitoes, due to their high viral loads. Graphic from FAQ: West Nile Virus, July 2013. American Society For Microbiology, 2013. 5.] [2

Table of West Nile Virus Host Competency of 25 Species of Birds [1x for now].
Species	Reservoir Competence Index*
Blue Jay	2.55
Common Grackle	2.04
House Finch	1.76
American Crow	1.62
House Sparrow	1.59
Ring-billed Gull	1.26
Black-billed Magpie	1.08
American Robin	1.08
Red-winged Blackbird	0.99
American Kestrel	0.93
Great Horned Owl	0.88
Killdeer	0.87
Fish Crow	0.73
Mallard	0.48
European Starling	0.22
Mourning Dove	0.19
Northern Flicker	0.06
Canada Goose	0.03
Rock Dove	0
American Coot	0
Japanese Quail	0
Northern Bobwhite	0
Ring-necked Pheasant	0
Monk Parakeet	0
*based on susceptibility, mean infectiousness and duration (days)

Over 300 species of birds have been found infected with West Nile Virus. Passerine birds (of the order Passeriformes, such as flycatchers, robins, wrens, mockingbirds, and finches), charadiiform birds (of the order Charadriiformes, such as gulls, plovers, and stilts), and many raptors have been found to be competent hosts for viral infection and spread. In particular, Corvids (species of the family Corviade, including crows, ravens, magpies, and jays), as well as a few other sensitive species, such as American Kestrels and Great Horned Owls, are competent and highly susceptible. Birds of the orders Anseriformes (ducks, geese), Columbiformes (pigeons), Galliformes (chickens, turkeys), Gruiformes (rails, coots), Piciformes (woodpeckers, toucans), and Psittaciformes (parrots) are generally less competent.

The table to the left shows analyzed viremia data from mosquito-exposed birds to calculate competence indexes. Species with higher reservoir competence have higher concentrations of infection virus particles in their blood in combination with a long duration of an infectious level of viremia. Thus, they are better “amplifying hosts” for the disease, and are more likely to spread the disease when fed on by mosquitos. [1x]

Occurrence of mortality due to infection increases with species competence. In early studies of WNV in the United States, up to 100% mortality was observed in American Crows (Corvus brachyrhynchos) experimentally inoculated with the virus.

It has been found that some species, such as crows, may contain a high enough viremia level that they are still able to transmit viral particles post-mortem to vector hosts that feed on their remains. [2?]



Birds.

birds yep.

wow such a large table I wish I knew how to collapse it

Infection Symptoms

An owl exhibiting symptoms of West Nile Virus infection: lethargy, head droop, and irregular body postition. DNR - West Nile Virus (WNV). Michigan Department of Natural Resources. Web. 10 Mar. 2014.

The majority of individual birds infected with West Nile Virus does not get sick or show symptoms. However, if they do become sick, clinical signs may include uncoordinated walking, lethargy, [ataxia] and weakness, tremors, inability to fly, anorexia followed by rapid weight loss, green waste products, blindness, lack of awareness, head droop, and abnormal body posture. They usually die within 24 to 48 hours of showing symptoms. In the final phase of infection preceding death, there birds may have severe tremors or seizers. These symptoms reflect virus infection and inflammation of the brain and spinal cord, as well as damage to other tissues, such as the heart, lungs, liver, etc. [xi, xii]

Through bird necropsies, organs found to be most frequently infected with virus particles are the spleen, kidney, skin, and eye. Usually, the virus first appears in the spleen, then rapidly spreads to other organs, and reaches the central nervous system later. Tissue distribution occurs earlier in more susceptible of the bird species. [Histologic] lesions, such as mild to severe [encephalitis], [necrosis], [inflammation], and [hemorrhages], have been found in multiple tissues, including the brain, spinal cord, eye, peripheral nervous system, heart, spleen, liver, kidney, lung, gastrointestinal tract, endocrine system, gonads, skeletal muscle, skin, and bone marrow. The speed of viral distribution and host death may affect the development of lesions, as hosts with higher susceptibility may die before lesions become observable. Macroscopic changes due to infection include emaciation, dehydration, multi-organ hemorrhages, and congestion. [xixix, xiii]

Host Factors for Susceptibility and Competence

Immunity/Genetics

Chickens!

Virus Effect on Bird Populations

Eagles! Endandered?? Crows decline come back. Allows other species to spread?

Further Reading

[Sample link] Ebola Hemorrhagic Fever—Centers for Disease Control and Prevention, Special Pathogens Branch

References

1x. Komar, N., S. Langevin, S. Hinten, N. Nemeth, E. Edwards, D. Hettler, B. Davis, R. Bowen, and M. Bunning. 2003. Experimental Infection of North American Birds with the New York 1999 Strain of West Nile Virus. Emerging Infectious Diseases 9(3): 311-322.

2. Greene, S. E., and A. Reid. (2013). FAQ: West Nile Virus, July 2013. American Society For Microbiology.

Xii "West Nile Virus." Seattle Audubon Society. Web. 9 Mar. 2014. <http://www.seattleaudubon.org/sas/LearnAboutBirds/SeasonalFacts/WestNileVirus.aspx>.

Xi "Frequently Asked Questions about West Nile Virus and Wildlife." USGS National Wildlife Health Center. N.p., n.d. Web. 9 Mar. 2014. <http://www.nwhc.usgs.gov/disease_information/west_nile_virus/frequently_asked_questions.jsp>.

Xiii Gamino, Virginia, and Ursula Höfle. "Pathology and tissue tropism of natural West Nile virus infection in birds: a review." Veterinary Research 44.1 (2013): 39. Print

Xixixi Nemeth, Nicole, Daniel Gould, Richard Bowen, and Nicholas Komar. "Natural And Experimental West Nile Virus Infection In Five Raptor Species." Journal of Wildlife Diseases 42.1 (2006): 1-13. Print.

xxi. Taylar, Ingrid. Crows. 2012. Seattle, WA. flickr. Web. 9 Mar. 2014. <http://www.flickr.com/photos/taylar/8516674909/>.

Edited by Leah Pomerantz, a student of Nora Sullivan in BIOL168L (Microbiology) in The Keck Science Department of the Claremont Colleges Spring 2014.