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Title: Obligate brood parasites show more functionally effective innate immune responses: An eco-immunological hypothesis

item HAHN, D - Patuxent Wildlife Research Center
item SUMMERS, SCOTT - Nature Conservancy
item Genovese, Kenneth - Ken
item He, Louis - Haiqi
item Kogut, Michael - Mike

Submitted to: Journal of Evolutionary Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/2/2013
Publication Date: 4/16/2013
Publication URL:
Citation: Hahn, D.C., Summers, S., Genovese, K.J., He, L.H., Kogut, M.H. 2013. Obligate brood parasites show more functionally effective innate immune responses: An eco-immunological hypothesis. Journal of Evolutionary Biology. doi: 10.1007/s11692-013-9231-x.

Interpretive Summary: Wild birds that can enter a chicken house to find feed can carry pathogens, such as Salmonella, that can infect baby chicks. These Salmonella can then get into chicken products and make people sick. The purpose of these experiments was to see whether the wild birds have a better immune defense system that will allow the bird be resistant to pathogens such as Salmonella. We found that the migratory cowbird has an excellent immune system that prevents the bacteria from growing in the cowbird. However, the bacteria can stay in the gut of the cowbird, and if the cowbird gets into a chicken house, it can shed the bacteria in its feces and expose chickens to the bacteria. This research shows the chicken farmer that keeping the chicken houses free from wild birds will help prevent food-borne pathogens from colonizing their chickens.

Technical Abstract: Design and functionality of the immune system may play a key role in the success of invasive species. We examined the relative effectiveness of functional innate immune defenses in the brown-headed cowbird (Molothrus ater, Icteridae), an invasive avian species that has shown unusual resistance to infection with West Nile virus, a virulent non-native pathogen. We hypothesized that the cowbird would show adaptations of the functional innate immune defenses since these are non-specific, fast-acting mechanisms that would be responsible for control of novel pathogens encountered by an invasive species in its new range. This cowbird is exposed to an exceptional diversity of parasites due to its life history strategy of brood parasitism in which it lays its eggs and raises its young in the nests of more than 200 different songbird species across North America. We have shown that this cowbird is exposed to a higher diversity of ectoparasites and blood parasites than are closely related, non-parasitic species in the same avian family. We demonstrated that the cowbird is significantly more resistant to both native and non-native viruses, including West Nile virus. We conducted a comparative study of the functional effectiveness of two innate immune responses in the invasive cowbird and a related, non-invasive species which has shown greater susceptibility to West Nile virus. This is the first study using measurements of oxidative burst and degranulation, two tactics that phagocytic leukocytes use against pathogens, to compare relative immunity in wild songbird species. We found the cowbird leukocytes were significantly more functionally efficient than the red-winged blackbird’s (Agelaius phoenicius, Icteridae) leukocytes and 20% more effective at killing the challenge bacteria. This is the first study linking superior functional effectiveness of innate immune mechanisms in a wild songbird to superior resistance to a real-world pathogen like West Nile virus.