A Behavioral and Ecological Approach to the Evolution of Immunity and Disease Resistance

We conduct field and laboratory studies on a variety of avian species to answer questions about emerging infectious diseases and the evolution of disease resistance and immune system defenses.  We integrate behavioral and ecological studies with genetic, endocrine, and virological techniques.  We are focused primarily on the brood parasitic Brown-headed Cowbird (Molothrus ater) and its non-parasitic relatives, which we study using wild populations, and on the Eastern Screech Owl (Megascops asio) which we study using a captive breeding colony at Patuxent Wildlife Research Center. 

Our collaborators include:
William K. Reisen, Center for Vectorborne Disease, University of California Davis
Nicole Nemeth and Richard Bowen, Colorado State University, School of Veterinary Medicine
Nicholas Komar, Center for Disease Control
John Wingfield and Brian Walker, University of Washington

Evolution of immunity and disease resistance
Our work on Brown-headed Cowbird (Molothrus ater) integrates behavioral and ecological studies of this parasitic bird to examine the evolution of disease resistance.  We conducted experimental infection studies using three serious diseases, West Nile virus (WNV), Western equine encephalomyelitis (WEEV), and St. Louis encephalitis (SLEV), to examine the suite of immune defenses used by cowbirds vs their non-parasitic relatives (see abstract below from Reisen and Hahn 2007).  Previous studies had established that cowbirds experience heightened exposure to parasites due to their life history strategy as brood parasites (Hahn et al. 2000  Auk 117:943-951) and their use of an unusual number of habitats types (Hahn and O’Connor 2002, in Predicting Species Occurrences:  Issues of Scale and Accuracy, edited by J.M. Scott et al.).

Mechanisms of resistance to West Nile Virus
West Nile virus becomes established in wildlife populations and they serve as a reservoir for mosquitoes to spread WNV to human populations.  The search for which avian species serve as principal WNV reservoirs has occupied wildlife researchers across North America as the disease has spread.  After field data indicated that raptors might serve as reservoirs for WNV, we tested the relative susceptibility of the eastern screech owl to WNV in an experimental infection study, showing that this owl was highly susceptible to infection and mortality (see abstract below from Nemeth et al. 2006). 

One of the ways West Nile virus infections drop in wildlife populations is the development of resistance following initial exposure.  We published the first demonstration of maternal transfer of West Nile Virus antibodies by documenting in screech owls that mothers confer enhanced resistance on their young through passive transfer of maternal antibodies in the yolk (see abstract below from Hahn et al.2006).  We are currently examining the development of the avian immune system in screech owls, with the goal of determining how susceptibility and resistance to West Nile Virus at different ages is related to the persistence of the disease in wild populations.

Comparison of immune responses of Brown-headed Cowbird and related blackbirds to West Nile and other mosquito borne encephalitis viruses

WK Reisen and DC Hahn, 2007, J. Wildlife Disease 43 (3)

The rapid geographic spread of West Nile Virus [WNV, Flaviviridae, Flavivirus] across the United States has stimulated interest in comparative host infection studies to delineate competent avian reservoir hosts critical for viral amplification. We compared the host competence of four taxonomically related blackbird species (Icteridae) following experimental infection with West Nile virus (WNV) and with two endemic, mosquito borne encephalitis viruses, western equine encephalomyelitis virus (WEEV) and St. Louis encephalitis virus (SLEV). We predicted differences in disease resistance among the blackbird species based on differences in life history, because they differ in geographic range and life history traits of that include mating and breeding systems. Differences were observed among the response of these hosts to all three viruses. Red-winged blackbirds were more susceptible to SLEV than Brewer’s blackbirds, whereas Brewer’s blackbirds were more susceptible to WEEV than red-winged blackbirds. In response to WNV infection, cowbirds showed the lowest mean viremias, cleared their infections faster, and showed lower antibody levels than concurrently infected species. Brown-headed cowbirds also exhibited significantly lower viremia responses following infection with SLEV and WEEV as well as co-infection with WEEV and WNV than concurrently infected Icterids. We concluded that cowbirds may be more resistant to infection to both native and introduced viruses, because they experience heightened exposure to a variety of pathogens of parenting birds during the course of their parasitic life style.

Passive West Nile virus antibody transfer from maternal Eastern Screech-Owls (Megascops asio) to progeny (PDF of paper)

D.C.Hahn, N. Nemeth, E. Edwards, P.R. Bright, N. Komar, (2006), Avian Diseases 50:454-455

Transovarial antibody transfer in birds has not been definitively demonstrated for WNV. We sampled chicks from captive adult Eastern Screech-Owls (Megascops asio) of known WNV infection history to evaluate the prevalence of transovarial maternal antibody transfer, as well as titers and duration of these antibodies. Seventeen owlets aged 1to 18 days old from five seropositive female owls circulated detectable antibodies with neutralizing antibody titers ranging from 1:20 to 1:1600 (median 1:60). Demonstrating that WNV antibodies are passively transferred transovarially is important for accurate interpretation of serologic data from young birds.

Blood samples taken from females and 5 day old chicks to demonstrate transovarial antibody transfer.

Newly hatched owlet with eggs and device for measuring incubation temperature.

Experimental West Nile virus infection in Eastern Screech Owls (Megascops asio)

Nicole M. Nemeth, D. Caldwell Hahn, Daniel H. Gould, and Richard A. Bowen. 2006. Avian Diseases 50 (1): 1-15. (PDF of paper)

Eastern screech owls (EASOs) were experimentally infected with the pathogenic New York 1999 strain of West Nile virus (WNV) by subcutaneous injection or per os. Two of nine needle-inoculates became severely morbid and either died or were euthanized. One of five orally exposed EASOs became infected. All needle-inoculates developed mosquito-infectious levels of viremia (>5 logs per ml serum); peak viremias ranged from 105.0 to 109.6 plaque forming units (PFU). All infected birds shed virus orally and cloacally. Early during the infection process, WNV targeted skin, spleen, esophagus and skeletal muscle. The two morbid owls had myocardial and skeletal muscle necrosis, and mild encephalitis and nephritis. Some birds had myocardial arteritis and renal phlebitis. WNV is a significant pathogen of EASOs, causing pathological lesions of varying clinical outcomes.

Patuxent has a captive breeding colony of screech owls that can be used for studies of West Nile Virus and other zoonotic diseases.

Course of infection in 10 experimentally infected screech owls

Human health and emerging infectious diseases

Emerging diseases like West Nile Virus that infect both wildlife and humans and that are actively transmitted between them require wildlife biologists to assist public health authorities. In the collaboration between wildlife scientists and epidemiologists, research on wildlife species addresses the ecological, physiological, and behavioral aspects of the disease in animals, providing insights into how wildlife species maintain and spread the disease to people. West Nile Virus emerged recently in North America and spread rapidly across the continent, with many wild bird species infected and an unknown number serving as significant reservoirs for the disease.

Red morphs predominate in the southeastern United States, and grey morphs predominate in the northeast and north central U.S. In Maryland, both morphs are common as well as the intermediate brown morphs.

We are working on several aspects of avian susceptibility and immunity to WNV, using different avian species with contrasting life history patterns to elucidate different aspects of the virus.  Eastern screech owls are a good study species for examining transmission of zoonoses that circulate among people, rodents and birds, because they prey on small rodents and are exposed to oral infection from infected prey as well as mosquito borne transmission via the main vector of WNV.  We utilize the captive breeding colony of screech owls at Patuxent Wildlife Research Center.  Brown-headed cowbirds are a good study species for examining evolutionary aspects of disease resistance, because their life history strategy as brood parasites exposes them to a elevated number of parasites.

Brown-headed cowbirds (Molothrus ater) are obligate brood parasites.