Wild Birds and Urban Ecology of Ticks and Tick-borne Pathogens, Chicago, Illinois, USA, 2005–2010

Sarah A. Hamer

, Tony L. Goldberg, Uriel D. Kitron, Jeffrey D. Brawn, Tavis K. Anderson, Scott R. Loss, Edward D. Walker, and Gabriel L. Hamer

Author affiliations: Michigan State University, East Lansing, Michigan, USA (S.A. Hamer, E.D. Walker, G.L. Hamer); Texas A&M University, College Station, Texas, USA (S.A. Hamer, G.L. Hamer); University of Wisconsin, Madison, Wisconsin, USA (T.L. Goldberg, T.K. Anderson); Emory University, Atlanta, Georgia, USA (U.D. Kitron); University of Illinois, Urbana, Illinois, USA (J.D. Brawn, S.R. Loss); and Smithsonian Migratory Bird Center, Washington, DC, USA (S.R. Loss)

Abstract

Bird-facilitated introduction of ticks and associated pathogens is postulated to promote invasion of tick-borne zoonotic diseases into urban areas. Results of a longitudinal study conducted in suburban Chicago, Illinois, USA, during 2005–2010 show that 1.6% of 6,180 wild birds captured in mist nets harbored ticks. Tick species in order of abundance were Haemaphysalis leporispalustris, Ixodes dentatus, and I. scapularis, but 2 neotropical tick species of the genus Amblyomma were sampled during the spring migration. I. scapularis ticks were absent at the beginning of the study but constituted the majority of ticks by study end and were found predominantly on birds captured in areas designated as urban green spaces. Of 120 ticks, 5 were infected with Borrelia burgdorferi, spanning 3 ribotypes, but none were infected with Anaplasma phagocytophilum. Results allow inferences about propagule pressure for introduction of tick-borne diseases and emphasize the large sample sizes required to estimate this pressure.

Wild birds can affect zoonotic disease risk to humans, wildlife, and domestic animals through their mobility and influence on the distribution and abundance of pathogens and vectors. Most notably, avian migration allows for rapid transcontinental transportation of novel pathogens and vectors that may seed new disease foci in receptive environments. For example, the spread of highly pathogenic avian influenza into and throughout most countries in Europe most likely occurred through the movement of migratory birds (1). Infected wild birds also contributed to the spread of West Nile virus (WNV) across North America (2). Thus, models of interseasonal connectivity among areas used by migratory birds can be used to forecast disease spread (3).
Over finer spatial scales, the patterns of bird use by blood-feeding vectors affect the prevalence of vector-borne pathogens. Host variation impacts the survival of vectors that feed on birds rather than on other vertebrates (4), and avian species exhibit differential reservoir competency for vector-borne pathogens (5). In combination, these factors influence disease risk; for example, just a few avian species that are heavily fed upon by mosquitoes and highly competent for WNV apparently drive most WNV transmission (6). Furthermore, host association of strains might help maintain pathogen diversity in some vector-borne diseases systems for which birds play critical roles (7).
Urban environments may promote pathogen transmission through increased host contact rates, high rates of pathogen introduction (i.e., propagule pressure), and warmer microclimates that are favorable to pathogens and vectors (8). These effects, in turn, may elevate disease risk to high-density urban human populations. Across gradients of urbanization, the incidence of some zoonotic pathogens has been found to be highest in urban cores (9). Reduced species richness in urban areas may contribute to elevated risk for diseases that are caused by multihost pathogens with generalist vectors (10), although the associations between biodiversity and disease risk are variable (11).
In humans, Lyme disease and anaplasmosis caused by infection with the bacteria Borrelia burgdorferi and Anaplasma phagocytophilum, respectively, are the 2 most common tick-borne diseases in the midwestern and northeastern United States, and both are emerging among human and canine populations (12,13). In eastern North America, both pathogens are maintained in blacklegged tick (Ixodes scapularis)–rodent cycles (14,15). We investigated the role of birds in the urban ecology of tick-borne zoonotic diseases. Our objectives were to 1) ascertain the prevalence of tick parasitism of birds in residential and urban green spaces in southwestern suburban Chicago, Illinois, USA, during a 6-year period; 2) estimate the infection prevalence of Borrelia spp. and A. phagocytophilum in ticks removed from birds; and 3) characterize the diversity of pathogens in ticks removed from birds by using genetic methods.