California Birds Loaded with Lyme Disease

California Birds Loaded with Lyme Disease

Something doesn’t add up.

We know birds have been carrying ticks all over the world and spreading Borrelia that way. But now, according to a study just released by the University of California at Berkeley, 623 birds representing 53 species yielded 284 I. pacificus larvae and nymphs, however, the species of Borrelia in the birds did not match the species in the nymphs.

This odd occurrence gives rise to the theory that the birds are reservoir hosts for the ticks, but also, the ticks and birds are likely exchanging species making both more infectious.

The study underscores the importance of bird behavior to explain local tick infestation and Borrelia infection in these animals, and suggest the potential for bird-mediated geographic spread of vector ticks and spirochetes in the far-western United States.

According to the study, the yellow-headed sparrow is one of the most infected of the birds studied (pictured above) and the more birds, that were studied, the more the researchers realized how the infected birds are contributing to the rise of infection (both chronic Lyme disease and acute Lyme disease).

The introduction states:

Lyme disease is the most prevalent vector-borne disease in the United States, with 20,000 to 30,000 confirmed cases reported annually from 2003–2011. Worldwide, clinical manifestations are caused by a subset of genospecies within the expanding Borrelia burgdorferi sensu lato (s.l.) spirochete complex. These bacteria are transmitted by ticks in the genus Ixodes. Larval or nymphal (subadult) ticks ingest spirochetes while feeding upon a bacteremic host, and may then infect other vertebrate hosts while feeding during the subsequent nymphal or adult stages. In North America, Lyme disease is caused by B. burgdorferi sensu stricto (s.s.). The primary bridging vectors to humans are the nymphal stages of the black-legged tick, Ixodes scapularis, in the east, and the western black-legged tick, Ixodes pacificus, in the far west.

In California, I. pacificus is known to infest >100 species of lizards, birds or mammals. The dusky-footed woodrat (Neotoma fuscipes) and the western gray squirrel (Sciurus griseus) serve as primary reservoirs for B. burgdorferi s.l., but little is known about the importance of non-mammalian tick hosts, particularly birds, as spirochete reservoirs. Studies attempting to identify potential avian reservoirs of B. burgdorferi s.l., based on tick loads or infection in birds or their attached ticks, have been conducted in the far-western United States, as well as in the eastern United States, Europe and Asia. Birds are potentially important spirochete reservoirs because they are species-rich and ecologically diverse, abundant, occur in the same habitats utilized by the primary mammalian reservoirs, are often infested by vector ticks, and are widely found to carry B. burgdorferi s.l. Birds therefore may be important local reservoirs for various B. burgdorferi s.l. spirochetes. However, the ability of spirochete-infected birds to infect feeding ticks is not well understood and varies among bird species.

Moreover, birds may play an important role in the geographic spread of ticks and borrelia. Dispersal distances for birds are generally much larger than for small mammalian hosts of Ixodes ticks, and because of their annual migrations, birds have been implicated as long-distance dispersal agents of vector ticks and pathogenic borrelia.

Bird communities also are changing radically due to human impacts, with large shifts in species composition, total abundance, and relative abundances among species. Geographic distributions of individual species are changing with climate, land use and introduction of exotic species. Although Lyme disease cases in the far western United States are rare compared to other regions of North America, birds, because of their high mobility, are likely to be the most important vertebrate hosts of B. burgdorferi s.l. in determining the northward spread of Lyme disease in western North America due to climate and related anthropogenic changes.

Although the primary reservoirs of B. burgdorferi s.l. in the United States are mammals, there is growing evidence that different vertebrate species contribute to the enzootic maintenance of Lyme disease spirochetes in the far west. As birds are species-rich (433 regularly-occurring species in California) and their community composition varies greatly among California’s diverse vegetation communities, clarifying the role of birds as reservoirs for B. burgdorferi s.l. is important both for more accurate modeling of the local ecology of Lyme disease spirochetes and to better understand the risk of human exposure to infected ticks.

The goals of this study are to identify bird species that are potentially important to the enzootic maintenance of Lyme disease spirochetes in northwestern California, to evaluate the individual nesting and feeding behaviors of birds that contribute to the transmission of spirochetes, and to test for the first time whether avian taxonomic relationships are predictive of I. pacificus larval and nymphal loads, and of B. burgdorferi s.l. infection.

As you can see, this study focused on Borrelia burgdorferi which is only one species of Lyme disease out of the 35 Lyme species and additional 9 relapsing fever forms of Lyme (as of 2014) see Genome Mapping for more information on the latest information from Dr. Luft at Stony Brook University Medical Center.


Newman EA, Eisen L, Eisen RJ, et al. Borrelia burgdorferi Sensu Lato Spirochetes in Wild Birds in Northwestern California: Associations with Ecological Factors, Bird Behavior and Tick Infestation. PLoS ONE. 2015;10(2):e0118146.

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