Birds may expand the range of Lyme disease and its vector tick

Lyme disease (Borrelia burgdorferi) is typically associated with mammals, but birds too can become infected by black-legged ticks (Ixodes scapularis), the principal vector of the pathogen.  Moreover, birds may figure significantly in the range expansion of both the Lyme bacterium and black-legged ticks.  So say Jory Brinkerhoff and colleagues of Yale University in a paper published today in Frontiers in Ecology and the Environment.

Black-legged adults and nymphs with straight pin. Photo: Jim Occi, BugPics, United States

As anyone who has been infected by the Lyme bacterium knows, it can cause any number of health problems including arthritis, nervous system abnormalities, and irregular heart rhythm.  According to the Centers for Disease Control and Prevention, Lyme disease is the most common vector-borne disease in the United States, with the number of reported human infections in the U.S. doubling between 1992 and 2006.  

B. burgdorferi occurs naturally in small mammalian hosts such as mice, squirrels, and shrews.  Immature (larval and nymphal) ticks can become infected with the bacterium when they feed on these small mammals.  During subsequent blood meals, infected nymphs and adult ticks can transmit the infection to other hosts, including humans.  Interestingly, the white-tailed deer-though it plays an important role in maintaining tick populations-is, in fact, a “dead end” for the Lyme bacterium because deer blood is immune to infection from it.  Enter the birds.

Gray catbird (top) and hermit thrush (below) carrying engorged immature ticks (red arrows). Photo: L Doss.

Brinkerhoff and colleagues found published records indicating that at least 70 North American bird species are parasitized by immature black-legged ticks.  I. scapularis most consistently parasitizes thrushes, brown thrashers, wrens, and wood warblers.  The authors estimate that as few as three individual birds are required to produce one infected black-legged tick.  As they write in Frontiers:

 White-tailed deer are probably responsible for the range expansion of I. scapularis, but they cannot transport B. burgdorferi.  Nomadic and post-breeding movements by birds, in addition to migration, facilitate the spread of B. burgdorferi and may result in northern range expansion of the pathogen and vector from the Northeast and in southern expansion from the Midwest.

What remains to be seen, say the authors, is whether the B. burgdorferi strains that can infect birds can also infect mammals.  If the answer is yes, they say, then the role of birds in the transmission of Lyme disease to humans could be profound.  

An abstract of the article can be accessed here and the complete paper is available here (subscription required).  Reporters interested in a copy of the paper or in speaking with the authors can contact Nadine Lymn at nadine@nullesa.org.

Brinkerhoff, R., Folsom-O’Keefe, C., Tsao, K., & Diuk-Wasser, M. (2009). Do birds affect Lyme disease risk? Range expansion of the vector-borne pathogen Borrelia burgdorferi Frontiers in Ecology and the Environment DOI: 10.1890/090062