Birds are more important than previously recognized as hosts for Lyme disease-causing bacteria, according to a recent study published in the journal PLOS ONE. The bacterium Borrelia burgdorferi, which is responsible for Lyme disease, was known to be carried by white-footed mice, wood rats, western gray squirrels, and other small mammals, but fewer studies have looked at the role of birds as reservoirs.
“The role of birds in the maintenance of Lyme disease bacteria in California is poorly understood,” said lead author Erica Newman, a UC Berkeley PhD student. “This is the most extensive study of the role of birds in Lyme disease ecology in the western United States, and the first to consider the diversity of bird species, their behaviors, and their habitats in identifying which birds are truly the most important as carriers.”
Moreover, the birds in the study that were found to be important hosts of Lyme disease bacteria — such as American robins, dark-eyed juncos and golden-crowned sparrows — are coincidentally ones that are commonly found in suburban environments.
According to the Centers for Disease Control and Prevention, Lyme disease is the most commonly reported tick-borne illness in the United States. There are approximately 300,000 cases each year, with the large majority occurring in the eastern United States.
Morgan Tingley, an ornithologist at the University of Connecticut who was not part of this study, underscored the significance of discovering which birds are carriers of Lyme-disease bacteria.
“Birds are much more capable of carrying diseases long distances than the small-mammal hosts typical of Lyme disease, and so may constitute an under-appreciated component of Lyme disease ecology,” said Tingley. “Particularly as we look to the future, birds may end up playing a larger role in disease ecology than other animals because of their ability to quickly and easily move long distances and to new habitats. In the same way that airplanes can help spread disease across nations, birds do the same thing for our ecosystems.”
Lyme disease is spread to humans through the bite of infected ticks. The black-legged deer tick (Ixodes scapularis) transmits B. burgdorferi in the eastern and north-central regions of the United States, while the western black-legged tick (Ixodes pacificus) spreads the bacteria in the West.
The bird and tick samples in this new study came from 14 sites in northwestern California. The study included multiple natural habitats, ranging from savannas and grasslands to chaparral and dense woodlands.
The researchers took blood samples from 623 birds representing 53 species, and they also carefully removed and identified ticks that they found on the birds. They collected a total of 284 juvenile ticks — more than 99 percent of which were western black-legged ticks — consisting of 192 larvae and 92 nymphs.
Lyme disease bacteria were detected in 57 of the 100 birds that carried ticks. Among the ticks themselves, 13 percent of the larvae and nearly 25 percent of the nymphs were infected with B. burgdorferi or related spirochetes.
Among the 23 species of birds that were infected, the study authors highlighted the lesser goldfinch, the oak titmouse, and the dark-eyed junco as birds that harbored more subtypes of Lyme disease bacteria than others. In addition, the golden-crowned sparrow was infected more frequently than other species.
The researchers also found, for the first time in birds, another species of Lyme disease spirochete that is closely related to Borrelia burgdorferi. That spirochete, named Borrelia bissettii, has been known to cause a Lyme disease-like illness in people in central and southern Europe. Furthermore, this bacterium was the most common of the Borrelia species found in birds.
“The fact that we found this particular bacterium for the first time in birds in California is notable because of the ease with which birds can distribute spirochetes to different regions,” said co-author Robert Lane, a medical entomologist at UC Berkeley. “It is worth watching to see if this spirochete expands in this state.”
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