Date Published: 26 February 2015

Study finds out which birds host Lyme disease bacteria in California

Small mammals such as wood rats and grey squirrels have been identified in various studies as wildlife hosts of the Lyme disease spirochete bacterium in the US state of California. Until recently few studies have investigated the role of birds as reservoirs of the corkscrew-shaped bacterium, Borrelia burgdorferi, responsible for Lyme disease, which is spread to humans through the bite of infected ticks.

A recent article in the scientific journal PLOS ONE has identified several species of birds as important hosts of Lyme disease bacteria. These included American robins, dark-eyed juncos and golden-crowned sparrows, which are commonly found in suburban habitats such as family gardens.

According to Erica Newman, lead author of the study at UC Berkeley (California, USA):

"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."

This information could be useful for human health because, according to the Centers for Disease Control and Prevention, Lyme disease is the most commonly reported tick-borne illness in the United States with approximately 30,000 cases reported each year - although the total number of cases may be considerably higher.

The possibility that birds might contribute to the geographical movement of the Borrelia burgdorferi bacterium is important because, due to their flight ranges, birds are able to carry such diseases over greater distances than the small-mammal hosts that are more commonly associated with the movement and incidence of Lyme disease. Therefore, according to ornithologist Prof. Morgan Tingley of the University of Connecticut, birds may constitute an underappreciated component of Lyme disease ecology.

" 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", explained Prof Tingley.

More about this research:

The bird and tick samples in the recent study came from 14 sites within the UC Hopland Research and Extension Center in northwestern California. The study included multiple natural habitats, including savannas, grasslands, chaparral and dense woodlands. The researchers took blood samples from 623 birds of 53 species. They also carefully removed and identified any ticks found on the birds. As a result of this process they collected 284 juvenile ticks, of which over 99% were western black-legged ticks (192 larvae and 92 nymphs).

Lyme disease spirochetes (i.e. the bacteria) were detected in 57 of the 100 birds that carried ticks. Of the ticks themselves, 13% of the larvae and almost 25% of the nymphs were infected with B. burgdorferi or related spirochetes. Of the 23 species of birds that were infected, the researchers found that the lesser goldfinch, oak titmouse and dark-eyed junco were the birds that harbored more subtypes of Lyme disease bacteria than others, while the golden-crowned sparrow was infected more frequently than other species. Previous studies that tested birds have also identified the dark-eyed junco as a likely source of Lyme disease bacteria for ticks that feed on them.

Tick-infested birds were found in all types of habitat studied. However, the the researchers unexpectedly found that chaparral correlated with the lowest counts of larvae and nymphs on birds among the ecosystems studies. This contributed to the suggestion that removing chaparral may contribute to increasing the incidence of Lyme disease in Califormia:

" Other studies have shown that there are plenty of ticks in chaparral, but that was not translating to transmission of Lyme disease bacteria in birds," said Newman.
" This is important because part of the fire management strategy in this state is to remove fire-prone chaparral. What this means for birds is that many species that only live in chaparral are then replaced by species from other habitats, some of which we also now know are more important carriers of Lyme disease bacteria. Our study suggests that by removing chaparral, we may be increasing the spread of Lyme disease in California."

Source(s): UC Berkeley PR

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