Researchers at the Yale School of Public Health have shown that birds have helped spread Lyme disease across North America.

A team led by School of Public Health researcher Maria Diuk-Wasser analyzed studies on 71 bird species that host the black legged tick, the main carrier of Lyme disease. They found that 58.6 percent of the bird species can infect the tick with the bacterium that is responsible for Lyme disease. The literature review was published online in December in the journal Frontiers in Ecology and the Environment.

“Although it has been known for some time that birds play some role in Lyme disease epidemiology, this study integrates all the available information and points at particular bird species that may have a critical role in dispersing the Lyme agent,” Diuk-Wasser said.

Traditionally, scientists thought small mammals such as white-footed mice, chipmunks and shrews were the main carriers of ticks, co-author Kimberly Tsao GRD ’13 said. But the study shows that the expansion of the range of Lyme disease in the Northeast and Midwest over the past 20 to 30 years can also be attributed to birds, she said. For example, nearly 70 percent of brown thrashers, a ground-dwelling species, carried the tick, post-doctoral researcher and main author Robert Brinkerhoff said.

Diuk-Wasser said differences in when tick larvae are active may result in birds transporting tick larvae northward in the Midwest and southward in the Northeast.

Because the bacterium that is responsible for Lyme disease has a lot of genetic variation, the researchers said the next step for scientists is to determine if birds carry the same strains that infect humans. Some strains, Brinkerhoff said, are more infectious than others.

Lyme disease was first reported in the United States in Old Lyme, Conn., in 1975. Lyme disease may result in skin rash, flu-like symptoms, arthritis and neurological symptoms.