The birthplace of Lyme disease

New Haven's neighbor, Lyme, Connecticut, is off the hook. Lyme disease originated in Europe, according to new research.

Lyme disease, which is transmitted to humans by ticks, is the most common vector-borne disease in the United States. It is caused by the bacterium Borrelia burgdorferi, which is more common in the United States than in Europe. Many researchers therefore believed the bacterium originated on this side of the Atlantic. (The first cluster of the disease in the United States was documented by Yale scientists in and around Lyme, giving the condition its name.)

Researchers from England's University of Bath and the Yale School of Public Health collected samples of Borrelia burgdorferi from various locations in this country and Europe. They then examined the samples to determine the sequence of eight so-called housekeeping genes, which control the bacterium's basic metabolic processes. By analyzing the number of mutations present in samples, the researchers were able to determine which strains were the oldest, and then draw a phylogenetic tree, a diagram of the samples' evolutionary history. The tree showed that the European samples of B. burgdorferi made up a group distinct from the American ones. The European lineage more closely resembled the common ancestor of all the samples -- leading the scientists to deduce that the bacterium first emerged in Europe and only later jumped the pond. (The research appeared on June 24 in the Proceedings of the National Academy of Sciences.) The Lyme organism, says Durland Fish, an epidemiologist at the School of Public Health and one of the study's lead authors, has "probably been here for hundreds of thousands of years."