Genetic study reveals surprises in the diversity of bacteria causing the infection.
Human activity might explain the explosion of Lyme disease cases in recent decades.
The Borrelia burgdorferi bacteria that cause the disease pass from parasitic ticks to vertebrates, including humans.
Katharine Walter of Yale University in New Haven, Connecticut, and her team sequenced the genomes of 146 Lyme disease bacteria, sampled from across the United States and Canada between 1984 and 2013. They found them to be highly diverse, suggesting that the bacteria had migrated over large distances, probably carried by their bird and mammal hosts.
Reconstructing the evolutionary history of B. burgdorferi, they found that this diversity dates back 60,000 years in North America.
This early diversity suggests that the recent epidemic of human Lyme disease has been fuelled not by evolutionary changes in the bacteria, but by ecological change — driven by disruption from human activity, such as increased deforestation and hunting, and by climate change — which has influenced the movement of Lyme-hosting birds and mammals.