Causes
In North America, Borrelia burgdorferi are carried by deer ticks of the species Ixodes scapularis, the western black-legged tick (Ixodes pacificus), and the lone star tick (Ixodes neotomae). While deer ticks do inhabit deer, the juvenile ticks that are most likely to transmit disease generally live on the white-footed mouse in North America. In Europe, Russia, China, and Japan, where the disease also occurs, different strains of Borrelia burgdorferi inhabit various other animals, especially sheep. They've also been found on dogs, which sometimes contract Lyme disease.
In North America, the areas of greatest risk are Pennsylvania, New Jersey, New York, Maine, Maryland, Massachusetts, Connecticut, Delaware, Rhode Island, Wisconsin, Minnesota, California, and Oregon. People who live or work in the woods are most at risk, while people living in cities are less likely to be bitten by ticks. However, household pets that spend time in the woods may carry ticks into city parks and bushes.
Different deer ticks in different regions are capable of transmitting Lyme disease. The Ixodes dammini (the deer tick) is found in the northeast and midwest, Ixodes scapularis (the black-legged tick) is found in the south, the Ixodes pacificus (the western black-legged tick) in the west, and amblyomma americanum (the lone star tick) is found in several regions. All are considered carriers of the disease, though not all ticks may be infected. There has also been some evidence that the American dog tick, dermacentor variabilis, may also be capable of transmitting the disease.
Lyme disease can take months to appear, so it's sometimes hard to tell if it's been contracted locally.