Field rats carry spirochetes in their bloodstream. In fact, field rats are the major reservoirs of Borrelia burgdorferi in nature. These spirochetes are transmitted to Ixodes ticks when they bite the rats. The tick has three life stages. It spends its first life stage, as a feeding larva, on mice. Its second life stage, as feeding nymph on a human. Its third life stage, it spends as an adult tick on a deer. The nymph stage emerges only during the summer, the time when people hike or hunt in the forests. The nymph transfers to the human and feeds on human blood. Infection begins when spirochetes in the tick’s saliva enter the wound created by the tick’s bite. Interestingly, only one percent of tick bites result in Lyme disease (Talaro 2008).
The tick that transmits Lyme disease on the Pacific coast of the US is the western black-legged tick Ixodes pacificus. On the Atlantic coast, and the rest of the US, the very small Ixodes scapularis is responsible for the transmission of the spirochete (Nester 2007).