Ecology and life cycle
Life cycle of ticks and transmission of Borrelia burgdorferi to humans. Source: CDC (https://www.cdc.gov/lyme/transmission/index.html)
Lyme disease is caused by the bacterium Borrelia burgdorferi. In nature, the bacterium is transmitted between the Ixodes species of tick and vertebrate hosts in an enzootic cycle. Ixodes ticks have three main stages: larva, nymph, and adult, and feed on vertebrate hosts once per stage. Adult ticks cannot pass B. burgdorferi through to their eggs. As a result, larvae ticks are uninfected when the hatch in the summer. These larvae can become infected with B. burgdorferi through feeding on an infected vertebrate, such as a wild mouse, squirrel or bird. Once infected, these larvae remain infected for the rest of their life cycle.
Following a blood meal, larvae molt into nymphs. If larvae fed on an uninfected host and did not acquire B. burgdorferi, uninfected nymphs can acquire the bacterium through their blood meal by feeding on an infected vertebrate the following spring. Similarly, infected nymphs can transmit the bacterium to uninfected hosts during their blood meal. This horizontal transmission from tick to vertebrate and vice versa allows for the survival of B. burgdorferi in nature. After a nymph blood meal, these ticks molt into adults. Adult ticks typically feed on larger mammals, such as deer. Although deer are not considered an important reservoir for B. burgdorferi, they are a primary site of tick mating and therefore help to maintain the tick population.
Transmission to humans
While all three stages of Ixodes tick can feed on humans, it is thought that nymphs are responsible for the majority of B. burgdorferi transmissions into humans. Nymphs typically feed in the late spring or early summer, and therefore there is generally a larger upswing in B. burgdorferi infections in June and July each year.
Because ticks cannot jump or fly, they rely on animals or humans brushing against grasses or shrubs, which allow them to climb onto their host. When a tick finds a suitable place to feed on a human, it attaches to the skin and can remain there for several days. The blood absorbed into the tick triggers signals that allow B. burgdorferi to migrate out of the tick and into the human bloodstream. This process takes time, however, so if you remove a tick within 24 hours of it being attached, there is a good chance that it will not have transmitted B. burgdorferi.