How Lyme disease is transmitted
Ixodes scapularis (previously called I. dammini ) is the tick that represents the major vector in the Northeastern and Midwest United States. I. scapularis egg mass is typically laid in the leaf clutter at the bottom of a forest; consequently, this tick does not thrive in dry climates.
Larvae (the first stage of tick development) emerge in the summer and fall. They require a blood meal that they obtain from rodents, an asymptomatic reservoir for Borrelia . After the tick larvae acquire Borrelia with their blood meal, they fall off the mouse and molt to nymphs (the second stage of tick development) that lay dormant until late spring and summer of the following year. Infected nymphs can then pass Borrelia on to other mice and humans (transmission of infection) when they take a blood meal. Subsequently, the nymphs become adults (final stage of tick development), and the females will feed on white-tailed deer (in the region of their shoulder, hence I. scapularis ) to get blood nutrients to make eggs. The eggs are laid in leaf clutter, completing the 2-year life cycle. Of note, the eggs are not infected with Borrelia even if the female tick is (the tick subsequently acquires the spirochete during the larvae stage from the rodent host population). Up to 50% of adult ticks are infected in endemic areas.
I. pacificus is the vector on the west coast of the United States, and its preferential host is the lizard, which is not a very good reservoir for Borrelia . Other Ixodid species are the vectors in other parts of the world. Ixodes species in Europe feed on mammals, birds, and reptiles. Birds are an important reservoir for B. garinii , and rodents are an important reservoir for B. afzelii .
The bite of the Ixodid tick is remembered by only half of the patients with Lyme disease—a key point to remember when taking a history. The tick is also very small (size of a freckle) and often simply overlooked. The intestinal tract of the tick is the reservoir for Borrelia . The blood meal ingested by the tick stimulates local spirochetes to enter a state that allows them to invade the salivary glands of the tick and subsequently be transmitted to the human (or rodent) upon which it is feeding, typically hours after initial attachment.
