OVERVIEW

During the two year life cycle, deer ticks generally feed as larvae, as nymphs, and as adults. Lyme spirochetes are typically transmitted to humans by nymphs.

Adult deer tick preparing to feed. (Link)

The nymph is the most likely transmitter because a larva most likely does not have a spirochete infection as it is most likely taking its first meal. The deer tick is not infected at birth, and so must acquire the infection from feeding on the blood of an animal already infected with the Borrelia burgdorferi. The nymph would have been infected while feeding as a larva. The adult is less likely to transmit spirochete infections to humans because of its size. The difference in size between an adult deer tick and a nymph is large enough to make the adult tick much more visible to the human. When spotted, the tick is likely removed. Since the adult is vastly larger, it is more likely to be noticed before it has fed for sufficient time to transmit infection.