Life cycle of the two-host ixodid (hard) ticks.

Life cycle of the two-host ixodid (hard) ticks.
Photographer: Centers for Disease Control/Division of Parasitic Diseases and MalariaRights holder: Centers for Disease Control/Division of Parasitic Diseases and Malaria

The adult of a two-host ixodid (hard) tick is considered the diagnostic stage, as identification to the species level is best achieved with adults.  An example of an ixodid tick of public health concern with this life cycle is Hyalomma marginatum, a vector of Crimean-Congo viral hemorrhagic fever.

Two-host ixodid ticks have a life cycle that usually extends over two years.  After feeding, gravid females drop off the second host to lay eggs (1), usually in the fall.  Eggs hatch into six-legged larvae (2) and overwinter in this stage.  The following spring, the larvae seek out and attach to the first host (3), usually a rodent (e.g., a mouse) or lagomorph (e.g., a rabbit).  The larvae molt into nymphs on the first host (3a - 3b).  Engorged nymphs drop off the first host, usually in the late summer or fall (4) and overwinter in the nymphal stage.  Nymphs molt into adults the following spring (5) and seek out the second host (6), which is usually a larger herbivore (bovids, cervids, etc).  Adults feed on the second host during the summer and mate.  In the fall, females drop off the second host to continue the cycle.  Females may reattach and feed multiple times.  Humans may serve as first or second hosts for ticks with this life cycle.  Also, the second host does not necessarily have to be a separate species, or even a separate individual, from the first host.

From Centers for Disease Control Parasites and Health website.