Ticks

Life cycle of one-host ixodid (hard) ticks. The adult is considered the diagnostic stage, as identification to the species level is best achieved with adults. Few ixodids of public health importance follow this pattern; an example is Rhipicephalus annulatus, which can serve as a vector for babesiosis.

One-host ixodid ticks remain on the same host for the larval, nymphal and adult stages, only leaving the host prior to laying eggs. Vertical transmission of Babesia via transovarial transmission has been demonstrated for some species of ticks. Gravid females lay eggs in the environment . The eggs hatch into six-legged larvae . Larvae seek out and attach to the host and after two molts, develop into adults - . Although humans may serve as incidental hosts for species normally found on other animals, they usually do not host all three stages. Females drop from the host to lay eggs and the cycle repeats.

Life cycle of the two-host ixodid (hard) ticks. The adult 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 spans over two years. Gravid females drop off the second host after feeding to lay eggs , usually in the fall. Eggs hatch into six-legged larvae and overwinter in this stage. The following spring, the larvae seek out and attach to the first host , usually a rodent or lagomorph. The larvae molt into nymphs on the first host - . Engorged nymphs drop off the first host, usually in the late summer or fall and overwinter in the nymphal stage. Nymphs molt into adults the following spring and seek out the second host , 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, as the first host.

Life cycle of three-host ixodid (hard) ticks. The adult is considered the diagnostic stage, as identification to the species level is best achieved with adults. Most ticks of public health importance follow this pattern, including members of the genera Ixodes (Lyme borreliosis, babesiosis, human granulocytic ehrlichiosis), Amblyomma (tularemia, ehrlichiosis and Rocky Mountain spotted fever), Dermacentor (Rocky Mountain spotted fever, Colorado tick fever, tularemia, tick paralysis), and Rhipicephalus (Rocky Mountain spotted fever, boutonneuse fever).

Three-host ixodid ticks have a life cycle that usually spans three years, although some species can complete the cycle in only two years. Adult females drop off the third host to lay eggs after feeding , usually in the fall. Eggs hatch into six-legged larvae and overwinter in the larval stage. In the spring, the larvae seek out and attach to the first host, usually a small rodent . Later in the summer, engorged larvae leave the first host and molt into nymphs , usually in the fall. The ticks overwinter in this stage. During the following spring, the nymphs seek out and attach to the second host , usually another rodent or lagomorph. The nymphs feed on the second host and drop off later in the summer . Nymphs molt into adults - off the host in the late summer or fall, and overwinter in this stage. The next spring, adults seek out and attach to a third host, which is usually a larger herbivore (including cervids and bovids), carnivore, or human . The adults feed and mate on the third host during the summer. Females drop off the host in the fall to continue the cycle. Females may reattach and feed multiple times. The three hosts do not necessarily have to be different species, or even different individuals. Also, humans may serve as first, second or third hosts.

Multihost life cycle for argasid (soft) ticks. Unlike the Ixodidae, members of the family Argasidae have two or more nymphal stages, each of which requires a blood meal. This pattern is referred to as the multihost life cycle. Two species of public health concern in the United States, Ornithodoros hermsi and O. turicata, are vectors of tick-borne relapsing fever (TBRF) spirochetes. In Africa and Asia, O. moubata is a vector of TBRF spirochetes. Members of the genus Carios are vectors of TBRF spirochetes in Central and South America.