[Amblyomma spp.] [Dermacentor spp.] [Ixodes spp.] [Ornithodoros spp.] [Rhipicephalus spp.]
There are many genera and species of ticks in the families Ixodidae (hard ticks) and Argasidae (soft ticks) that are of public health importance. Some representative genera, and diseases they are known vectors for, include: Amblyomma (tularemia, ehrlichiosis, Rocky Mountain spotted fever (RMSF), and boutonneuse fever); Dermacentor (RMSF, Colorado tick fever, tularemia, Siberian tick typhus, and Central European tick-borne encephalitis, as well as being an agent of tick paralysis); Hyalomma (Siberian tick typhus, Crimean-Congo hemorrhagic fever); Ixodes (Lyme disease, babesiosis, human granulocytic ehrlichiosis, and Russian spring-summer encephalitis); Rhipicephalus (RMSF and boutonneuse fever); Ornithodoros (tick-borne relapsing fever); Carios (tick-borne relapsing fever).
Most tick species undergo one of four different life cycles. Members of the family Ixodidae undergo either one-host, two-host or three-host life cycles. During the one-host life cycle, ticks remain on the same host for the larval, nymphal and adult stages, only leaving the host prior to laying eggs. During the two-host life cycle, the tick molts from larva to nymph on the first host, but will leave the host between the nymphal and adult stages. The second host may be the same individual as the first host, the same species, or even a second species. Most ticks of public health importance undergo the three-host life cycle, whereby the tick leaves the host after the larval and nymphal stages. The three hosts are not always the same species, but may be the same species, or even the same individual, depending on host availability for the tick. Members of the family Argasidae undergo what is called a multihost life cycle. Argasid ticks have two or more nymphal stages, each requiring a blood meal from a host. Unlike the ixodid ticks, which stay attached to their hosts for up to several days while feeding, argasid ticks are adapted to feeding rapidly (about an hour) and then promptly leaving the host.
One-Host Ixodid Tick Life Cycle
Two-Host Ixodid Tick Life Cycle
Three-Host Ixodid Tick Life Cycle
Multihost Argasid Tick Life Cycle
Mating usually occurs, and egg-laying always occurs, off the host in a sheltered area (usually an animal nest). Eggs hatch into six-legged larvae in the parents’ sheltered area. They quest for a host in the vicinity of the sheltered area. Once a suitable host is found, they feed for anywhere from one hour to several days, depending on the species . After feeding, the larvae leave the host and molt into the first nymphal instars in the sheltered area – . The nymphs quest for, and feed on, the second host rapidly (usually about an hour). The second host is usually the same species, and often the same individual, as the first host. The first nymphal instars leave the host and molt into the next nymphal instars in the sheltered area – . This cycle can continue to accommodate up to seven nymphal instars , depending on the species. After the last nymphal instar has fed, it leaves the host and molts into an adult – in the sheltered area. Adults may continue to feed on the host , feeding rapidly and detaching after each blood meal. Females of some species lay egg batches after each meal. Humans are usually only incidental hosts for argasid ticks and may be fed upon by any of the stages.
While ticks as a whole are worldwide in distribution, most species are restricted to various regions. All major biogeographic regions (except Antarctica) have tick species of public health importance.
Most ticks do not elicit any response from their host while feeding. Ticks in the genera Dermacentor and Ixodes have been implicated in tick paralysis, a condition characterized by an acute, ascending, flaccid motor paralysis that can result in death if the tick is not removed. The condition is believed to be caused by toxins in the ticks’ saliva.
Amblyomma americanum and Amblyomma spp.
Members of the genus, Amblyomma, are known vectors of a variety of diseases in humans. In North America, A. americanum transmits Francisella tularensis (tularemia), Ehrlichia chaffensis (ehrlichiosis), and Rickettsia rickettsii (Rocky Mountain spotted fever, or RMSF). In Africa, A. hebraeum transmits Rickettsia conorii (boutonneuse fever); in Central and South America, A. cajennense transmits RMSF. Members of the genus Amblyomma are characterized by having mouthparts noticeably longer than the basis capituli, a usually ornate dorsal shield, eyes present on the dorsal shield, and festoons (which may be difficult to see in engorged specimens). Adult females of A. americanum have a distinctive white spot near the posterior end of the dorsal shield.
Dermacentor andersoni and D. variabilis.
Ixodes scapularis and Ixodes spp.
Rhipicephalus sanguineus is known as the brown dog tick and is found nearly worldwide. Humans are not the usual host, but there are increasing reports of disease transmission with this species, including Rickettsia rickettsii (Rocky Mountain spotted fever, or RMSF) and Rickettsia conorii (boutonneuse fever). Adults are characterized by having a laterally-produced, angulate basis capituli, a dorsal shield with eyes, festoons, and deeply-cleft front coxae. The festoons and anal groove may be difficult to see in engorged specimens. The mouthparts are relatively short, in relation to the basis capituli.
Ornithodoros moubata and O. turicata.
Members of the genus, Ornithodoros, are known vectors of Borrelia hermsi (tick-borne relapsing fever, or TBRF) in North America and several TBRF spirochetes in Africa. Like most members of the Argasidae, or soft ticks, Ornithodoros spp. are characterized by lacking a dorsal shield and not displaying marked sexual dimorphism. Their mouthparts are subterminally attached and not visible from above. Adults and developing nymphs do not remain attached to their hosts, as do members of Ixodidae; rather they are adapted to feeding rapidly and leaving the host promptly.
In most cases, ticks need only to be identified to the genus level to rule-out potential vectors of disease. Ticks are best identified by examining non-engorged adults. Nymphs may lack certain diagnostic features and in engorged individuals, certain features (festoons, anal groove) may be difficult to see. The geographic location of the tick can also be important for accessing the public health importance.
DPDx is an education resource designed for health professionals and laboratory scientists. For an overview including prevention and control visit www.cdc.gov/parasites/.