Ticks are large blood-sucking Acari, sometimes >1cm in length. There are three families of ticks, Argasidae, Ixodidae, and Nutalliellidae, consisting of a total of about 800 species. The latter family contains only one species of no known medical significance. The development of ticks is from an egg via one (Ixodidae) or more (Argasidae) nymphal stages to the adult stage. The larva has six legs while the nymph and adult have eight legs. The soft ticks
(Argasidae) differ from the hard ticks (Ixodidae) by lacking a dorsal shield (scutum). Moreover, in the soft ticks the mouth-parts are not visible when the tick is observed from above. Between the larval stage and the adult stage, soft ticks can have up to five nymphal stages. The eggs are laid when the female has ingested a blood meal (Sonenshine 1991, 1993; Varma 1993).
Ticks of the genus Ornithodoros can be distinguished from other soft ticks (Argas, Otobius) by the presence of mamillae (minute, regular, usually hemispherical elevations) of the integument and the absence of a distinct lateral margin to the body (Varma 1993) (Figure 23.7). Most species of Ornithodoros inhabit animal burrows, nests, dens, and caves where they feed on wild mammals or seabirds. These ticks are the main vectors and reservoirs of human relapsing fever (Borrelia spp.) also denoted as tick-borne (endemic) relapsing fever. The foci of this infection are very localized and restricted to huts, caves, etc. The spirochaetes are transmitted both transstadially and transovarially in the tick population. To the ticks' blood hosts the spirochaetes are transmitted by tick bite but sometimes also by contaminated coxal fluid from the ticks. Human infections play no role in the dynamics of this zoonosis among the natural hosts of the ticks, that is, small mammals. In the US, three Ornithodoros species (O. hermsi, O. parkeri, and O. turicata) are vectors of relapsing fever Borrelia spp. (B. hermsi, B. parkeri, and B. turicatae). From Israel to western China O. tholozani is the vector of relapsing fever. In Africa and Latin America, other Ornithodoros species transmit other species of relapsing fever borreliae. A typical infection in a human patient includes three to seven attacks at 4-7-day intervals. The clinical picture of the disease is similar to the one exhibited by louse-borne relapsing fever. Ornithodoros, parasitizing seabirds, are also vectors of several arboviruses which can cause human disease.
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