Ticks are currently considered to be second only to mosquitoes as vectors of human infectious diseases in the world. Each tick species has preferred environmental conditions and biotopes that determine the geographic distribution of the ticks and, consequently, the risk areas for tickborne diseases. This is particularly the case when ticks are vectors and reservoirs of the pathogens. Since the identification of Borrelia burgdorferi as the agent of Lyme disease in 1982, 15 ixodid-borne bacterial pathogens have been described throughout the world, including 8 rickettsiae, 3 ehrlichiae, and 4 species of the Borrelia burgdorferi complex.

Ticks are obligate hematophagous arthropods that parasitize every class of vertebrates in almost every region of the world. By 1 January 1996, 869 species or subspecies of ticks had been recorded. There are 2 major tick families: the Ixodidae, or “hard ticks,” so called because of their sclerotized dorsal plate, which are the most important family in numerical terms and in medical importance, and the Argasidae or “soft ticks,” so called because of their flexible cuticle. A third family, the Nuttalliellidae, is represented by only a single species that is confined to southern Africa. Ticks have 3 basic life stages: the larval, nymphal, and adult (male and female). Ixodid and argasid ticks differ both anatomically and in their life cycle.

 Ixodids have a number of attributes that enhance their vector potential. They feed for relatively long periods (several days), during which they remain firmly attached to the host. Also, their bite is usually painless and they may go unnoticed for lengthy periods of time. Each stage of the tick feeds only once, and this feeding may involve a great variety of vertebrates that occupy very diverse habitats. On the other hand, Argasids feed briefly and often, usually on a single host species. They tend to live in dry areas, and most species live in sheltered sites near their hosts.

Ticks have been recognized as human parasites for thousands of years and were described by ancient Greek writers, including Homer and Aristotle. Although ticks have been studied for a long time, the first demonstration that they may transmit infectious diseases was made at the end of the 19th century, when Smith and Kilbourne demonstrated that Boophilus annulatus transmitted the protozoan Babesia bigemina, the agent of Texas cattle fever.

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Microbiology: Current Research
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