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A genus of spirochetes that have a unique genome composed of a linear chromosome and numerous linear and circular plasmids. Borreliae are motile, helical organisms with 4–30 uneven, irregular coils, and are 5–25 micrometers long and 0.2–0.5 μm wide. All borreliae are arthropod-borne. Of the 24 recognized species, 21 cause relapsing fever and similar diseases in human and rodent hosts; two are responsible for infections in ruminants and horses; and the remaining one, for borreliosis in birds. See Bacteria
The borreliae of human relapsing fevers are transmitted by the body louse or by a large variety of soft-shelled ticks of the genus Ornithodoros. The species B. burgdorferi, the etiologic agent of Lyme disease and related disorders, is transmitted by ticks of the genus Ixodes. Borrelia anserina, which causes spirochetosis in chickens and other birds, is propagated by ticks of the genus Argas. Various species of ixodid ticks are responsible for transmitting B. theileri among cattle, horses, and sheep. Borrelia coriaceae, isolated from O. coriaceus, is the putative cause of epizootic bovine abortion in the western United States.
Polyacrylamide gel electrophoresis of spirochetes has shown that the outer surface of the microorganisms contains numerous variable lipoproteins of which at least two are abundant. The antigenic variability is well known for the relapsing fever borreliae. A switch in the major outer-surface proteins leads to recurrent spirochetemias. Tetracyclines, penicillins, and doxycycline are the most effective antibiotics for treatment of spirochetes. Two vaccines consisting of recombinant B. burgdorferi have been evaluated in subjects of risk for Lyme disease. Both proved safe and effective in the prevention of this disease. See Antibiotic, Medical bacteriology