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Some spirochetes are free living; some are members of the normal flora of humans and animals. The oral cavity, particularly the dental crevice, harbors a number of species of the genera Treponema and Borrelia as part of its normal flora. Under un-usual conditions these spirochetes together with anaerobes in the normal flora can cause necrotizing, ulcerative infection of the gums, oral cavity, or pharynx (Vincent’s infection, trench mouth). The pathogenesis of these opportunistic infections is not un-derstood but they are correlated with immunocompromise, severe malnutrition, and neglect of basic hygiene. The term “trench mouth” refers to the occurrence of these in-fections in troops under the appalling conditions that existed in the trenches during World War I.
The major spirochetal diseases are caused by selected species of three genera which are not found in the normal flora, Treponema (T. pallidum), Leptospira (L. interrogans), and Borrelia (B. recurrentis, B. hermsii, and B. burgdorferi). Most Borrelia and Lep-tospirainfections are zoonoses transmitted from wild and domestic animals. T. pallidum is a strict human pathogen transmitted by sexual contact.
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