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Leptospirosis is a multisystem febrile disease; its underlying pathogenesis is a vasculitis. All leptospires belong to one species of spirochete, Leptospira interrogans. It causes a variety of clinical syndromes including Weil disease (occasionally used to refer to the most severe form of leptospirosis), swamp fever, and field fever in Europe; 7-day fever (nanukayami) and autumnal fever (Hasami-Netsu) in Japan; canefield fever in Australia; and Bushy Creek fever and Fort Bragg fever in the United States; all described before discovery of the causative agent. There is ample epidemiologic evidence that swine, cattle, dogs, and rodents serve as reservoirs.1

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All leptospires belong to one species, L interrogans, which consists of two complexes, interrogans and biflexa. The pathogenic strains belong to the interrogans complex, and at least a dozen strains are known to infect humans. Only three, however—the icterohaemorrhagiae serogroup (from rats), the pomona serogroup (from swine), and the canicola serogroup (from cattle and dogs)—do so with any frequency. The hardjo serovariant from cattle has also caused disease in humans.

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Although uncommon, leptospirosis is the most common zoonosis in the world and may be an emerging infection. Although only 54 patients were reported to the Centers for Disease Control in 1988, an outbreak of this infection in 1995 in Nicaragua affected several thousand people. In 1997, an outbreak in India of an epidemic febrile illness associated with uveitis was diagnosed as leptospirosis. Clinical leptospirosis is not always recognized, as evidenced by antileptospiral antibodies in individuals who have no history of leptospirosis. Active surveillance on two Hawaiian islands revealed an annual incidence rate of 128 per 100,000. Among the factors most strongly associated with this infection are household use of rainwater catchment systems and contact with animal tissues and cattle.1-3

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The reservoir animals retain leptospires in their renal tubules and shed large numbers of these organisms in the urine for months after infection. Humans become infected through contact with animal urine, either directly or secondarily through contaminated soil or water. Leptospiras are very sensitive to acid and perish in solutions of low pH in a few hours; however, in alkaline or neutral medium, they persist for weeks if the temperature is above 22°C (71.6°F). During the warm seasons, stagnant waters and moist soil are common sources of infection. Infection can be acquired through cut or abraded skin, or through respiratory or conjunctival epithelium with immersion. The disease is associated with farming, abattoirs, and sewers, as well as with camping, fishing, and contact with pet dogs. It is more frequent in summer and early fall, and has a 3:1 predominance of males. Seventy percent of infections occur in individuals between 10 and 40 years of age.1-3

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Clinical Manifestations

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Clinical manifestations vary somewhat with the infecting serogroup. The canicola and pomona groups tend to cause less severe disease than does the icterohaemorrhagiae group. Nevertheless, in an outbreak of the hardjo infection ...

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