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Leptospira is a genus of spiral-shaped, gram-negative bacteria in family Leptospiraceae, order Spirochaetales.  Before 1989 the different strains, or serotypes, were organized into two species: L. interrogans, the pathogenic varieties, and L. biflexa, which are harmless.  Genetic techniques have since helped group the more than 250 pathogenic known “serovars” (types, based mostly on cell surface proteins, and generally using specific mammalian or marsupial hosts) and more than 50 non-pathogenic known serovars into 21 species for formal classification.  Because serovars regularly exchange genetic information, this serotype system is still used, especially in clinical practice.  

Leptospira serotypes are found world wide, except in Antarctica.  They are common in warm wet climates especially where sanitation is poor, as the bacteria are spread in the urine of infected hosts.  Common hosts are rats, and other rodents, cattle and muskrats.  In humans and other vertebrates, pathogenetic Leptospira cause leptospirosis.  Infection occurs by ingesting the spirochete bacteria in contaminated food, water, or through skin contact, especially when broken skin comes in contact with contaminated water.  Outbreaks regularly occur in developing countries especially in wet seasons or after flooding.  Most cases are mild or show no symptoms, but leptospirosis is a potentially life-threatening disease which usually manifests as flu-like symptoms in early stage, progressing to symptoms including encephalitis and dysfunction of kidneys, liver, reproductive organs and lungs and ultimately leading to the failure of these organs. Number of human cases is estimated at up to 100 cases per 100,000 persons per year in topical climates, fewer in temperate locales.  Though diagnosis can be difficult, once diagnosed leptospirosis can be effectively treated with antibiotics.  Leptospirosis also affects a variety of mammals including farm animals, pets (dogs and cats), opposums, rats, deer, South American camelids, and while generally rare among wild animals, wild pinnipeds regularly become infected.  Leptospira bacteria cannot multiply outside their hosts but in warm conditions can remain viable in water or wet soil for months (Levett 2001; Norris 2014; Ren et al. 2003; Spickler and Leedom Larson 2013; WHO 2014; Zimmer 2014).

Levett (2001) provides a detailed review of the biology, taxonomy, epidemiology, clinical features, and pathogenesis of leptospires.

Recent high tech sequencing and bioinformatics methods diagnosed a Leptospira infection in a patient whose symptoms could not be identified, a first use of new powerful medical technology (Naccache et al. 2014; Norris 2014; Wilson et al. 2014; Zimmer 2014).

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