Leptospirosis: A Worldwide Zoonotic Disease

By Ian Veitch

Introduction

Leptospires are Gram-negative spirochetes that all share a distinctive cell structure consisting of a long, tight spiral that hooks at both ends (1, figure 1). Leptospires are a well-known spirochete mainly because they cause leptospirosis, the most widespread zoonotic disease in the world that has been found in almost all species of mammals examined (2). Chronic carriers of leptospirosis are usually wild or domestic animals, such as rats, dogs, cattle, and pigs (3). Humans can contract the disease; however, leptospirosis in humans is always acquired from an animal source. Pathogenic leptospires reside in the proximal renal tubules of the kidneys of carriers and are subsequently excreted in urine. Urine tainted with leptospires then can contaminate soils, surface water, streams and rivers (3). It is pertinent to note that not all leptospira species are pathogenic. There are twenty documented species in the genus Leptospira (figure 2). These twenty species are classified as pathogenic, non-pathogenic, or intermediate/opportunistic. While only nine of twenty species are labeled as pathogenic, more than 500,000 cases of severe leptospirosis are reported each year, with mortality rates exceeding 10% (4). Furthermore, the incidence of leptospirosis is higher in the tropics than in temperature regions because of longer survival of leptospires in the environment in warm and humid conditions (5). Preventing and diagnosing leptospirosis can be a complicated process, partly due to the sheer diversity of bacteria within the genus Leptospira .