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INTRODUCTION

Leptospirosis is a zoonotic bacterial infection with a ubiquitous worldwide distribution. Weil’s disease, or severe icteric leptospirosis associated with renal failure, was first described in 1886. However, reports of illnesses that most likely representing leptospiral jaundice date back to the early 1800s. The causative organism of leptospirosis, Leptospira species, was recognized independently in Japan and Germany in 1915 through the investigation of illnesses related to occupational and environmental exposures.1 Leptospirosis is the likely etiology for other long recognized occupational hazards among rice field workers in China and Japan, and was referred to as “Autumn Fever.”1 Now considered an emerging neglected infectious disease, it is estimated to cause 1 million severe cases and 60,000 deaths annually.2 Prompt recognition and treatment of leptospirosis continue to be problematic due to the wide spectrum of clinical manifestations and imperfect diagnostic tests. Due to the complexity of the organism, global distribution, and the lack of robust surveillance data, the reported burden of leptospirosis worldwide is underestimated and the epidemiology poorly described.

THE AGENT

Leptospires are slow growing, obligate aerobe, flagellated, and highly motile spirochetes.1 They have a distinct morphologic appearance, which resulted in the species nomenclature of “interrogans,” due to their question mark-like microscopic appearance (Fig. 150-1). Surface lipopolysaccharides (LPS) contribute to host immune response, facilitate characterization of antigenically related serovars of Leptospira into serogroups, and have a role in virulence.1,3,4 Virulence traits of Lepotspires are not well described but include LPS and hemolysins, among others. Based on phenotypic LPS characterization, the genus Leptospira has historically been divided into two species: Leptospira biflexa, containing the nonpathogenic serovars, and Leptospira interrogans, containing more than 200 known pathogenic serovars comprising 24 serogroups.5,6

FIGURE 150-1

Scanning electron microscopic (SEM) image of Leptospira interrogans. (Source: Janice Haney Carr/NCID/HIP/CDC.)

More recently, Leptospira have been classified into 22 distinct genomospecies based on DNA relatedness, 10 of which are pathogenic.6,7 Genomospecies do not equate with traditional serovar and serogroup classifications, or with the traditional species divisions. Genomospecies may contain both pathogenic and nonpathogenic serovars of Leptospira, as well as serovars from more than one serogroup. Serovar and serogroup designations will likely remain in use, therefore, as they are valuable in both the serologic diagnosis and epidemiologic characterization of leptospirosis. The complexity of phenotypic characterization and lack of a standardized molecular subtyping method limit public health investigation and response. Traditional temporal, spatial, and epidemiological determinants most often identify outbreaks rather than use of molecular subtyping.

Molecular typing methods have been applied to leptospires. Pulsed-field gel electrophoresis and other genetic subtyping methods, including whole genome sequencing, hold promise as clinically and epidemiologically relevant techniques for identification and subtyping of leptospiral strains in the future.6

Historical discovery and diagnosis ...

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