Leptospirosis is a worldwide zoonotic disease caused by infection with the spirochete bacterium Leptospira interrogans. L. interrogans is further divided on the basis of antigenic relatedness into serovars. Approximately 200 different serovars of L. interrogans have been identified and within a geographic area, certain serovars are prevalent and become adapted to a particular maintenance host (Table 1). In the United States the serovars of L. interrogans most commonly associated with bovine leptospirosis are hardjo, pomona and grippotyphosa. Serovars icterohaemorrhagiae, canicola, and bratislava are also occasionally associated with leptospirosis in cattle.