**2. Etiology**

Leptospirosis is a prominent communicable disease caused by spirochete bacteria. The bacterial species belong to genus *Leptospira* that have ability to cause a disease in a variety of wild and domestic animal bases [3]. Spirochetes bacteria are motile having hook form or question-mark shape and range in size from 6 to 20 μm in length and 0.1 μm in thickness [4]. Family Leptospiraceae includes genus *Leptospira*, which is further divided into two strains, that is, pathogenic and saprophytic [4, 15]. Pathogenic *Leptospira* have 21 species, 25 serogroups, and 250 serovars [3, 5]. *Leptospira* spp. are obligate aerobes having sluggish growth. Ideal growth temperature for *Leptospira* is 28–30°C [16]. There are different other characteristics of *Leptospira* like size, number of genes and pseudogenes, etc. (**Figure 1**). Serovars "pomona and grippotyphosa" are expectedly found to be the most prevalent candidates [17]. Leptospirosis in cattle is caused by *Leptospira borgpetersenii* and *Leptospira interrogans* serovar Hardjo, strains (well adapted to cattle) Hardjo bovis and Hardjo prajitno [18]. In Brazil as well as in Latin America, L. borgpetersenii serovar Hardjo strain Hardjo bovis were isolated from naturally infected cattle. Before this study, only serological studies had shown reactive animals with the serovar Hardjo in various countries [20, 21]. In Brazil, Chile, England, and Columbia, serovar Hardjo was found most prevalent among cattle [22]. Between 1988 and 2007 in France, serovar Serjoe (34%) was most common in cattle [23]. There are different reservoir hosts of *Leptospira* (**Table 1**).
