# Serological Investigation and Epidemiological Analysis of Bovine Leptospirosis in Egypt
## 1 Introduction  
Leptospirosis is a re-emerging zoonosis of worldwide distribution with increased prevalence in tropical, subtropical, and temperate countries [1]. This disease is caused by spirocheltal pathogens of the genus _Leptospira_, which comprise pathogenic and saprophytic species. Although rodents are considered the main reservoirs [2; 3], the epidemiological profile of the pathogen also includes a wide range of domestic and wild animals, besides humans, that can be considered natural reservoirs and carriers of _Leptospira_. As such, the pathogen is transmitted mainly by direct contact of mucosa or damaged skin to exposed water or soil contaminated with the urine of infected animals, subsequently colonizing the proximal renal tubules of various mammals [4]. According to genetic relatedness, numerous immunologically distinct servovars of _Leptospira_ spp. have been identified and classified into over 300 serovars grouped into almost 30 serogroups [5]. Certain servovars may adapt to specific animal hosts, such as _L. interrogans_ serogroup Hardjo serovar Hardjo in cattle, which appeared as an asymptomatic carrier animal. In contrast, the same animal species can become an accidental host of another serovar, such as _L. interrogans serogroup Pomona_ serovar Pomona, _L. interrogans_ serogroup _Grippotynphos_ serovar _Grippotynphos_ and _L. interrogans_ serogroup _Icterohaemorrhagiae_ serovar _Icterohaemorrhagiae_, as in the case of bovine leptospirosis [6; 7]. Regarding its clinical impact, leptospirosis is definitely a cause of significant wealth and health losses in animals and humans [8]. In animals, the disease can range from insignificant and asymptomatic to deadly, depending on the causative species [9]. Among others, bovine leptospirosis has been associated with multiple symptoms that include septicemia, abortion, infertility, hemoglobinuria, drop in milk production, and mastitis [10; 11]. Given its zoonotic potential, studies have shown that human leptospirosis presents with various signs, some of which are mild (i.e., fever, headache, and mylgia) and some that are severe (i.e., liver and kidney failure). In addition, death in some cases of human leptospirosis frequently occurs due to pulmonary hemorrhage [12; 13]. Pathogen detection in a host has been considered one of the main approaches for controlling the infection [14; 15; 16; 17; 18]. Isolation of the causative agent can be used to definitively diagnose leptospirosis. However, bacterial isolation has been associated with some drawbacks considering that this method is time consuming and necessitates pathogen livability and viability of bacteria growth on culture media. As such, several serological techniques have been adopted for detecting anti-_Leptospir_ antibodies worldwide, which have provided several advantages over classical detection and isolation methods [19]. Among others, the enzyme-linked immunosorbent assay (ELISA) technique has been widely accepted as a standard serological test for detecting antibodies against a wide range of bacterial pathogens, including _Leptospira_ spp. [20]. Considering this, ELISA has some advantages over other remaining techniques, such as the microscopic agglutination test (MAT). In this respect, ELISA is relatively sensitive, specific, and semi-automated, utilizes killed antigens, and allows for objective interpretation of results [21]. Moreover, ELISA has been considered practical, fast, and affordable and has a greater throughput. It is therefore not surprising that ELISA has been regularly used for initial serological screening in large-scale surveys for the identification of specific antibodies against _Leptospira_ spp. [20].