Leptospirosis is a worldwide zoonotic disease of major importance caused by pathogenic spirochetes of the genus Leptospira. To date, 38 pathogenic Leptospira species have been described (composing subclades 1 and 2, previously referred to as pathogenic and intermediary Leptospira, respectively) (1). Among the currently known pathogenic Leptospira taxa, more than 300 serovars have been identified and classified into serogroups according to their antigenic similarities (2).
In both humans and other animals, Leptospira infection can cause mild or strong clinical signs or be asymptomatic. In swine, acute and chronic infections are described mainly in regards to the reproductive impairments (abortion, stillbirth, perinatal mortality) responsible for economic losses; however, deterioration of general condition is also described, including haemorrhage, haematuria, renal damage and death (3). Asymptomatic carriage also appears to occur in pigs, allowing for the undetected expansion and maintenance of the bacteria on farms (4).
Transmission generally occurs by contact with water spoiled by contaminated urine, in which the bacteria can survive for several months (5). The main hosts of the Leptospira bacteria are rodents; however, other wild and domestic mammal species can also be involved in the transmission pathways. Because pig farming is mainly conducted indoors in France, contamination is most likely to occur via the introduction of an infected individual or through contact with commensal rodents. Furthermore, as close contact between animals can promote intra-herd transmission of the bacteria, the regulation that has recently mandated that pregnant sows be grouped together (Directive 2008/120/EC) could lead to an increased Leptospira incidence.
The incidence of Leptospira on pig farms in Europe remains unknown. However, epidemiological information has been extracted from data recorded in laboratories offering MAT for diagnostic purposes. According to these data, between 19 % and 26 % of tested pigs were seropositive in Italy, Germany and France (6–9). However, the inclusion criteria (e.g., the inclusion or not of pigs without clinical suspicion of infection) and the panel of serogroups included may vary among studies, leading to study variation in seroprevalence and limiting reliable comparisons of the results.
In addition, the previous studies did not report how the MAT results, including the cross sero-reactivity results, were managed (6–9). According to previous studies, MAT result interpretation is subjective and should consider possible cross reactions (10, 11). Chappel et al. 2004 recommended that in the presence of cross-reactions, the serogroup associated with the maximum titre should be considered the predominant serogroup. In addition, Miller et al. (2011) showed that the values of a single dilution titre vary among operators. Thus, a strict difference of two dilutions between the maximum titre and the others should be considered to account for this variability and ensure the putative infective serogroup is defined with sufficient evidence. As no consensus currently exists in the literature regarding the definition of the predominant serogroup, this paper suggests the use of weighted averages to report the serogroup distribution from large sets of MAT surveillance data.
Given that several mammalian species may act as reservoirs for swine leptospirosis, disease source control is complex. Vaccination may be a promising way to reduce the health and economic consequences of Leptospira infections. Vaccines provide protection against homologous or closely related but not heterologous serovars; therefore, particular attention should be paid to identifying relevant serovar antigens for vaccine development. In France, a swine vaccine has been commercialized since 2019 that protects against the following serogroups: Icterohaemorrhagiae, Australis, Grippotyphosa, Pomona and Tarassovi. However, a comprehensive survey of all serogroups circulating in swine herds in France is required to assess the relevance of including a given serogroup in a putative vaccine. The last such survey dates back to 2007 (6), and in light of the spatio-temporal variation in serogroup distribution observed in domestic animals (12, 13), updated data are required. This paper reports the results of a nationwide survey of the serogroups in swine herds in France.