Towards a Tool for Retrospective Assessment of Exposure of Non-weaned Calves to the Bovine Respiratory Syncytial Virus (brsv)

Introduction Respiratory disorders are usually considered as a major health issue in cattle, particularly in young calves [1, 8, 13]. Among the great number of viruses and bacteria potentially involved, the Bovine Respiratory Syncytial Virus (BRSV) appears to be the most common and significant aetiological agent of respiratory disorders [8, 9, 13]. Oubreaks of BRSV infection mostly occur in nonweaned calves less than three months of age [14]. Currently, the most widespread method of indirect detection of BRSV infection relies on detection of total immunoglobulins G (IgG) specific of BRSV by Enzyme Linked Immunosorbent Assay (ELISA). Evidence of an active immune response to BRSV is a specific rise in titer of total IgG from at least 400% between two blood samples three weeks apart [5]. However, taking into account the very strong seroprevalence of specific BRSV antibodies in cows (between 70% and 95%) the frequency of calves with BRSV specific maternallyderived antibody is very high [4, 9]. Maternally-derived antibody suppresses antigen-specific serum antibody response during the first months of life. So, reliance on paired-serological testing to identify BRSV in calves less than 3 months of age is not recommended [15]. The passively transferred antibodies are mainly of the IgG1 isotype, whatever the etiologic agent concerned [4, 6]. Amount of IgG2 is very low in the colostrum [2, 7, 10]. Moreover, if IgG2 seem to appear only 3 weeks after the infection, they remain detectable during more than 80 days [7, 11, 15]. However, the only study dealing with IgG2 specific of BRSV in calves less than 3 months and in spontaneous BRSV natural infections was carried out on only five calves [11]. This study aimed at describing, under field conditions, the level of total IgG and IgG2 specific of BRSV obtained from a large sample of sera from young calves before and after colostrum intake and from their dams.