Association of Acute-Phase Proteins and IgG with Bovine Respiratory Disease, Seroconversion to Respiratory Infections and Farm-Level Factors in Rearing Calves

This study investigated the associations between acute-phase proteins (APPs) such as serum amyloid A (SAA), haptoglobin (Hp), and albumin (Alb) as well as immunoglobulin G (IgG) with bovine respiratory disease (BRD), seroconversion to respiratory infections and farm-level factors in rearing calves. Datasets were obtained from a randomised trial of 476 calves in Finland that compared morbidity in large- versus small-group housing. Calves were assessed for clinical BRD, and their blood was sampled three times during the first 50 rearing days to measure APPs and IgG concentrations and virus-specific antibodies against <i>Mycoplasma bovis</i> (<i>M. bovis</i>), bovine respiratory syncytial virus (BRSV), bovine parainfluenza virus 3 (BPIV3), and bovine coronavirus (BCV). Linear mixed-effects regression models showed higher serum SAA and Hp concentrations in calves with clinical BRD. BRSV seroconversion was associated with increased serum SAA and lower Alb while <i>M. bovis</i> seroconversion with increased serum Hp. Calves in larger groups had lower serum Hp, SAA and Alb, and pens with higher BRD cases were associated with increased serum SAA and lower Alb. IgG concentration was associated with BRSV seroconversion. These results suggest that early immune monitoring using APPs and IgG could help guide targeted management strategies to improve calf health and welfare.