11. Association of Serum Igg Concentration and Serum Brix Values with Neonatal Lamb Mortality in a Commercial Rideau Arcott Flock.

Abstract Neonatal lamb mortality remains a significant welfare and economic challenge in sheep production systems. Adequate passive immunity acquired through colostrum ingestion is essential for neonatal health, as lambs are born essentially devoid of circulating immunoglobulins due to the epitheliochorial structure of the ovine placenta. Serum immunoglobulin G (IgG) concentration is widely used to assess passive immunity status; however, relatively few studies have quantified the relationship between serum IgG concentration and lamb mortality under commercial production conditions. Serum Brix measurements obtained using digital refractometers have also been proposed as a practical on-farm proxy for passive immunity status. The objective of this cohort study was to evaluate the association between immunity indicators, measured as serum IgG concentration and serum Brix values, and neonatal lamb mortality in a commercial sheep flock. This study was conducted on a commercial Rideau Arcott flock in Ontario, Canada from August 2024 to June 2025. Lambs were sampled once between 24 and 166 hours of age via jugular venipuncture. Serum Brix measurements were obtained on-farm using a digital refractometer and IgG concentrations were quantified using radial immunodiffusion. Lamb survival was monitored until 100 days of age. Mixed-effects logistic regression models evaluated associations between passive immunity indicators and mortality while accounting for clustering by ewe, and Cox proportional hazards models assessed time-to-event associations with survival. A total of 637 lambs were included in the analysis, with 140 deaths recorded during the observation period. In mixed-effects logistic regression models higher serum Brix values were associated with reduced odds of mortality (OR = 0.78, P = 0.001). In Cox proportional hazards models, higher serum Brix values were also associated with a reduced hazard of death over time (HR = 0.81, 95% CI: 0.70–0.93, P = 0.003). When serum IgG concentration was evaluated as the exposure in the same models, increasing IgG concentration was similarly associated with reduced mortality risk, with lower odds of death observed in logistic regression (OR = 0.96, P < 0.001) and reduced hazard of death in the Cox proportional hazards model (HR = 0.97, 95% CI: 0.95–0.98, P < 0.001). The consistency in direction and magnitude of these associations indicates that serum Brix measurements provide comparable predictive information to serum IgG for lamb survival. These findings indicate that higher serum IgG concentrations are consistently associated with reduced mortality risk, emphasizing the importance of achieving adequate passive immunity in neonatal lambs and supporting the use of serum Brix measurements as a practical on-farm proxy for passive immunity status in commercial sheep production systems.