145. Antibody Response to Booster Clostridial Vaccination Is Negatively Correlated with PFEC EBV Values in Katahdin Lambs

Abstract To determine the relationship between antibody response to booster clostridial vaccine and parasite resistance in Katahdin lambs, blood samples were collected 1 week after booster clostridial vaccination. Lambs were divergently bred for postweaning fecal egg count (PFEC) estimated breeding values and fecal samples were collected for EBV calculation at weaning and again after weaning. Blood was collected into serum separator tubes from 157 spring-born Katahdin lambs in 2024 and serum was collected and stored at -20 °C until analysis. To quantify immunoglobulin-G (IgG) the Valita Titer™ assay was used where serum was diluted 1:20 in MOPS buffer and measured using fluorescence polarization to determine IgG concentration. There was a significant negative correlation between IgG and PFEC EBV of -0.24 (P = 0.002). Then, lambs were sorted into 3 groups based on PFEC EBVs (High > +100, Med < 100 >-75, Low < -75). While mean IgG of all lambs was 42.29mg/ml, lambs in the Low PFEC group had significantly greater IgG than lambs in the high group (44.52mg/ml vs 39.38mg/ml; P = 0.01), whereas lambs in the Med group had intermediate IgG concentration (42.59 mg/ml) and were not different from other groups. The second grouping was based on IgG concentration where ½ of the SD was added to and subtracted from the mean to determine 3 groups (High > 43.67mg/ml, Med 43.67 - 37.86 mg/ml, Low < 37.86 mg/ml). Estimated breeding values for lambs within the IgG groups was significantly different between High and Low IgG groups (-38.04 vs 34.01; P = 0.009), and again the Med group was intermediate and not significantly different than either high or low group (9.53; P > 0.05). Collectively these data demonstrate that PFEC EBVs are predictive of antibody level when EBV values are extreme but were not predictive for PFEC EBV values between -75 and +100. Identification of sheep with superior antibody response to a common vaccine may provide a tool for producers to select for generalized disease resistance ultimately reducing the need for and use of antibiotics or anthelmintics.