36 Differential haptoglobin responsiveness to a Mannheimia haemolytica challenge altered immunologic, physiologic, and behavioral responses in beef steers

A retrospective analysis of data from a previous study (Kayser et al., J. Anim. Sci. 97:596; 2019) revealed that steers challenged with Mannheimia haemolytica (MH) had divergent serum haptoglobin (HPT) despite having similar leukocyte and temperature responses. In that study, 36 steers (BW 352 ± 23 kg) were fitted with rumen boluses and were fed from electronic bunks to measure DMI and feeding behavior prior to inoculation with saline or MH. Whole blood was collected on days -4, 0, 1, 2, 3, 5, 7, 10 and 14 relative to MH inoculation. The MH steers were retrospectively classified as HPT responsive (RES; n = 9; mean AUC = 62.6 mg/dL/d) or HPT non-responsive (NON; n = 9; mean AUC = 4.5 mg/dL/d). The current objective was to determine if the HPT responsive phenotype altered other immunologic, physiologic, or behavioral responses, compared with saline controls (CON; n = 18). The magnitude of increase in neutrophils (P< 0.01) and total leukocytes (P< 0.05) was greater in RES than NON or CON on d1. All MH-challenged steers experienced a transient febrile response, but temperature was greater (P< 0.01) in RES on days 0 and 4 compared to NON and CON. Intake was depressed in all three groups d0, but magnitude of depression in RES was greater (P< 0.01) than NON or CON, and remained lower (P < 0.01) d 1, 2, 3, 6, and 8. Bunk-visit duration was decreased (P < 0.01) in all MH-challenged steers on day 0, but RES were greater (P < 0.02) than NON and CON d 11 and 12. Correspondingly, bunk-visit eating rate of RES was decreased (P < 0.01) from day 2 - 14. Had this not been a sub-clinical challenge model, the greater reduction in intake and increased leukocyte recruitment may have resulted in performance differences between RES and NON steers. These results suggest that differences in HPT responsiveness may be associated with differences in innate immunocompetence.