The relationship between the occurrence of undifferentiated bovine respiratory disease and titer changes to Haemophilus somnus and Mannheimia haemolytica at 3 Ontario feedlots

The association between exposure to Haemophilus somnus and Mannheimia haemo lytica (formerly Pasteurella haemolytica) and the risk of undifferentiated bovine respiratory disease (UBRD) was investigated using serological eviden ce of exposure coupled with a factorial design vaccine field trial. Measure s of previous exposure (titer at arrival) and current exposure (titer incre ase in the study period) to these agents were used. The vaccine field trial involved systematic allocation of animals into groups that received either a M. haemolytica vaccine, an H. somnus vaccine, a combined M. haemolytica and H. sommus vaccine, and an unvaccinated control group. Serum was collect ed from the 852 animals enrolled to determine titers to H. sommus, M. haemo lytica, bovine coronavirus and bovine viral diarrhea virus. Vaccination wit h H. somnus in combination with M. haemolytica and with M. haemolytica alon e reduced the risk of UBRD. The odds ratio for vaccination with H. somnus a lone and UBRD risk suggested some sparing effect, but the 95% confidence li mits included unity. There was no association between serological evidence of concurrent exposure to M. haemolytica and UBRD occurrence, There was an association between titer change to H. somnus and UBRD risk. However, the a ssociation changed with time of BRD treatment; animals diagnosed and treate d for UBRD on or after day 10 showed little evidence of exposure to H. somn us, despite evidence of natural H. somnus exposure in the unvaccinated grou p. The association between titer change to H. somnus and UBRD occurrence se en in this study may be a consequence of prolonged exposure to antibiotics, rather than a causal association.