THE ASSOCIATION OF TITERS TO HAEMOPHILUS-SOMNUS, AND OTHER PUTATIVE PATHOGENS, WITH THE OCCURRENCE OF BOVINE RESPIRATORY-DISEASE AND WEIGHT-GAIN IN FEEDLOT CALVES

Serum samples were obtained from 602 calves (from 19 groups in four fe edlots: three in Ontario, and one in Alberta) upon arrival at the feed lot and 28 d later. Of these calves, 202 developed bovine respiratory disease (BRD) and 400 did not develop BRD. Based on high antibody tite rs noted upon arrival, we infer that most calves were exposed to Haemo philus somnus prior to arrival at the feedlot. Within a group, calves with high titers on arrival had a reduced risk of developing BRD later . Most calves did not experience titer increases after arrival; howeve r, calves that had stable or increasing titers had a relatively low ri sk of contracting BRD. The calves at greatest risk of BRD were those w ith titers on arrival of less than 6.8 units and subsequent titer decr eases of more than 1 unit. The effects of both the titer on arrival an d the titer change after arrival were stable when the serologic effect s of a number of viruses and Mycoplasma agents were considered. Neithe r antibody titer on arrival nor titer change was related to weight gai n differences among calves. Calves with BRD or calves with lower weigh t on arrival had decreased weight gains in the first 28-day feeding pe riod. The high titers on arrival may have protected most calves agains t further infection with H. somnus. However, since the calves that dev eloped BRD had large titer increases to a number of viruses and to Pas teurella haemolytica, while having decreased antibody titers to H. som nus, we infer that the existing antibodies were ''used up'' in combatt ing the agents, including H. somnus, which may have ''caused'' the BRD . Calves which were able to increase their antibody levels to H. somnu s tended to have a reduced risk of BRD.