Transmission of a vaccinal strain of infectious bovine rhinotracheitis virus from intranasally vaccinated steers commingled with nonvaccinated steers.

Ninety-seven feeder steers, averaging 7 months of age, were allotted to 3 groups. Group I (n = 33) was vaccinated intranasally with an infectious bovine rhinotracheitis virus (IBRV) vaccine on postinoculation day (PID) 0; group II (n = 31) was not vaccinated on PID 0 but was commingled with group I; and group III (n = 33) served as controls housed in the same facility, but was physically separated from groups I and II. On PID 20, all steers were given a modified-live IBRV vaccine IM. Virus isolation attempts from nasal swab specimens collected on PID 10 resulted in IBRV isolation from 19 (57.6%) of group I, 4 (12.9%) of group II, and 0 of group III. By PID 20, geometric mean titer for serum antibody to IBRV had increased in group I but had decreased in groups II and III. By PID 40, geometric mean titer for serum antibody to IBRV had increased in the 3 groups in response to IM vaccination given on PID 20. Seemingly, transmission of a vaccinal strain of IBRV to nonvaccinated steers did not take place at a frequency that elicited a humoral immune response similar to that of vaccinated steers.