Excretion and reexcretion of thermosensitive and wild-type strains of infectious bovine rhinotracheitis virus after co-infection or two successive infections

Twelve cattle were divided into 2 groups. The first was intranasally co-infected with 2 strains of infectious bovine rhinotracheitis virus (Bovine herpesvirus 1; BHV 1): the thermosensitive vaccine strain IBR/ts RLB106 and a Belgian field isolate IBR/Cu5. Reactivation of BHV 1 was induced by dexamethasone treatment 2 months later and again 5 months later for 3 animals that only reexcreted small quantities of virus during the first dexamethasone treatment. The second group was intranasally infected with IBR/Cu5. Two months later, an attempt to reinfect this group with IBR/ts RLB106 failed. Four months after the primary infection, these cattle were treated with dexamethasone. Except after reinfection and at the beginning or the end of the (re)excretion periods, excreted and reexcreted viruses replicated at 35, 37 and 40 degrees C, indicating the presence of the wild-type virus. Only one isolate, out of 116 cloned from the nasal exudates collected during the excretion and reexcretion periods, expressed the thermosensitive phenotype. This isolate was characterized by its mean plaque size as the IBR/ts RLB106 strain. The epizootiological significance of these findings is discussed, with emphasis on the weak spreading capacity of the ts vaccine strain and the possibility of emergence of recombinant viruses.