SITES OF REPLICATION OF BOVINE RESPIRATORY SYNCYTIAL VIRUS IN NATURALLY INFECTED CALVES AS DETERMINED BY IN-SITU HYBRIDIZATION

Replication of bovine respiratory syncytial virus (BRSV) was studied i n three naturally infected calves by in situ hybridization using stran d-specific RNA probes. One of the calves was a 5-month-old Friesian, t he other two calves were a 3-month-old and a 2-week-old Jersey. Two Je rsey calves, 3 months and 3 weeks of age, served as controls. Replicat ion of BRSV took place in the luminal lining of the respiratory tract. In one of the BRSV infected animals (calf No. 1), replication was esp ecially seen in the bronchi, whereas in the two other animals (calf No s. 2 and 3) replication of BRSV was demonstrated in the bronchiolar ep ithelial cells and in alveolar cells. Syncytia were often observed in the bronchiolar walls and in alveoli and such syncytia were always rep licating BRSV. By immunohistochemistry it was possible to demonstrate BRSV antigen at the same location as replication of BRSV was detected. In tissue outside the respiratory tract neither BRSV antigen nor repl ication of BRSV could be demonstrated.