EXPERIMENTAL REPRODUCTION OF RESPIRATORY-DISEASE IN CALVES WITH NON-CELL-CULTURE-PASSAGED BOVINE RESPIRATORY SYNCYTIAL VIRUS

To reproduce experimentally clinical bovine respiratory syncytial viru s (BRSV) infections in cattle, we isolated BRSV from a calf in the fie ld that suffered from acute respiratory disease, Cell culture passage of the virus was avoided to prevent any modification of the biological properties of the virus. The isolated BRSV was passaged in specific-p athogen-free (SPF) calves, Lung lavage fluids of these calves, which c ontained at least 10(3) TCID50/ml BRSV and which were found to be free of other known respiratory pathogens, were collected and pooled for e xperimental infection. To reproduce a clinical BRSV infection, two gro ups of six SPF calves were inoculated intranasally with 2 mi of 10(3.9 ) TCID50/ml BRSV of the obtained virus stock. Another five calves, whi ch were persistently infected with bovine virus diarrhoea virus (BVDV) , were given the same inoculum, One group of six calves served as mock -infected controls. Clinical signs were closely monitored from 1 week before until 16 days after inoculation. Reproducible clinical signs co nsisting of significantly (p<0.05) increased respiratory rates and ele vated body temperatures were recorded but not in all BRSV-inoculated c alves, Although clinical signs were induced by experimental infection with non-cell-culture-passaged BRSV, the respiratory signs were not as serious as in the most severe cases in the field.