Pathogenesis of bovine respiratory syncytial virus in experimentally infected lambs.

Twenty-four 6-8-week-old conventionally reared lambs were inoculated intranasally and intratracheally with bovine respiratory syncytial virus. Infected lambs showed mild clinical signs characterized by slight serous nasal discharge, coughing, lachrymation and bronchovascular sounds on the middle part of the lung 5-9 days post-inoculation (PI). Virus was isolated in nasal swabs from 9 of 24 lambs between 3 and 7 days PI. However, virus was recovered from tracheal and lung tissue of all lambs killed between 3 and 11 days PI. Virus-specific antibodies appeared as early 6 days PI but high titres were attained 14-21 days PI. Lungs of lambs killed on different days PI had multifocal areas of consolidation. There was an increase of lymphocytes with a T-suppressor cell marker and a decrease in those with a T-helper marker in lung lavages obtained 5 days PI.