Serological indication for persistence of bovine respiratory syncytial virus in cattle and attempts to detect the virus.

To identify putative persistent bovine respiratory syncytial virus (BRSV) infections in cattle, seven cattle that had experienced BRSV infections were treated with corticosteroids for two periods of 5 days. During the 5-day periods and the 3 weeks after treatment, attempts were made to isolate BRSV from lung lavage fluid and nasal swab specimens. Fluorescent antibody tests were used to detect BRSV antigen in lung lavage cells. A BRSV specific polymerase chain reaction (PCR) assay was developed, and was performed on lung lavage samples of all seven cattle as well as on various tissues of five of the cattle. In addition, nasal swabs of 74 over-one-year-old cattle, in a closed dairy herd were also assayed by PCR. The virus or its RNA was not detected in putative carriers, by any of the methods used, whereas all positive controls were positive. After corticosteroid treatment, three of the seven cattle showed a fourfold rise in antibody titre, suggesting induction of virus replication. BRSV-seronegative sentinel calves, that were housed together with each corticosteroid-treated animal, did not develop antibodies to BRSV indicating that BRSV was not shed by corticosteroid-treated cattle, or was shed at a very low level. In addition BRSV was not detected in seropositive cattle in a closed farm in summer. Although we consider the rises in antibody titres against BRSV an indication for persistence of BRSV in cattle, BRSV or its RNA was not detected in infected cattle.