Serology for diagnosis and epizootiological studies of bovine respiratory syncytial virus infections.

The complement fixation test (CFT) and the virus neutralisation test (VNT), performed as a plaque reduction test, were employed to measure antibodies to bovine respiratory syncytial virus. The CFT with bovine sera was performed with supplementation of the complement factors in fresh guinea pig serum by an adequate amount of Clq-factor of the bovine species. Kinetics of maternally derived antibodies and the antibody response after spontaneous and experimental infections and after intramuscular vaccination were studied by both tests. Patterns of development of complement fixing and virus neutralising antibodies were generally similar and titres equalled each other in the test systems that are described. However a VNT detected antibodies a few days earlier after an infection than a CFT and peak-levels reached after a naturally acquired infection decreased faster in a CFT than in a VNT: a mean decrease of 3.1 and 1.4 log2 units was found in 13 weeks respectively. Mean half-life of passive antibodies was 25 days in a VNT. An infection with bovine respiratory syncytial virus could be diagnosed by serology, using a CFT on acute and convalescent serum samples of a number of animals in a group. Serology is preferable to virus isolation for routine diagnosis of bovine respiratory syncytial virus infections. Paired sera, collected at 14-day intervals and examined by CFT, are recommended for the diagnosis of the cause of respiratory disease. A VNT is preferable if low antibody levels are to be detected because non-specific reactions occur in a CFT at low serum dilutions.