Direct staining of nasopharyngeal smears with hyperimmune bovine serum raised against respiratory syncytial virus (RSV) and conjugated with fluorescein isothyocyanate was compared with virus isolation from 14 experimentally infected animals. Twenty-four out of 107 specimens examined were positive by the fluorescent antibody (FA) method and 21 out of 107 by virus isolation. Most of the FA positives (81 per cent) were detected after the ninth day of infection whereas 80 per cent of virus isolations were made before this time. Only one false positive (one out of 57 specimens) was detected by the FA method in nine uninfected control animals. Non-specific fluorescence presented some problems when examining nasopharyngeal material. The same conjugate was found to be more useful in detecting RSV antigen in lung tissue of eight experimentally infected animals and 11 out of 22 naturally occurring cases of calf pneumonia from five outbreaks of disease. In three of the outbreaks the diagnosis was confirmed by virus isolation and serology. The dominating histopathological response in both the experimental and the natural disease was an acute bronchiolitis and alveolitis. The finding of RSV antigen in association with these lesions provides further evidence for the role of RSV as a respiratory pathogen of cattle.
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