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Abstract
Extract
The results of three field trials of a live, cold-adapted, parainfluenza virus type 3 vaccine supported the hypothesis that infection with this virus initiates some ovine pneumonias.
The serological response following vaccination was poor and only 2244% of vaccinates developed a serum antibody response by two weeks after vaccination. Nevertheless, the prevalence of pneumonia was lower in vaccinated lambs in all three trials at the time that natural infection with PICitation3 was detected in the non-vaccinated lambs hut the differences were significant in only one trial. In some of the trials the number of deaths and the number of carcases downgraded or condemned because of severe pleural lesions were also lower and growth rates were higher in vaccinated Iambs.
The vaccine was specific for PICitation3; thus there was little effect on the prevalence of pneumonias occurring at other times in the absence of new PICitation3 infections. Nevertheless, this is the first direct evidence that any virus has a role in naturally occurring ovine pneumonias and the first indication that the prevalence of the naturally occurring disease may be reduced by vaccination.