The performances of two cellular
vaccines, one sparsely piliated and the other well piliated, were compared on irrigated pasture with those of
vaccines containing their respective, purified pili. There were statistically significant differences among the four
vaccines in the mean numbers of feet per sheep which developed severe
foot rot during 27 weeks of exposure. The incidence of severe
foot rot was significantly lower in the two pili-vaccinated groups than in the group vaccinated with well-piliated cells, which were in turn substantially more effective than the sparsely piliated. However, the two pili
vaccines did not prevent
infection of the interdigital skin so that the differences in
vaccine performance were much less marked when interdigital skin lesions were included. The number of feet affected by blowfly strike was closely associated with the presence of the most severe lesions and consequently there were far fewer struck feet in the two pili
vaccine groups than in the two cellular
vaccine groups. Analysis of serum
agglutinin titres led to the same assessment of vaccinal efficacy as that derived from the analysis of severe
foot rot lesions. Individually the
agglutinin response to vaccination was not universally associated with resistance or susceptibility to severe
foot rot but mean titres were significantly higher in the two pili
vaccine groups than in the highly piliated-cell
vaccine group, which were all in turn significantly higher than in the poorly piliated-cell
vaccine group.