Observations on the infectivity and pathogenicity of three isolates of Ostertagia spp sensu lato in calves.

After experimental infection of three groups of calves with three different defined isolates of Ostertagia spp sensu lato a marked variation in the infectivity and degree of pathological change was noted. The most highly infective and pathogenic isolate contained approximately 30 per cent O leptospicularis in addition to O ostertagi (70 per cent) and a negligible number of Skrjabinagia lyrata. The other two isolates consisted solely of O ostertagi (approximately 90 per cent) and S lyrata (approximately 10 per cent) and showed comparable infectivity and pathogenicity to that commonly recorded. The greater pathogenicity of the isolate containing O leptospicularis was attributed to the higher infectivity and the greater damage caused to the gastric mucosa by individual parasites when compared with O ostertagi.