Duration of resistance to experimental footrot infection in Romney and Merino sheep vaccinated withBacteroides nodosusoil adjuvant vaccine

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Abstract
Groups of 10–12 Romney and Merino wethers were challenged simultaneously with homologous experimental footrot infection after having received the second of 2 doses of Bacteroides nodosus(strain 198) vaccine 0, 4, 8, 12, 16 and 20 weeks previously. Inoculations were carried out 28 days apart and unvaccinated sheep of both breeds were challenged as controls. Most Romneys that had been vaccinated up to 16 weeks prior to challenge were resistant to footrot whereas 8 of 10 controls were susceptible. This resistance was lost by about 20 weeks after vaccination. By contrast, protection against challenge in vaccinated Merinos lasted only about 4–5 weeks, although residual benefits of vaccination were apparent after longer intervals from the reduced number and severity of foot lesions among vaccinates compared with controls. Agglutinin titres, which did not differ markedly after similar intervals between Romneys and Merinos, reached maximum levels between 4 and 8 weeks after the second vaccine dose and subsequently declined. Although peak titres were generally recorded at the time of maximum protection in Merinos, the relationship between agglutinin levels and resistance in the Romneys was ill-defined.