Contagious ecthyma virus-vaccination failures.

Cross-protection experiments were undertaken to investigate reasons for contagious ecthyma (CE) virus-vaccination failures. Vaccination with sheep-passaged or with cell culture-passaged virus did not protect lambs against development of lesions after challenge inoculation with sheep-passaged virus. However, lesions which developed after challenge exposure with sheep-passaged CE virus healed significantly faster than did those induced by the initial vaccination with sheep-passaged virus (P less than 0.001). A significant decrease in the healing time was not observed for lambs initially vaccinated with cell culture-passaged CE virus after challenge exposure with sheep-passaged virus (P greater than 0.05). Protection was evident when lambs were challenge inoculated with the less virulent cell culture-passaged virus and cross-protection between the ST and CSL isolates was detected. This study indicated that complete protection even against homologous strain challenge was not achieved. Antigenic differences between vaccinal and field strains appeared an unlikely cause of vaccination failures. It also is evident that cell culture-propagated CE virus preparations are less effective for vaccination purposes than are those propagated in sheep.