Recombinant-Derived Modified-Live Herpesvirus Vaccines

Abstract

Swine pseudorabies (PR; Aujezky’s disease [AD]) and infectious bovine rhinotracheitis (IBR) are important veterinary diseases, each of which cost American farmers over 30 million dollars annually. Killed and modified-live virus (MLV) vaccines have been used in the United States and in other countries for over 25 years to control AD and IBR, but these conventional vaccines have had numerous shortcomings with regard to safety and efficacy. First, despite the intensive use of the conventional vaccines, AD- and IBR morbidity and mortality have increased. For example, about 10% of the sixty million swine in the United States are now infected with PRV compared with about 1% in 1972. Second, animals vaccinated with conventional killed or MLV vaccines remained susceptible to infection at a later time with virulent field strains, and often shed virulent virus, thereby spreading infection in the herd. Third, the intramuscular (IM) administration of MLV IBR vaccines has been hampered by the hazards of vaccine-induced abortions. As an alternative, temperature-sensitive IBR vaccines have been used, but these specialized vaccines could only be administered by the inconvenient and laborious intranasal (IN) route. Virus replication in the deep tissues of the body to amplify the immune response was, by definition, restricted. Fourth, some of the conventional AD vaccines, though safe for swine, were unsafe for oother animal species.