Trials of an inactivated equid herpesvirus 1 vaccine: challenge with a subtype 2 virus

Serological responses following two and three doses of an inactivated equid herpesvirus 1 ( EHV -1) vaccine containing a subtype 1 strain were examined in yearling ponies. Complement fixing antibody responses were significantly higher against the subtype 1 vaccine strain than against a subtype 2 virus. Complement fixing antibody responses declined rapidly after the second dose of vaccine and had returned to almost pre-vaccination levels eight weeks after the second dose of vaccine. Complement fixing antibody titres to the heterologous subtype 2 strain increased after each successive dose of vaccine. The neutralising antibody responses following vaccination were weak although less strain specific than the complement fixing antibody responses. When challenged with a subtype 2 strain, 15 ponies which had received three doses of vaccine, two, eight and 12 weeks previously resisted challenge infection. However, 11 out of 15 ponies which had received two doses of vaccine eight and 12 weeks previously were susceptible to infection. While two doses of vaccine did not reduce duration of virus excretion or febrile responses in ponies challenged eight weeks after the second dose, the amount of virus excreted was significantly reduced.