Immunity and the level of neutralization antibodies in foals and mares vaccinated with a modified live-virus rhinopneumonitis vaccine.

Foals (farms 1 and 2) and mares (farms 1 and 3) vaccinated with a commercially available rhinopneumonitis vaccine were tested for the level of antibodies and for protection against the natural infection. The serum-neutralization (SN) antibody titers against equine herpesvirus type 1 were 1:8 or less in the majority of foals and 1:16 or higher in about 10%. The level of the nasal secretory antibody in the foals (farm 2) was less than 1:4. The vaccinated foals did not show any apparent respiratory tract infection for about 6 months and then an epizootic of respiratory tract disease appeared on farm 1 involving 3 foals. From each of these 3 foals herpesvirus was isolated. On farm 1, about 50% of the vaccinated mares aborted, and on farm 3, there were no abortions. Isolations of herpesvirus were made from 5 of the 6 aborted fetuses (farm 1). The SN antibody titers of the pregnant mares in the 2 farms at the time of foaling were high.