Equine rhinopneumonitis vaccine: immunogenicity and safety in foals.

Abstract: Immunogenicity and safety of an equine herpesvirus 1 (ehv-1) vaccine were studied in 111 foals varying in age from 1 to 122 days. Each of 88 principals was given 1 im injection of vaccine. Five of the 88 foals were revaccinated; 69 of the vaccinated principals and 23 nonvaccinated foals (serving as controls) were challenge exposed intranasally with virulent ehv-1. The vaccine failed to cause adverse local or systemic reaction in 88 principals with serunirneutralization (sn) titers against ehv-1 varying between 0 to 1:256 at time of vaccination. After vaccination, the foals' body temperatures remained normal and peripheral blood leukocyte counts were normal or only slightly depressed. Attenuated ehv-1 was generally isolated from peripheral blood leukocytes obtained from foals after vaccination. None of 36 vaccinated foals, including those with maternal ehv-1 antibody, had signs of equine rhinopneumonitis (erp) following challenge exposure on experimental day 120. In contrast, all of the nonvaccinated controls had anorexia, depression, nasal discharge, occasional coughing, fever, and marked leukopenia after challenge exposure. Foals vaccinated when they were 1 to 12 days old and nonvaccinated controls had classic signs of erp after challenge exposure on experimental day 365. Following challenge exposure, vaccinated foals and controls developed fever and leukopenia, and virulent ehv-1 was isolated regularly from nasal and buffy coat specimens. Foals vaccinated when they were 7 to 42 days old failed to exhibit clinical signs of erp, but did develop mild fever and leukopenia when challenge exposed on experimental day 365. Furthermore, virulent ehv-1 was isolated regularly from paired nasal and blood specimens. Foals vaccinated at an average age of 122 days not only failed to exhibit clinical signs of erp, but also did not have fever and leukopenia after challenge exposure on experimental day 365. Virulent ehv-1 was not isolated from 42 paired specimens obtained 3 to 14 days after challenge exposure. Vaccination of foals with low sn titers (1:10 to 1:30) of ehv-1 antibody gave rise to gradual increase in titer over a 35-day period, whereas vaccination of foals with higher sn titers (1:171) produced a decline consistent with maternal antibody decay. Maternal ehv-1 antibody was found to be depleted within 56 to 130 days, depending directly upon the initial sn titer.