Latency and reactivation of a glycoprotein E negative bovine herpesvirus type 1 vaccine: influence of virus load and effect of specific maternal antibodies

The effects of the vaccination of neonatal calves with a glycoprotein E (gE)-negative bovine herpesvirus type 1 (BHV-1) were investigated in naïve and passively immunised calves either with the recommended dose or a 5-fold concentrated one. After inoculation (PI), all calves excreted the virus vaccine except three passively immunised calves inoculated with the lower titre. No antibody response could be detected in passively immunised calves, whatever the dose used, and they all became BHV-1 seronegative and remained so after dexamethasone treatment (PDT). Nevertheless, as shown by a gamma-interferon assay, all calves that excreted the vaccine PI developed a cell-mediated immune response and a booster response was observed PDT, suggesting viral reactivation. The vaccine virus was recovered PDT from nasal secretions in two calves and BHV-1 DNA were detected in trigeminal ganglia from five calves belonging to all inoculated groups. The results show that the BHV-1 gE-negative vaccine can establish latency not only in naïve but also in passively immunised neonatal calves after a single intranasal inoculation. Moreover, this study shows for the first time that the gE-negative vaccine, when used in passively immunised calves, can lead to seronegative vaccine virus carriers.