identification of an introduced bovine herpesvirus type 1 strain in a closed dairy herd by experimental virus reactivation followed by DNA restriction enzyme analysis

In a closed dairy herd in the province of Utrecht in 1995, nine replacement heifers were erroneously intramuscularly vaccinated with Tracherine(R), a live virus IBR vaccine. More than 18 months later, serology of the herd rev ealed that a large part of the herd had developed an antibody response towa rds BHV1 (62 of 87 animals). To investigate whether Tracherine(R) had recir culated on the farm, four BHV1 antibody positive animals, of which two had been vaccinated with Tracherine(R), were treated with corticosteroids to re activate latent BHV1. Two virus isolates were obtained and subsequently ana lysed by restriction enzyme analysis. Both isolates were identified as BHV1 .1 subtypes. One of the isolates was clearly distinct from Tracherine(R) an d was most likely a BHV1 field virus. A BHV1 field virus was most likely in troduced into the farm even though the herd was closed, the animals had not been in contact with other cattle, and preventive hygienic measures had be en implemented. There was no indication that Tracherine(R) had recirculated .