Reintroduction of bovine herpes virus type 1 into Danish cattle herds during the period 1991-1995: a review of the investigations in the infected herds.

In Denmark a programme for the systematic eradication of bovine herpes virus 1 (BHV-1) was completed during the years 1984 to 1991, but outbreaks due to new introductions of BHV-1 were seen. Between January 1991 and May 1994, 22 herds became infected with BHV-1, all located closely to the German border. In 1995, 61 herds were detected BHV-1 antibody positive, but they were situated in many different parts of Denmark. In order to find the source of infection owners of infected herds were interviewed, and restriction fragment pattern analysis (RFP-analysis) was performed on virus isolates from the herds with clinical outbreaks. Isolates from clinical outbreaks up to 1995 were identified as a Cooper-like strain, while 2 of those in 1995 had characteristics of a "new" strain, which had never before been identified in Denmark or elsewhere in Europe. In the described situation different transmission routes for virus seemed possible. One being a sporadic introduction of virus due to accidental contact with infected cattle near the German border or maybe due to an airborne transmission of virus over longer distance. The other, presumably a result of import of an infected animal despite the national regulations. The latter, due to an extensive trade pattern, resulted in the introduction of infected cattle into 51 BHV-1 seronegative cattle herds.