Genome differences among field isolates and vaccine strains of pseudorabies virus

The DNA of field isolates and vaccine strains of pseudorabies virus (PRV) was analysed by digestion with the restriction endonuclease BamHI. A number of distinct restriction profiles of the field isolates obtained from different locations within Europe were observed. As for herpes simplex virus, the variations could be classified into two types: first, alterations in the mobility of fragments due to the presence of additional sequences and/or the occurrence of deletions, a phenomenon most apparent in fragments containing part or whole of the repeat sequence of PRV DNA; second, generation of differently sized fragments due to loss and/or gain of restriction endonuclease cleavage sites. By analysis of several strains with BamHI a small number of variable cleavage sites were identified within particular regions of the unique long (UL) segment of the PRV genome. Compared to wild-type PRV, the restriction fragment patterns of vaccine strains showed characteristic alterations, including the absence of bands, which were non-variable in wild-type strains, and/or the presence of new bands some of which were submolar. Some of these characteristics could be explained by a deletion in the unique short (US) region of the genome of most vaccine strains and the occurrence of closely related variants in the uncloned vaccine virus stocks.