Classical swine fever: genetic detection and analysis of differences between virus isolates

Two pairs of oligonucleotide primers were designed that specifically amplified regions of the classical swine fever virus genome. These products, corresponding to a 671 bp portion of the genes encoding the E1 and E2 (gp33 and gp55) proteins and a 1090 bp portion of the putative polymerase gene, were amplified from eight virus isolates which had been responsible for a series of classical swine fever outbreaks in Italy involving both domestic pigs and wild boar. For each virus the fragments were partially sequenced to give 475 bp of the E1/E2 glycoprotein and 212 bp of the putative polymerase gene sequences. The data from each set of fragments were compared with one another and with reference strains. This allowed us confidently to assign most of the viruses to one of three subgroups. An analysis of the same viruses with a panel of monoclonal antibodies was much less informative. The subgrouping of the isolates suggested that, in this region of Italy, there had been at least two separate introductions of classical swine fever over a 7 year period and that virus had been transmitted between domestic pigs and wild boar. A consensus nucleotide sequence derived from the glycoprotein fragments of all the viruses examined revealed conservation at the wobble position of some codons.