Plasmid constructs containing specific viral target fragments were used as templates for optimisation of multiplex PCR and to measure the limit of detection for each virus by single and multiplex PCR. First round amplification products (Table
1) were separately cloned using the pGEM-T Easy Vector System II (Promega) and constructs were extracted using the Rapid Plasmid Daily-Prep Kit (V-Gene). The constructs were verified by PCR and DNA sequencing (Sangon). The DNA concentrations of the constructs were determined spectrophotometrically (Molecular Devices).The limit of detection for each target was defined as the highest dilution detected of a series of serially diluted (1:10) plasmid constructs of the target sequence. The multiplex PCR was optimised and standardised using various concentrations of MgCl2 (1–2 mM) and primers (0.2–1 mM), annealing temperatures (50–62 C) and numbers of cycles (30, 35, 40, 45) using the positive control plasmid constructs (data not shown). The optimum product yield was achieved with 1.5 mM MgCl2, 0.5 mM primer for PCV-2, PPV and PRRSV and 0.25 mM primer for CSFV,an annealing temperature of 55 C and 35 cycles.