Polymerase chain reaction for differentiation between pathogenic and non-pathogenic serotype 1 Marek's disease viruses (MDV) and vaccine viruses of MDV-serotypes 2 and 3.

A polymerase chain reaction (PCR) test based on primers flanking the 132 bp tandem repeat in pathogenic MDV-1 DNA was developed. These primers amplify a dimer or a trimer 132 bp repeat in pathogenic MDV-1 DNA from blood and organs of commercial chickens with Marek's disease (MD) symptoms. Using the same primers in a radioactive PCR test, it was possible to distinguish between vvMDV-1 and the non-pathogenic MDV-1 CVI-988 vaccine in which the 132 bp repeats in the DNA were increased up to 9 repeats. The MDV-1 specific primers did not amplify MDV-2 (SB1) and MDV-3 (HVT) DNA. Primers prepared according to the nucleotide sequence of MDV-1 antigen A gene amplified MDV-1 DNA only. Specific primers prepared according to the nucleotide sequence of MDV-3 (HVT) antigen A gene amplified MDV-3 DNA but not MDV-1 nor MDV-2 DNA. The results of the present study show that the PCR tests can be used for the early identification of vvMDV-1 DNA in pathological samples from diseased commercial chickens and to distinguish between the vvMDV-1 and the three types of virus vaccines used to immunize chickens. The tests are accurate and can be performed in the presence of vaccine virus DNA in the sample.