Woźniakowski et al. Acta Veterinaria Scandinavica 2011, 53:10 http://www.actavetscand.com/content/53/1/10 R E S E A R C H Open Access Molecular characteristics of Polish field strains of Marek’s disease herpesvirus isolated from vaccinated chickens Grzegorz Woźniakowski*, Elżbieta Samorek-Salamonowicz, Wojciech Kozdruń Abstract Background: Twenty-nine Marek’s disease virus (MDV) strains were isolated during a 3 year period (2007-2010) from vaccinated and infected chicken flocks in Poland. These strains had caused severe clinical symptoms and lesions. In spite of proper vaccination with mono- or bivalent vaccines against Marek’s disease (MD), the chickens developed symptoms of MD with paralysis. Because of this we decided to investigate possible changes and mutations in the field strains that could potentially increase their virulence. We supposed that such mutations may have been caused by recombination with retroviruses of poultry - especially reticuloendotheliosis virus (REV). Methods: In order to detect the possible reasons of recent changes in virulence of MDV strains, polymerase chain reaction (PCR) analyses for meq oncogene and for long-terminal repeat (LTR) region of REV were conducted. The obtained PCR products were sequenced and compared with other MDV and REV strains isolated worldwide and accessible in the GeneBank database. Results: Sequencing of the meq oncogene showed a 68 basepair insertion and frame shift within 12 of 24 field strains. Interestingly, the analyses also showed 0.78, 0.8, 0.82, 1.6 kb and other random LTR-REV insertions into the MDV genome in 28 of 29 of strains. These genetic inserts were present after passage in chicken embryo kidney cells suggesting LTR integration into a non-functional region of the MDV genome. Conclusion: The results indicate the presence of a recombination between MDV and REV under field conditions in Polish chicken farms. The genetic changes within the MDV genome may influence the virus replication and its features in vivo. However, there is no evidence that meq alteration and REV insertions are related to the strains’ virulence. Background Neoplastic diseases of poultry may be related to infec- tion with Marek’s disease virus (MDV) and with the group of avian retroviruses. MDV is a highly cell- associated herpesvirus with lymphotropic properties but its molecular and genomic structures are similar to Alphaherpesvirinae subfamily [1]. The MDV’s genome consists of double stranded DNA about 180 kb and it is divided into unique long (UL) and unique short (Us) regions flanked by internal (ITR) and terminal (TR) repeats. * Correspondence: grzegorz.wozniakowski@piwet.pulawy.pl National Veterinary Research Institute, Department of Poultry Viral Diseases, Partyzantow 57 Avenue, 24-100 Pulawy, Poland The main oncoprotein of MDV is MEQ with leucine zipper characteristic for all transcriptional regulators and potential oncogens. MEQ is present in two copies located within the TRL region and between the junction of IRL and IRs. Other MDV genes taking part in replica- tion and oncogenesis are ICP4 (immediately-early pro- moter), pp38 (phosphoprotein from pp38-pp24 complex) and LAT (latency associated transcripts), which down regulate MDV replication during latency [2]. The MDV genome is known to be susceptible to recombination and insertion of parts of foreign viral genomes especially retroviruses like the avian leukosis virus (ALV) and reticuloendotheliosis virus (REV) [3-5]. Retroviruses are another group of oncogenic viruses. The Retroviridae family falls into a few genera. ALVs © 2011 Woźniakowski et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.