Light Shed on Cause of Marek's Disease Outbreaks in Vaccinated Broilers

Recombination appears to have taken place between Marek's disease virus (MDV) and reticuloendotheliosis virus (REV) under field conditions in Polish chicken farms, and this may account for sporadic outbreaks of disease in the country in recent years, according to new research.

25 February 2011

3 minute read

The molecular characteristics of Polish field strains of Marek's disease herpesvirus isolated from vaccinated chickens in Poland have been investigated by Grzegorz Wozniakowski and colleagues at the country's National Veterinary Research Institute's Department of Poultry Viral Diseases in Pulawy. Their paper has been published in the journal, Acta Veterinaria Scandinavica.

The researchers explain that 29 Marek's disease virus (MDV) strains were isolated over a three-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, the Pulawy group investigated 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 PCR products obtained were then 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 base-pair insertion and frame shift within 12 of the 24 field strains.

Interestingly, the analyses also showed 0.78, 0.8, 0.82, 1.6kb and other random LTR-REV insertions into the MDV genome in 28 of 29 of strains, according to the paper's authors. 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, concluded Wozniakowski and colleagues. 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, they added.

Reference

Wozniakowski G., E. Samorek-Salamonowicz and W. Kozdrun. 2011. Molecular characteristics of Polish field strains of Marek's disease herpesvirus isolated from vaccinated chickens. Acta Veterinaria Scandinavica, 53:10. doi:10.1186/1751-0147-53-10.