Comparison of Marek's disease virus challenge strains and bird types for vaccine licensing

Authors: Dunn, John; Mays, Jody; Hearn, Cari; HARTMAN, ANGELA - Zoetis

Submitted to: Avian Diseases
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/8/2021
Publication Date: 6/1/2021
Citation: Dunn, J.R., Mays, J.K., Hearn, C.J., Hartman, A. 2021. Comparison of Marek's disease virus challenge strains and bird types for vaccine licensing. Avian Diseases. 65(2):241-249. https://doi.org/10.1637/aviandiseases-D-20-00122.
DOI: https://doi.org/10.1637/aviandiseases-D-20-00122

Interpretive Summary: Marek’s disease virus (MDV) is controlled through widespread vaccination of poultry, but continued evolution of the virus require ongoing improvements to available vaccine strains. Recently, several stakeholders have reported failures in vaccine licensing trials of positive controls in MDV challenge trials to reach appropriately high levels of virulence MD incidence. To investigate and develop solutions to this problem, we evaluated new combinations of novel MDV challenge strains with multiple commercial specific pathogen-free chicken lines. Although MDV strain 617A had the most potential as an alternative to strains that are currently approved for licensing trials, no combination of bird line and challenge virus consistently met the goals for a successful challenge model in all study replicates, indicating that high variability is an inherent difficulty in MDV challenge studies, at least when commercial outbred birds are used.

Technical Abstract: Marek’s disease virus (MDV) is an important poultry pathogen which is controlled through widespread vaccination with avirulent and attenuated strains, but continued evolution of field viruses to higher virulence has required ongoing improvement of available vaccine strains, and these vaccine strains also offer an attractive platform for designing recombinant vector vaccines with cross-protection against MDV and additional pathogens. Recent reports of failures in vaccine licensing trials of positive controls in MDV challenge trials to reach appropriately high levels of virulence MD incidence prompted us to evaluate possible combinations of outbred specific pathogen-free bird lines and alternative virulent challenge strains which could provide more consistent models for serotype-3 vaccine development. Choice of bird line and virulent MDV challenge strain each contributed to the ability of a challenge model to reach 80 percent virulence in unvaccinated positive control groups in the majority of trials without overwhelming serotype-3 vaccine protection in vaccinated groups. Conversely, reducing challenge virus dose by a factor of four, or vaccine dose by half, had no consistent effect across these models. Although MDV strain 617A had the most potential as an alternative to strains that are currently approved for licensing trials, no combination of bird line and challenge virus consistently met the goals for a successful challenge model in all study replicates, indicating that high variability is an inherent difficulty in MDV challenge studies, at least when outbred birds are used.