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 to reach appropriately high levels of MD incidence prompted us to evaluate possible combinations of outbred specific pathogen-free (SPF) layer lines and alternative virulent challenge strains which could provide more consistent models for serotype-3 vectored vaccine development. Choice of layer 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 vectored 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 layer 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.

SUMMARY Turkey herpesvirus (HVT) is widely used as a vaccine against Marek's disease in chickens and recently as a vector for foreign genes from infectious bursal disease virus, Newcastle disease (ND) virus, infectious laryngotracheitis (ILT) virus, and avian influenza virus. Advantages of HVT-vector vaccines are that the vaccines do not contain live respiratory viruses or live infectious bursal disease virus able to replicate and cause disease or embryo mortality, they can be administered at hatch or in ovo , and they are relatively insensitive to interference from maternally derived antibodies. As producers have tried to combine HVT-vector vaccines to protect against additional diseases, reports have indicated that applying two vectored vaccines using the same HVT vector is reported to reduce the efficacy of one or both vaccines. To confirm this interference, we evaluated commercial vaccines from multiple companies, including products with inserts designed to protect against ND, infectious ILT, and infectious bursal disease (IBD). Using a standard dosage, we found that the ILT product was most severely affected by the addition of other vaccines, as demonstrated by a significant increase in clinical signs, significant decrease in weight gain, and increase in quantity of challenge virus observed from tracheal swabs collected from Days 3–5 postchallenge. The ND and IBD products were also affected by the addition of other vaccines, although in most cases differences compared to vaccination with the vector alone were not statistically significant. This study demonstrates the importance of following manufacturer guidelines and the need for validating alternative strategies to benefit from the high level of protection offered by vector vaccines.