Genetic and vaccination studies reveal feather viral load as a potential biomarker for individual and flock-level protection against Marek's disease in chicken.

Marek's disease (MD) remains a major threat to poultry, despite control efforts through biosecurity, vaccination, and genetic selection. Marek's disease virus (MDV), the causative agent, continues to persist and evolve to increased virulence, indicating that current measures are suboptimal. While vaccination-even with 'leaky' vaccines-has been shown to reduce MD symptoms and mortality at the flock level by decreasing virus shedding, it is unclear whether genetic resistance offers similar indirect protection. Using natural transmission experiments in inbred layer chickens, we compared the effects of vaccination and genetic resistance, defined by reduced tumour incidence and increased survival, on virus shedding and MD outcomes in both virus-inoculated shedder birds and unvaccinated MD susceptible contact birds. Shedder birds with genetic or vaccine-mediated resistance were less likely to develop disease and die, but only vaccinated shedders showed significantly reduced feather viral load (FVL). Nearly all contact birds became infected, suggesting that neither genetic resistance nor vaccination prevented transmission. However, significant reductions in symptomatic MD incidence and mortality were only observed in contacts exposed to vaccinated shedders. Exposure timing relative to shedders' infection time also influenced contact outcomes. Process analyses and dose-response curves revealed that higher shedder or group mate FVL correlated strongly with MD incidence and mortality and shedding in contact birds. FVL emerged as a promising indicator for both individual and flock-level protection and could serve as a useful phenotype for future breeding and vaccination strategies aimed at reducing MDV transmission and limiting virulence evolution.