Marek's disease is a highly contagious viral disease of chickens causing neurological symptoms and tumors. Vaccination programs have dramatically improved poultry welfare and production outcomes.
Marek's disease vaccination represents one of poultry welfare science's great successes - a disease that caused widespread suffering and mortality now largely prevented through routine vaccination. The welfare improvement from vaccination is enormous: Marek's disease paralysis causes prolonged suffering from inability to access food and water, followed by emaciation. The vaccine has been so effective that its welfare impact in commercial flocks is now primarily the background threat it poses in unvaccinated backyard flocks. Owners of backyard chickens who do not vaccinate expose their birds to unnecessary welfare risk - vaccination is a straightforward welfare intervention for all chicken keepers.