Marek's Disease in Poultry: Welfare Implications

Marek's disease is a highly contagious herpesvirus causing tumors, paralysis, and immune failure in chickens — a major welfare concern in unvaccinated flocks.

Key Facts

• Marek's disease virus (MDV) spreads via feather dander and is highly environmentally resistant
• Classic Marek's causes leg and wing paralysis from nerve tumors in young chickens
• Visceral tumors cause organ failure, weight loss, and death in affected birds
• Vaccination is highly effective and used routinely in commercial poultry worldwide
• Backyard and small flocks often have lower vaccination rates, increasing welfare risks

Welfare Considerations

Marek's disease causes significant suffering in affected birds. Nerve tumors cause progressive leg and wing paralysis — birds become unable to reach food and water, suffer from dehydration and starvation, and are vulnerable to trampling by flock mates. The immune suppression caused by MDV predisposes to secondary infections that add to the welfare burden. In visceral cases, birds experience organ dysfunction, wasting, and death. Welfare-focused flock management requires vaccination of all day-old chicks, prompt identification and humane culling of affected birds, and biosecurity measures to reduce environmental viral load. The availability of effective vaccines makes vaccine-preventable Marek's disease suffering ethically unjustifiable.

What You Can Do

• Vaccinate all chicks at day one against Marek's disease, including backyard flocks
• Purchase chicks from hatcheries that vaccinate as standard practice
• Monitor flocks for early signs: leg weakness, wing droop, eye changes (grey iris)
• Cull affected birds humanely and promptly to prevent suffering and reduce viral shedding
• Maintain biosecurity measures including clean litter management to reduce feather dander accumulation