Marek's Disease: What Every Pullet Buyer Needs to Know

The Disease Most Backyard Keepers Don't Think About Until It's Too Late

Marek's Disease is a highly contagious herpesvirus that affects chickens worldwide. It's also the single most common cause of death in backyard flocks — and the majority of keepers who lose birds to it never saw it coming.

If you're buying pullets, hatching chicks, or adding new birds to an existing flock, understanding Marek's Disease is not optional. It's one of the most important things you can know as a flock keeper.

What Is Marek's Disease?

Marek's Disease Virus (MDV) is a herpesvirus that attacks the nervous system and immune system of chickens. It spreads through feather dander — the fine dust shed from a bird's feathers — which means it travels easily through the air, on clothing, on equipment, and on your hands.

Once MDV is in your environment, it can survive for years in the dust and debris of your coop. There is no treatment. Birds that develop clinical Marek's will not recover.

The good news: there is an effective vaccine, and it works — when given correctly and at the right time.

Symptoms of Marek's Disease

Marek's presents in several forms, which is part of why it's often misdiagnosed:

• Classical (neural) form: Progressive paralysis of legs, wings, or neck. Birds may develop a characteristic "splits" posture with one leg forward and one back. This is the most recognizable presentation.
• Ocular form: Iris turns gray or irregular; bird loses vision partially or completely.
• Visceral form: Internal tumors on organs — often no external symptoms until the bird is found dead.
• Cutaneous form: Lesions and tumors on feather follicles; may be visible on skin.

Symptoms typically appear in birds between 6 weeks and 30 weeks of age, though the virus can activate later in immunocompromised birds. Once clinical signs appear, the prognosis is poor.

The Vaccination Window: Why It Matters So Much

The Marek's vaccine must be given within the first 24 hours of a chick's life to be effective. After that window closes, vaccination provides little to no protection.

This is why vaccination status is one of the most critical questions to ask when buying pullets or chicks:

• Were the birds vaccinated for Marek's?
• At what age was the vaccine administered?
• Which vaccine was used? (HVT, bivalent, or Rispens — Rispens provides the strongest protection against virulent strains)

At Stafford Hill Farms, all pullets are vaccinated for Marek's Disease at hatch. This is a non-negotiable part of our flock health protocol — not an upsell or an optional add-on.

Vaccinated Birds Can Still Get Marek's — Here's Why

This is the part that confuses most keepers: a vaccinated bird can still contract Marek's Disease and show symptoms. The vaccine does not prevent infection — it prevents the development of tumors and clinical disease in the majority of vaccinated birds.

This matters for two reasons:

1. Vaccinated birds can still shed the virus and expose unvaccinated birds.
2. If you introduce unvaccinated birds into an environment where Marek's is present (including any environment that has housed chickens previously), those birds are at extremely high risk.

Never mix vaccinated and unvaccinated flocks. And never introduce unvaccinated birds into a coop that has previously housed chickens without thorough sanitization — and even then, residual virus in the environment is a real risk.

Biosecurity Practices That Reduce Marek's Risk

• Buy vaccinated pullets only — from hatcheries or farms that vaccinate at hatch
• Quarantine all new birds for a minimum of 30 days before introducing to your flock
• Change clothes and wash hands after visiting other flocks or poultry operations
• Clean and disinfect coops thoroughly between flocks
• Avoid bringing in used equipment from other operations
• Keep wild birds out of your coop — they can carry and spread MDV

If You Suspect Marek's in Your Flock

If you have a bird showing progressive paralysis, gray iris, or sudden unexplained death in young birds, contact your state veterinarian's office or your local cooperative extension service. A necropsy (bird autopsy) can confirm Marek's and help you understand your flock's exposure level.

Remove affected birds from the flock immediately — not to prevent spread (the virus is already in your environment if one bird is symptomatic), but to prevent suffering and secondary infections.

Document everything: which birds are affected, onset date, symptoms, and any recent additions to your flock. This record is critical for your vet consultation and for protecting the rest of your operation.

The Bottom Line for Pullet Buyers

Ask about Marek's vaccination before you buy. Buy from producers who vaccinate at hatch. Quarantine new birds. Keep records of your flock's health history from day one.

Flock Steward's health logging tools let you document vaccination status, quarantine periods, and any health events — so you always know exactly what your birds have been through and when.

Explore Flock Steward and our vaccinated pullet offerings at Stafford Hill Farms →