What Happens When Vaccinated Chickens Are Introduced to a Marek’s Positive Flock?

Introducing Marek’s-vaccinated chickens to a Marek’s-positive flock can be done, but it requires careful planning and management. Here's what happens and what you need to know:

How Marek’s Vaccination Works

• The Marek’s vaccine does not prevent chickens from getting infected with the Marek’s virus. Instead, it prevents the development of the tumors and severe symptoms associated with the disease.
• Vaccinated chickens can still become infected with the Marek’s virus from your positive flock and may shed the virus into the environment.
• The vaccine protects birds by allowing their immune systems to fight off the disease’s harmful effects, which keeps them healthy despite being exposed.

What Happens When Vaccinated Chickens Are Introduced to a Positive Flock?

1. Exposure to the Virus:
   • Vaccinated chickens will almost certainly be exposed to the Marek’s virus present in your positive flock. This is unavoidable since the virus spreads through feather dust and dander in the environment.
2. Vaccination Effectiveness:
   • If the vaccination was administered correctly (within the first 24 hours after hatch) and the birds had time to build immunity (at least 7-14 days in a virus-free environment), the vaccine should protect them from developing Marek’s symptoms, such as paralysis and tumors.
3. Virus Shedding:
   • Once exposed, vaccinated birds may become carriers of the virus. They can shed the virus into the environment, but because your flock is already positive, this won’t change the overall risk in your setup.
4. Risk of Breakthrough Infection:
   • If the birds were vaccinated improperly, or if the exposure to the virus is overwhelming (due to heavy contamination), there’s a chance the vaccine may not fully protect them, and they could develop symptoms of Marek’s.

Key Considerations Before Introducing Vaccinated Birds

If you want to successfully introduce vaccinated chickens into your Marek’s-positive flock, here are some steps to take:

1. Ensure Proper Vaccination Timing

• The Marek’s vaccine is most effective when given within 24 hours after hatching. It must be administered before the chicks are exposed to the Marek’s virus.
• If you’re bringing in vaccinated birds, confirm they were vaccinated at hatch and allowed at least 7-14 days to build immunity in a virus-free environment before joining your flock.

2. Quarantine New Birds

• Quarantine the vaccinated birds for at least 2 weeks in a clean, virus-free environment to give their immune systems time to respond to the vaccine. This helps ensure they are fully protected before exposure to your Marek’s-positive flock.

3. Monitor for Symptoms

• Keep a close eye on the newly introduced birds for any signs of illness or stress, especially during the first month after introduction.
• Signs of Marek’s include leg or wing paralysis, weight loss, or changes in behavior.

4. Minimize Stress

• Stress weakens the immune system, making vaccinated birds more vulnerable to disease. Provide a calm, clean environment, proper nutrition, and gradual integration to reduce stress during introduction.

5. Clean and Manage the Environment

• Even though your flock is already Marek’s-positive, maintaining a clean coop with proper ventilation reduces the amount of virus in the environment and improves the overall health of your birds.

Risks to Keep in Mind

1. Breakthrough Infections:
   While rare, there’s a chance that vaccinated birds may still develop Marek’s symptoms if exposed to a very high viral load or if their immune systems are compromised.
2. Shedding to New Flocks:
   If you ever sell or give away birds or eggs from your flock, you must disclose that the flock is Marek’s-positive. Even vaccinated birds can carry and spread the virus to other flocks.
3. Potential for Genetic Resistance to be Missed:
   Vaccinated birds may not exhibit signs of illness, which could make it harder to identify birds with natural resistance to Marek’s if you’re also breeding for resistance.

Best Practices for Introducing Vaccinated Birds

1. Start with Properly Vaccinated Chicks:
   • Purchase chicks from a trusted hatchery that vaccinates against Marek’s and provides documentation.
2. Prepare the Environment:
   • Deep clean the coop and reduce feather dust before introducing the new birds to limit viral exposure.
3. Integrate Gradually:
   • Introduce the new birds slowly to reduce stress. Use a partition to allow them to see each other before full integration.
4. Focus on Immune Health:
   • Provide high-quality feed, clean water, and supplements like probiotics and vitamins to support the immune systems of all your birds.

Can It Be Done Successfully?

Yes, vaccinated birds can thrive in a Marek’s-positive flock as long as:

• They were vaccinated properly and given time to develop immunity.
• You maintain good biosecurity and reduce stress.
• You accept that these birds, like your existing flock, will carry and potentially shed the virus for life.

With proper care and management, vaccinated birds can integrate well into a Marek’s-positive flock, live healthy lives, and contribute to your flock’s productivity.