Marek's & MG Vaccinations in Chicks: Not A Cure-all

When buying chicks, you’ll often see “vaccinated for Marek’s” or “MG-clean / MG-vaccinated” listed as major selling points. While vaccinations absolutely have value, they are often misunderstood. Vaccines are tools for risk reduction, not magic shields—and they do not replace good management, biosecurity, or genetics.

This article explains what Marek’s disease and MG actually are, how the vaccines work, why vaccination alone is not the end-all, be-all solution, and our approach at the Ranch.

What Is Marek’s Disease?

Marek’s disease is a highly contagious viral disease of chickens caused by a herpesvirus. It primarily affects young birds, most commonly between 6 weeks and 6 months of age, though exposure can happen much earlier.

How Marek’s Spreads

Marek’s is spread through dander (skin flakes) that infected birds shed into the environment. Once present, the virus can survive in litter, dust, and housing for a very long time—sometimes years.

Chicks are usually exposed by:

• Contaminated brooders or coops

• Adult birds that are carriers

• Dust carried on clothing, shoes, or equipment

  
  

Symptoms of Marek’s Disease

Symptoms vary depending on the form of the disease, but may include:

• Progressive paralysis (legs, wings, neck)

• Weight loss despite eating

• Grey or misshapen pupils (ocular Marek’s)

• Tumors in internal organs

• Sudden death with no warning

  
  

Some birds may carry the virus and never show symptoms, while others become severely affected.

The Marek’s Vaccine: What It Does (and Doesn’t Do)

The Marek’s vaccine is typically given at hatch, often within the first 24 hours of life.

What the Vaccine Does

• Reduces the severity of the disease

• Helps prevent tumor development

• Allows birds to survive exposure that might otherwise be fatal

  
  

What the Vaccine Does Not Do

• It does not prevent infection

• It does not stop viral shedding

• It does not eliminate Marek’s from your flock or environment

  
  

Vaccinated birds can still become infected and still spread the virus—they are simply less likely to show severe symptoms.

What Is MG (Mycoplasma gallisepticum)?

Mycoplasma gallisepticum (MG) is a bacterial respiratory disease that affects chickens, turkeys, and other poultry. Unlike Marek’s, MG is not a virus—and that distinction matters.

How MG Spreads

MG is spread through:

• Direct bird-to-bird contact

• Respiratory droplets

• Contaminated equipment

• Vertical transmission (from hen to chick through the egg)

  
  

Once MG enters a flock, it is extremely difficult to eliminate.

Symptoms of MG

Symptoms may include:

• Sneezing, coughing, or wheezing

• Nasal discharge or bubbles in the eyes

• Swollen sinuses

• Reduced growth and feed efficiency

• Drop in egg production

  
  

Stress (cold, heat, overcrowding, shipping, breeding pressure) often triggers flare-ups in birds that previously appeared healthy.

MG Vaccination & “MG-Clean” Flocks

MG management is very different from Marek’s management.

MG Vaccines

MG vaccines exist, but they:

• Do not cure MG

• Do not prevent infection entirely

• Can complicate testing and diagnosis

  
  

Vaccinated birds may still carry and transmit MG.

MG-Clean vs MG-Positive

• MG-clean flocks are maintained through strict testing, biosecurity, and closed breeding programs

• MG-positive flocks may look healthy but can pass MG to offspring or spread it under stress

Once MG is introduced, eradication usually requires complete depopulation—which is why serious breeders place such importance on prevention.

Biosecurity: Your First and Strongest Line of Defense

Vaccines can reduce risk, but biosecurity is what actually keeps diseases out of your flock. Marek’s and MG are most often introduced through everyday, well-meaning actions that poultry keepers don’t realize carry risk.

Biosecurity doesn’t have to be extreme—but it does need to be intentional.

Control What Comes Onto Your Property

The most common way disease enters a flock is through new birds, equipment, or people.

Best practices include:

• Quarantining all new birds for a minimum of 30 days (longer is better)

• Keeping new birds physically separated with no shared air space if possible

• Not sharing feeders, waterers, crates, or tools between flocks

• Avoiding poultry swaps, auctions, and mixed-bird sales when possible

  
  

Even birds that look perfectly healthy can be carriers.

Limit Exposure From Other Flocks

MG and Marek’s can hitch a ride on clothing, footwear, and hands.

Simple habits that make a big difference:

• Dedicated coop boots and clothing

• Washing hands before and after flock chores

• Changing clothes after visiting other poultry keepers or feed stores

• Cleaning and disinfecting equipment that leaves and re-enters your property

  
  

Manage Age Groups Carefully

Mixing age groups increases disease pressure.

To reduce risk:

• Avoid placing chicks into housing that previously held adult birds

• Raise chicks in clean, well-sanitized brooders

• Practice all-in, all-out flock management when possible

  
  

Reduce Stress to Reduce Disease Expression

Stress does not cause Marek’s or MG—but it activates them.

Common stressors include:

• Overcrowding

• Temperature extremes

• Poor ventilation

• Shipping, handling, or frequent changes

• Breeding pressure

Low-stress birds are better able to cope with exposure and less likely to show clinical illness.

Source Birds Responsibly

Where your birds come from matters just as much as how you raise them.

When sourcing chicks or breeding stock:

• Ask about disease testing and flock status

• Understand the difference between vaccinated, clean, and tested

• Avoid impulse purchases from unknown or mixed-source sellers

A single addition can change your entire flock’s health status.

Our Approach at the Ranch

At the Ranch, we have chosen not to routinely vaccinate our chicks for Marek’s or MG.

This decision is not based on the belief that vaccines are ineffective—but on the understanding that vaccination does not prevent infection or transmission of these diseases. Instead, our focus is on prevention through biosecurity, management, and selective breeding.

Our program prioritizes:

• Strict biosecurity and limited flock exposure

• Closed or carefully managed breeding groups

• Clean brooding environments

• Low-stress rearing practices

• Sourcing and maintaining birds with strong overall resilience

  
  

By managing disease pressure at its source, rather than masking it, we aim to raise birds that are well-suited for responsible, biosecure environments.

Because unvaccinated birds may be more vulnerable if exposed to high disease pressure, our chicks are best suited for:

• Buyers with good biosecurity practices

• Flocks that are not frequently mixed with outside birds

• Keepers who understand the importance of quarantine and stress reduction

We believe in transparency, education, and informed choice—so customers can decide what approach best fits their own flock and management style.

The Takeaway

Marek’s and MG vaccinations can be valuable tools—but they do not replace good flock management or disease awareness.

Understanding what these diseases are, how they spread, and what vaccines realistically do allows poultry keepers and breeders to make informed decisions, rather than relying on a false sense of security.

Healthy flocks are built through education, prevention, and responsible practices—not vaccines alone.

Education empowers better poultry keeping. Vaccines help—but knowledge is the strongest line of defense.