A brown sick chicken with her eyes closed and laying down

Marek’s Disease in Chickens: Signs, Symptoms, and Prevention

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Marek’s disease is a viral illness caused by the highly contagious Marek’s disease virus (MDV), a type of herpesvirus. It primarily affects young chickens but can also impact older birds, leading to devastating consequences if left unchecked. Here, we’ll delve into the best practices for safeguarding your flock against this perilous poultry disease.

Understanding Marek’s Disease

Marek’s disease primarily targets the lymphoid tissues and feather follicles of infected birds. The virus is incredibly resilient and can survive in the environment for long periods, making it challenging to eradicate once introduced to a flock. It is highly contagious as it spreads through feather dander and can even be carried by seemingly healthy chickens, acting as silent carriers of the disease.

Sick, brown hen with missing feathers on back of neck

What Causes Merek’s Disease in Chickens?

Marek’s disease in chickens is caused by an alphaherpesvirus known as Marek’s disease virus (MDV). This lymphoid leucosis virus primarily targets the immune system, specifically the lymphoid tissues, and can also affect the peripheral nerves and feather follicles of infected birds. As mentioned, MDV is highly contagious and can spread rapidly within a flock through various means. Here are some ways chickens transmit it to other chickens.

  1. Inhalation: Chickens can inhale the virus shed in feather dander or respiratory secretions from unvaccinated birds that are infected with the Marek’s virus.
  2. Ingestion: Contaminated feed, water, or surfaces can serve as sources of infection if ingested by susceptible birds.
  3. Direct Contact: Close contact between infected and susceptible birds can facilitate the transmission of the virus, especially in crowded or poorly ventilated environments.
  4. Vertical Transmission: Infected hens can pass the virus to their offspring through the egg, leading to the infection of day-old chicks.

Once the virus enters the chicken’s body, it replicates primarily in the feather follicles and lymphoid tissues, where it disrupts the normal function of the immune system and promotes the development of tumors or lymphomas. These tumors can affect various organs, including the liver, spleen, kidneys, and nerves, leading to a range of clinical signs and symptoms associated with Marek’s disease.

white chicken eating out of someone's hand and some is spilling

Can Humans Contract Marek’s Disease?

No, humans cannot contract Marek’s disease. Marek’s disease is a viral illness which primarily affects chickens and occasionally other birds in the order Galliformes, such as turkeys, quails, and pheasants. MDV is a type of herpesvirus that is highly species-specific, meaning it only infects birds and does not pose any risk to humans or other animals outside of the avian family.

While herpesviruses can infect a wide range of species, each type of herpesvirus typically has a specific host range and does not cross species barriers easily.  The Marek disease virus has evolved to target the immune systems of birds, particularly chickens, and does not have the ability to infect or cause disease in humans.

a white chicken that is sick, with her head down and eyes closed

Recognizing the Symptoms of Marek

Identifying Marek’s disease in its early stages is crucial for effective containment. Marek’s disease can manifest in various ways in chickens, and the appearance of symptoms can vary depending on the stage of the disease and the affected bird’s immune response. Here are some common signs and symptoms to look out for.

  1. Weight Loss: Infected chickens may experience rapid weight loss despite maintaining a normal appetite. This is often one of the earliest signs of Marek’s disease.
  2. Loss of Appetite: Affected chickens may show a decreased interest in food, leading to a decline in overall nutrition and health.
  3. Pale Combs: The comb and wattles of infected birds may become pale or discolored due to reduced blood flow and oxygenation.
  4. Signs of Depression: Infected chickens may appear lethargic, withdrawn, or less active than usual. They may spend more time resting or isolated from the rest of the flock.
  5. Lack of Coordination: As the disease progresses, chickens may exhibit difficulty walking or maintaining balance. This can manifest as stumbling, falling, or even paralysis in severe cases.
  6. Enlarged Feather Follicles: Marek’s disease virus targets the feather follicles, leading to the development of tumors or nodules around the skin. These may appear as raised, lumpy areas under the feathers.
  7. Paralysis: In advanced stages of the disease, affected chickens may experience partial or complete paralysis, especially in the legs or wings. This paralysis is often irreversible and can significantly impact the bird’s quality of life.
  8. Tumors: Marek’s disease can cause tumors to form in the chicken’s internal organs, including the liver, spleen, and kidneys. These tumors may not be externally visible but can contribute to the bird’s overall decline in health.
  9. Vision Problems: In some cases, Marek’s disease can affect the eyes, leading to vision impairment or blindness in infected chickens.

It’s important to note that not all infected chickens will display all of these symptoms, and the severity of the disease can vary widely among individuals. Additionally, some chickens may be carriers of Marek’s disease without showing any outward signs of illness, making it crucial to implement preventive measures such as vaccination and biosecurity to protect the entire flock. If you suspect Marek’s disease or notice any concerning symptoms in your chickens, consult with a poultry veterinarian for proper diagnosis and treatment.

a close up of many different colored baby chicks

How Common is Merek’s Disease?

Sadly, Marek’s disease is relatively common in chickens, particularly in commercial poultry operations and backyard flocks worldwide. It is considered one of the most economically significant diseases affecting the poultry industry due to its widespread prevalence and significant economic losses associated with mortality, reduced productivity, and costs of prevention and control measures.

In regions where Marek’s disease is endemic, such as North America, Europe, and parts of Asia, the virus can be found in the environment and may affect a significant proportion of chicken populations, especially in unvaccinated flocks or those with inadequate biosecurity measures.

Veterinarian with stethoscope holding and examining chicken on ranch background.

Is Merek’s Disease Treatable?

Unfortunately, there is no specific treatment for Marek’s disease in chickens. Once a chicken becomes infected with MDV, the virus establishes a lifelong infection, and there is currently no cure to eliminate the virus from an infected bird’s body.

Since there is no treatment to directly combat the virus, management strategies typically focus on supportive care and prevention of secondary infections. Here are some steps that poultry owners can take to help manage Marek’s disease in their flocks.

  1. Isolation: Infected birds should be promptly isolated from healthy birds to prevent the spread of the virus to healthy individuals. This helps reduce the risk of transmission and minimizes the impact on the overall flock.
  2. Supportive Care: Providing the sick bird with supportive care, such as maintaining a comfortable and stress-free environment, offering a balanced diet, and ensuring access to clean water, can help improve their overall welfare and quality of life.
  3. Biosecurity: Implementing strict biosecurity measures can help prevent the spread of Marek’s disease virus within the flock. This includes controlling access to the poultry premises, disinfecting equipment and footwear, and minimizing contact with wild birds and other potential sources of infection.
  4. Culling: In severe cases where infected birds are suffering and unlikely to recover, euthanasia or culling may be considered to prevent further suffering and reduce the risk of spreading the virus to other birds.

It’s important to note that while these management strategies can help mitigate the impact of Marek’s disease, they cannot eliminate the virus from the flock entirely. Prevention remains the best approach for protecting chicken flocks.

A brown hen sleeping on pine shavings

What is the Mortality Rate for Infected Birds?

The mortality rate for chickens with Marek’s disease can vary depending on several factors, including the strain of the virus, the age and health status of the birds, and the presence of other concurrent infections or stressors. Generally, Marek’s disease can cause significant mortality in affected flocks, particularly in unvaccinated or inadequately protected populations.

In severe cases of Marek’s disease, where chickens develop tumors or paralysis, mortality rates can be quite high, approaching 100% in some instances. However, not all chickens infected with Marek’s disease will exhibit severe clinical signs or succumb to the disease. Some birds may become carriers of the virus, showing no outward symptoms of illness but shedding the virus and potentially transmitting it to other susceptible birds.

A hand with white glove injecting a syringe into a yellow baby chick

Preventing Marek’s Disease

Preventing Marek’s disease is the best way to safeguard your flock’s health. Here are some essential steps to take.

  1. Vaccination: Administering Marek’s disease vaccine to day-old chicks is the most effective preventive measure. Vaccination helps bolster the chicken’s immune system, reducing the likelihood of infection and minimizing the spread of the disease within your flock.
  2. Biosecurity Measures: Implement strict biosecurity protocols to prevent the introduction and spread of Marek’s disease virus. This includes maintaining a clean environment, minimizing contact with wild birds, and practicing good hygiene when handling chickens or entering the coop.
  3. Quarantine New Birds: Before introducing new chickens to your flock, quarantine them for a minimum of three weeks to monitor for signs of illness. This allows you to identify and isolate any potentially infected birds before they can spread the disease to your existing flock.
  4. Regular Health Checks: Routinely inspect your chickens for any signs of illness and promptly isolate sick birds from the rest of the flock. Early detection can help prevent the spread of Marek’s disease and other contagious infections.
  5. Strong Immune System: Ensure your chickens maintain a strong immune system by providing a balanced diet, clean water, and adequate shelter. A healthy chicken is better equipped to fight off infections and resist the effects of Marek’s disease virus.

Bottom Line

Preventing Marek’s disease in chickens is essential for maintaining the health and well-being of your flock. By implementing a combination of vaccination, biosecurity measures, and vigilant monitoring, you can significantly reduce the risk of infection and safeguard your chickens against this devastating disease. Remember, early intervention is key, so stay informed, stay proactive, and keep your flock healthy and happy.

Remember, consult with your local vet or poultry specialist for personalized advice and guidance tailored to your specific flock’s needs. Together, we can work towards ensuring the long-term health and vitality of backyard poultry everywhere.


What questions do you have? Let me know in the comments!

Related Posts:
Why Chickens Get Eggbound and How to Support Them
Everything You Need to Know About Chicken Poop
Can Chickens Contract Rabies?
Are Chickens Mammals or Birds?

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