What is Marek’s Disease in Chickens?
by Alexa Lehr | The Pioneer Chicks | October 24, 2023
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Marek’s disease a common but little known poultry disease. It can take on many shapes and forms, so it is often a confusing and hard to diagnose ailment. The disease can be asymptomatic, transient, and eventually fatal. Understanding how Marek’s disease works and what symptoms to watch can help you be prepared for recognizing Marek’s disease in the homestead flock. Unfortunately, Marek’s disease has no known cure, but knowing how to deal with Marek’s disease can help you prevent it from becoming a big issue in your flock.

What is Marek’s Disease?

Marek’s disease is a viral disease that affects few other animals besides chickens. The nature of this virus makes it a very complex chicken disease to manage and diagnosis. Some strains of the virus are asymptomatic, meaning the chicken will carry the virus but won’t show any symptoms of being a carrier. Other strains of the virus cause severe illness and eventually death.

In order for a chicken to be asymptomatic, the chicken must have a strong immune system. The immune system works to fight against harmful pathogens, bacteria, viruses, and protozoa that cause chickens to get sick. If the immune system is strong, the chicken’s body will be able to fight the Marek’s virus and make it go dormant. The dormant virus will reside in a chicken’s DNA, making the chicken a carrier for life. Even though the virus won’t cause the chicken to exhibit symptoms of illness, the asymptomatic chicken can still spread the virus to other chickens.

Chickens who don’t have a strong immune system will become symptomatically affected by Marek’s disease. They may exhibit one or more of the various symptoms that different strains of the disease can cause. Chronic diseases and parasites can make a chicken even more susceptible to Marek’s disease since the immune system will already be compromised.

To make things more complicated, Marek’s disease symptoms can come and go, or they can be strictly internal. A chicken may show symptoms of Marek’s disease, then appear to recover, only to show the same or different symptoms later on. When Marek’s disease is strictly internal, the virus often causes slow growing tumors to form which eventually leads to death. Internal Marek’s disease is often only diagnosed through a postmortem examination.

Is Marek’s Disease Common in Chickens?

Marek’s disease is a very common virus. It is so common that many avian experts believe that most flocks are carriers of the virus even if they show no symptoms. This is possible if your flock has built up immunity to the virus and is able to keep the virus dormant. Different flocks can build up immunity to different strains of the virus. So your flock may be immune to one strain of the virus, but susceptible to a different strain of the virus.  

As you might suspect, Marek’s disease commonly affect young and growing chickens. Chickens between the ages of 4 weeks to 6 months are the most susceptible to succumbing to the virus. During this stage of life, the young birds are still building up a strong immune response. If their immune systems aren’t strong when they are exposed to the Marek’s virus they can become symptomatic.

However, even mature birds are susceptible to Marek’s disease. Mature chickens can be carriers of the virus, but they can also be affected by the virus if they are exposed to a different strain or if the virus is activated by stressors.

How is Marek’s Disease Activated?

Dormant Marek’s disease virus can become symptomatic if a chicken’s body is under stress. The stress factor can be natural, such as the natural stress a chicken’s body experiences as it grows, or the stress created by the natural cycle of reproduction. Or the stress factors can be environmental, such as boredom, being transported, cold stress, heat stress, parasites, or another disease. Any stress factor can activate the dormant Marek’s disease.

Since the Marek’s virus can be activated by stressors, such as other diseases and parasites, it can often seemingly make those diseases or parasites the cause of illness of death. However, Marek’s disease can also work internally without the chicken showing any initial symptoms at first. If the slow-growing tumors start to develop internally, the chicken won’t show any symptoms until those tumors get large enough to cause serious problems and eventually death.

Causes of Marek’s Disease

Marek’s disease is caused by several strains of herpesviruses and is considered a viral tumor disease that affects a chicken’s DNA. The virus works by targeting a chicken’s tumor-blocking genes and antibody-producing cells. By inactivating these genes and cells, the virus renders the chicken’s body defenseless against tumors or fighting the virus.

Without antibodies, the chicken’s immune system is severely weakened, enabling the virus and any other disease or parasite to thrive. Since the virus can be active at the same time as another disease or parasite, it makes the virus even harder to diagnosis in chickens. Marek’s disease shares symptoms with many other poultry diseases and often coexists with other ailments such as coccidiosis or chronic respiratory disease. The presence of the virus can make these diseases even worse since the chicken’s immune system will already be compromised.

How Does Marek’s Disease Spread?

So, you know that Marek’s disease is caused by a virus, but how is the virus spread? Marek’s disease is spread though exposure. The virus can reside on chicken dander and is commonly spread through inhalation of the dander. Chicken dander can be spread through the air, on our clothing and shoes, in litter, or even on equipment.

Marek’s disease is highly contagious, and even asymptomatic chickens can spread the virus. Chickens who are exposed to other chickens who were diagnosed with Marek’s disease must be considered carriers of the virus for life even if they never exhibit any symptoms.

Despite affecting the DNA of a chicken, Marek’s disease is not transmittable through hatching eggs. A hen who is a carrier of the virus will not pass the virus to her eggs. Chicks who hatch from a virus-carrying hen won’t necessarily be carriers of the virus themselves as long as they are hatched and raised away from adult birds. Raising the chicks away from any exposure to adult birds up until they are five months old can help them become immune to the virus as adults. During that time, the young birds have enough time to build a strong immune system to fight the virus once they are exposed.

It has been found that some strains and breeds of chickens are more resistant to the virus than others. Silkies, Polish, Sebrights, and other exotic chicken breeds tend to be more susceptible to the virus. However, well-bred chickens with a strong genetic pool will always be more resistant to any disease than poorly bred birds.  

Marek’s Disease Symptoms

Since the Marek’s disease virus can be temporarily asymptomatic, dormant, or symptomatic it makes it a very challenging disease to diagnosis. A chicken infected with the virus can show no symptoms, show a single symptom, show multiple symptoms, or only show symptoms sporadically. It can also be active when another disease or parasite is causing a chicken to exhibit symptoms of illness.

Different forms of the Marek’s disease virus are characterized by different symptoms. However, there are two symptoms that are characteristic of all strains of the virus:

• Immunosuppression– A chicken can have transient immunosuppression or full immunosuppression. Transient immunosuppression will leave the chicken susceptible to disease for short periods of time before the immune system becomes strong again. Full immunosuppression will leave a chicken with no immune response to fight diseases or parasites.
• Transient Symptoms– All Marek’s disease virus strains can cause come-and-go, or transient, symptoms. A chicken will exhibit signs of sickness then appear to recover, only to exhibit the same symptoms or different symptoms at any time again.

The only way for Marek’s disease to be diagnosed positively is through a DNA blood test or a PCR test. Marek’s disease cannot be positively diagnosed while a chicken is still alive. Postmortem examinations can reveal tumors caused by the virus, but the presence of the virus can only be confirmed through a blood test or a PCR test.

Marek’s disease is not a zoonotic virus, meaning it will not affect other species of animals. The virus is specific to chickens, and only on rare occasions has it been reported to affect other poultry species such as quail or turkeys. People cannot get Marek’s disease. Marek’s disease cannot be transmitted through a chicken’s meat or eggs.

Forms of Marek’s Disease

While all strains of the virus cause the symptoms mentioned above, different forms of Marek’s disease affect a chicken differently. There are 5 recognized forms of Marek’s disease:

• Eye Form (aka gray eye, ocular lymphatosis)– Rare. Causes graying & shrunken iris, irregular-shaped pupil, un-equal pupil sizes, and blindness in one or both eyes.
• Nerve Form (aka neural leukosis)– Causes lack of coordination or walking stilted, muscle spasms, progressive paralysis of neck, legs or wings, weight loss, paleness, enlarged crop, and gasping.
• Transient Nerve Form (aka pseudobotulism)- Causes temporary limp paralysis of the neck or legs.
• Skin Form (aka cutaneous Marek’s)– Rare. Causes enlarged feather follicles (especially on legs) and reddened, bloody looking shanks.
• Visceral Form (aka visceral leukosis)– Causes rapid weight loss, sometimes greenish diarrhea, and massive internal tumors.

The most common forms of Marek’s disease to affect chickens are the transient nerve form and the visceral form. Since the transient nerve form only causes sporadic symptoms, it rarely leads to death. However, the visceral form is much more fatal and usually causes a 60-80% mortality rate of the birds it infects. The eye form and nerve form can also lead to death.

How to Treat Marek’s Disease

Unfortunately, there is no known treatment for the Marek’s virus. Once a chicken is exposed to Marek’s disease, it is considered a carrier of the virus for life. If the virus does not affect the chicken, the chicken should be able to carry on with normal behaviors. If the virus becomes symptomatic, there is no known treatment to stop the virus from being fatal.

A strong immune system is a chicken’s best defense against Marek’s disease. When the immune system is strong, it will fight the virus and cause it to go dormant. As long as the immune system stays strong, it will be able to prevent the dormant virus from becoming active.

If you suspect that one or more of your chickens has Marek’s disease, it is a good idea to explore other ailments that could cause similar symptoms. Since you can’t accurately diagnose Marek’s in a live chicken, exploring treatment options for similar disease is always beneficial.

Here are some common poultry ailments that can cause similar symptoms to Marek’s disease.

Diseases Similar to Marek’s:

• Vitamin Deficiencies– find out the symptoms of vitamin deficiencies here!
• Avian Lymphoid Leukosis– causes emancipation due to internal tumors
• Toxicity or Poisoning– heavy metal & lead toxicity or poisoning from aflatoxins in moldy or stale feed
• Botulism– caused by a toxin that attacks the body’s nervous system
• Egg Binding– occurs when a hen is unable to lay an egg
• Hypocalcemia– often called layer fatigue and is due to dangerously low calcium levels
• Internal & External Parasites– mites, lice, & internal worms
• Ear Infections– will sometimes cause a chicken to hold its head at an odd angle
• Avian Encephalomyelitis– can cause paralysis
• Viral Tenosynovitis– poultry version of arthritis and can cause difficulty walking
• Mycoplasma Infection– a respiratory disease that can also be asymptomatic
• Bumblefoot– staph infection of the foot pad that can cause a chicken to limp or walk funny

Is the Marek’s Disease Vaccine Affective?

The Marek’s disease vaccine can only be used as a preventative measure for Marek’s disease. It is not a treatment for chickens who already have the virus. Even with the Marek’s vaccine, chickens can still get the virus regardless if they have been vaccinated.

Basically, the vaccine helps chickens build up immunity to the virus. When the chicken does get exposed to the virus, the vaccine can help the chicken’s body respond in a way to cause the virus to be asymptomatic. Chickens of any age can be vaccinated for Marek’s disease. However, the vaccine will not be effective if the chickens have already been exposed to the virus. Vaccinating chicks within 24 hours of hatching is the most effective way to use the Marek’s vaccine.

The Marek’s disease vaccine is not 100% effective. About 5% of chickens who receive the vaccine end up getting the virus anyways. Like most viruses, as the virus becomes more common new strains of the virus will develop. Current Marek’s vaccines are not effective against new strains of the Marek’s virus.

What Marek’s Vaccine Can be Used for Homestead Chickens?

The most common Marek’s disease vaccine to administer to non-commercial chickens is the MDV-3 vaccine. This vaccine is derived from a natural virus found in turkeys. The turkey virus prevents the Marek’s disease virus from forming the slow-growing, deadly tumors. The MDV-3 vaccine will not infect a chicken with Marek’s or spread the Marek’s disease virus.

Learning about Marek’s disease and how it works can help you become more knowledgeable about how the virus can affect your homestead flock. The best way to prevent Marek’s disease from becoming a serious issue is to keep your flock’s immune systems strong. A strong immune system will make the virus go dormant even if your flock gets exposed. Be mindful of how the virus can be spread to new or young chickens. Practice good biosecurity and keep your flock healthy with holistic supplements to protect your homestead flock!

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