You finally have your coop up and running. Your cute feathered friends are clucking away. You’re practicing egg-cellent biosecurity measures and feeding the right diet but what else can you do to protect your birds?

SoCal Nestbox aims to provide you with helpful tools, such as  Find A Vet, on our site along with articles (like this one!) to support your happy and healthy human-animal bond. We are here to share important information with you about one of the most common and most devastating avian infectious diseases. Please read on to learn about Marek’s Disease and what you can do to protect your backyard flock from the virus that causes it.

Marek’s Disease is a highly contagious (spreadable) viral disease of poultry, especially chickens. It was first described by a Hungarian veterinarian, Jozef Marek, when he observed the signs in his backyard roosters in 1907 (Note: the virus that causes Marek’s disease was not discovered until 1967!). Marek’s Disease is known to cause tumors and nerve swelling leading to paralysis. Below, we will discuss some basics, how the disease spreads, signs and symptoms to watch for, diagnosis of the disease, and how to prevent it in your flock.

Some Virus Basics

Marek’s Disease, also known as Fowl Paralysis, Range Paralysis, Polyneuritis, or Neurolymphomatosis, is an important illness to recognize in chickens because this disease is highly contagious and found in most poultry worldwide. In chickens, this disease is often fatal with no cure currently available. The Marek’s Disease virus, is a herpes virus (not the type that humans can get)– more specifically, it is within the genus Mardivirus and the virulent (most harmful) strains are all Gallid alphaherpesvirus 2. Like other herpes viruses, the Marek’s Disease virus cannot survive very well outside of the body; however, cool moist environmental conditions promote extended survival while windy conditions may promote aerosol (virus particles carried by air) transmission over longer distances. Like the current COVID-19 virus, a chicken infected with Marek’s may be asymptomatic (no signs or symptoms of illness). This is important to note as an asymptomatic chicken is still an infected chicken that can spread the disease!

Although chickens are the most important natural host for this virus, other bird species can become infected. According to an article by a veterinary epidemiologist, Dr. John Dunn, quails may become naturally infected while turkeys have only been experimentally infected. He adds that turkeys are commonly infected with a non-harmful strain of a turkey herpesvirus that is related to the Marek’s Disease virus. This point is important because the herpesvirus of turkeys (HVT) is commonly used in a vaccination for chickens against Marek’s Disease. We will discuss this more later in this article. For our readers concerned about human health risks when working with either Marek’s Disease or HVT, there are currently none recognized by the OIE (World Organisation for Animal Health).

Transmission – How the Disease Spreads

Marek disease is typically considered a disease of young chickens, but a chicken of any age can still get sick. As previously mentioned, MDV is highly contagious and easily transmitted (spread) among flocks.  It is of utmost importance to note that even vaccinated birds that come into contact with the virus can still become infected, and, while protected from serious illness, still have the potential to spread the virus to other birds through close contact, shared equipment (e.g. food and water dispensers), or even contact with wild birds. Since herpes viruses like MDV, do not survive well outside of the body, spread of the disease is usually through close contact. Some highlights of disease transmission are outlined below:

  • Inhaled dust or dander (skin flakes) particles from infected chickens are the most effective way for the virus to spread
  • Virus particles shed in chicken dander are not as infectious but may survive in the environment (e.g. poultry house dust or litter) for months
  • Virus particles shed directly from feather follicles are very infectious but are vulnerable in the environment
  • Infected chickens become disease carriers for life
Flock of chickens gathered outside coop.

Flock of chickens gathered outside coop.

Signs and Symptoms to Watch For

Signs and symptoms of Marek disease can be tricky. Chickens may show little to no signs of illness when infected with the virus. Some subtle signs of illness may include decreased growth rates and egg production or even only depression just before death. For more information on subtle signs of illness in birds, you can find our (hopefully) helpful article here.

A more obvious sign of Marek disease in chickens is paralysis. The virus may affect the nerves causing them to swell and eventually lead to paralysis, especially in the legs. Other body parts may be affected as well. Some chickens may have a milder form of paralysis called ataxia (incoordination, abnormal movements like a toddler walking).

Death may occur as a result of paralysis as it may prevent their ability to reach food and water. Additional signs and symptoms that may be difficult to find without the help of your trusted veterinarian are visceral lymphoma (cancerous tumors inside the body) and skin leukosis (enlarged feather follicles). Some infected chickens may show one or more signs, while others may have no symptoms yet still spread the disease within your flock.

  • Most common signs of Marek disease
    • Paralysis (partial to full)
      • Paralysis might be one sided or symmetrical (e.g. both legs)
      • Ataxia (toddler-walking) or full paralysis of a limb, lameness, torticollis (twisted neck position), and/or drooping wings
    • Lymphoma in the eye(s)
      • Gray iris(es) aka Gray eye, Cat’s eye, or Pearl eye
      • Partial or total blindness
    • Lymphoma in the skin
      • Enlarged feather follicles
    • Internal tumors


If your chickens are showing any of the signs or symptoms listed above, please contact your trusted avian veterinarian or use our Find A Vet tool. To make a diagnosis of Marek Disease, your veterinarian will likely gather background information on your sick chicken and/or flock including age and vaccination records, perform a thorough physical exam, and may request to run some diagnostic tests. In the unfortunate event that you lose one of your chickens and your trusted veterinarian suspects Marek disease, he/she may recommend performing a necropsy (dissection of a dead animal to determine cause of death) to obtain a conclusive diagnosis. Confirmation of Marek’s Disease will be important to protect the rest of your flock. Having copies of records on hand for your veterinarian will be tremendously helpful in making an accurate diagnosis as there are other diseases that can have similar signs and symptoms.

How to Prevent Marek’s Disease in Your Flock

Currently, there is no cure or effective treatment for Marek’s Disease; therefore, prevention is the best medicine for your flock! There are two pillars to a Marek’s Disease prevention program. The first is to habitually follow good biosecurity practices. The second pillar is to consistently follow an effective vaccination program.

Please refer to our disease prevention tab here for more information on what biosecurity is and how you can achieve the best practices in protecting your beloved flock.

In addition to practicing good biosecurity, vaccination against the Marek’s Disease virus is the most common method for prevention and control of this disease. The most widely used vaccines include the following:

  • Turkey herpesvirus (HVT), naturally avirulent (non-harmful)
  • SB-1 or 301B/1, naturally avirulent (non-harmful)
  • Rispens (CVI988)
Chick standing next to Marek's HVT + IBD vaccine ampule. Photo by Vaxxitek.

Chick standing next to Marek’s HVT + IBD vaccine ampule. Photo by Vaxxitek.

Vaccines must be administered on the day the chicks hatch or in ovo (embryo inside the egg) during egg incubation in order to be most effective. For a good explanation of which vaccines to use and how to properly use them please follow this link. With proper handling of vaccine during transport, storage and administration effectiveness is typically over 90%. Since the start of vaccine use, Marek’s Disease outbreaks have significantly decreased. Many of our readers will likely be purchasing through a hatchery or feed store, so be sure to ask about the vaccination status of the birds you would like to purchase in order to avoid future heartache.

In Summary

  • Marek disease is a highly contagious, devastating, and global disease of poultry that may lead to tumors and/or nerve enlargement leading to paralysis (particularly in the legs) and to pre-mature death.
  • Your veterinarian can make a reasonable diagnosis based on the information provided (including vaccination records), signs and symptoms, and testing.
  • Prevention is the best protection. Vaccines are protective if given by day one and should be used in combination with good biosecurity measures.