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Marek's Disease

Marek’s disease (MD) is a widespread, highly contagious, tumor-causing viral disease caused by a the Marek’s disease virus (MDV), also known as the Gallid herpesvirus 2 (GaHV-2)---which is a type of alpha-herpesvirus that affects chickens. There are lots of different strains of the MDV, ranging from non-pathogenic (doesn't cause the chicken to develop MD) to highly virulent (very harmful to the bird). MDV is known for causing harm to the chicken's immune system, demyelination of peripheral nerves (leg and wing), and the development of tumors throughout their organs and tissue.

What Causes Marek's Disease

MDV is a type of herpes virus. Herpes viruses are known for gravitating towards (and replicating in) lymphoid tissue---where in humans and other mammals, this refers to the lymph nodes. Chickens do not possess lymph nodes in confined areas in their bodies like mammals, instead their lymphoid tissue is spread throughout their body. MDV closely resembles the Herpes Simplex Virus (HSV) responsible for oral and genital herpes, and Varicella Zoster Virus (VZV) that causes shingles in humans.

How Chickens are Infected

The MDV is not transmitted through the parents to the egg. Chickens become infected with the virus from exposure to it in their environment. Both infected (this includes vaccinated birds) and recovered chickens are lifelong carriers of the MDV, and will continue to shed the virus in their feather dander and oral and nasal secretions for life. Therefore, it is important to know if any of the birds in your flock are infected, not to "cull" them but simply to keep in mind that if you bring any new chickens to the premises, that they are going to be at risk of becoming infected with MDV if they are kept in the same environment as the birds confirmed to be infected with the MDV. The MDV has been known to survive for at least one year in the environment. MDV infections can be detected through virus isolation and by the demonstration of the presence of viral antigens or antibodies, which can be accomplished by testing feather follicles.

Clinical Signs of Marek's Disease

Most chickens with MD begin to develop noticeable signs when they are about 12-24 weeks (3 to 6 months) old---which is also about the same time their bodies undergo stress as a result of hormonal growth changes such as start of egg laying (in hens) and crowing (in roosters). Sometimes older birds may have a delayed presentation of signs, where they appear fine until a stressful event occurs (predator attack, rough handling, inner flock fighting, change of ownership or management, extreme weather changes, high parasite loads, etc.), causing them suddenly go lame. This initial lameness is often mistaken as an injury, which progressively worsens until the bird can barely or is unable to walk. These birds may eventually develop the classic Marek's disease pose, where one leg is positioned straight out in front of their bodies, with the other straight out behind. These birds may even be seen trying to use their wings to balance themselves, in an attempt to move on their own.

Different Forms of Marek's Disease

MD can develop in several different forms with the most common being as a lymphoproliferative syndrome, associated with the growth of lymphomas throughout the body, most commonly resulting in enlargement of the peripheral nerves, and the presence of lymphomas in various visceral organs and tissues, including the skin and eye.

FormGross pathologyClinical signs
Neurological formEnlarged or inflammation of the peripheral nerves. In some cases may be 2-3 times normal size with loss of crossstriations and gray or yellow discoloration.Incoordination or stilted gait as an initial first sign, followed by progressive, often unilateral paralysis of the legs (one leg stretched forward and the other leg backwards), wings (wing drooping), or crop (gasping and darkening or purplish coloring of comb resulting from lack of oxygen).
Cutaneous (skin) formAssociated with enlarged feather follicles, often scattered or clustered together, especially on the legs, shanks and feetReddened, bloody looking shanks
Ocular (eye) formOcular lesionsGray, often misshapen or shrunken iris or eyes with unequally-shaped pupils. Sometimes conjunctivitis and edema are observed. Chickens often progress to develop blindness in one or both eyes
Transient paralysis (TD)Vasogenic brain edemaIncoordination (ataxia), partial to complete paralysis lasting only temporary (1 to 2 days), afterwhich the chicken recovers. However many of these cases result in chickens developing clinical MD several weeks later.
Visceral (internal) formLymphomas occur in visceral organsOften chickens will die suddenly without any apparent cause. In other cases, when signs do appear they may include depression and stunted growth.

How Marek's Disease is Diagnosed

There are a number of laboratory tests, offered by different diagnostic testing facilities, that can be done to find the Marek's disease virus (MDV) in chickens. Tests for MDV can be done using different types of samples, including blood, tissue, and feathers. In the United States, North Carolina State University Poultry Tumor Lab has an extensive list of available tests.


One leg stretched forward and the other back
Leg weakness
Nervous tics
Drooping wings
Inward curving of the toes
Squatting position
Enlarged crop
Delayed crop emptying
Irregular-shaped or unequally sized pupils
Change in eye color
Weight loss
Enlarged feather follicles, especially on legs
Reddened, bloody looking shanks
Loss in appetite


  • History
  • Clinical signs
  • PCR
  • Necropsy


Supportive careIsolate the bird from the flock and place in a safe, comfortable, warm location (your own chicken "intensive care unit") with easy access to water and food. Limit stress. Call your veterinarian.
Sodium tanshinone IIA sulfonate (STS)A type of Chinese medicinal herb. Studies show evidence that it helps to promote apoptosis of cells in chickens infected by MDVWang J et al., 2016; Sun Y et al., 2014; Sun N et al., 2014; Zhang JQ et al., 2013
Dipotassium glycyrrhizinate (DG)A type of Chinese medicinal herb. Studies show evidence that it helps to promote apoptosis of cells in chickens infected by MDVSun Y et al., 2014
Acyclovir (Zovirax®)An anti-viral drug used to treat herpes simplex virus in humans. Studies show dosage of 120 mg/kg orally every 12 hours in tragopans was a safe dosage for treatment of antiviral infections.E Rush et al., 2005; T Norton et al., 1992;
St Johns wort (Hypericum Perforatum)3 doses of 30xBackyard Chickens member
Thymulin 5cHDiluted into drinking waterSato C et al., 2012
HomeopathyCausticum for paralysis, Calc flour and carb for thickened skin and feather follicles, Arg nit and Euphrasia for gray eye, and Kali phos 6x and Kali Sulph 6x for sciatic nerve enlargement


  • Vaccinate chicks - Administer MDV vaccine to chicks the day they hatch. It is given subcutaneously (under the skin) at the back of the neck.
  • Newly hatched chicks should not be put in the same environment as older flock members until they are at least 6 months old.

Scientific References

Good Overviews


Age Range

Chickens between 12 to 24 weeks old are most commonly affected.

Risk Factors

  • Silkie, Sebrights, and some strains of Polish chicken breeds are very susceptible to infection with MDV.
  • Brooding chicks under 5 months of age in the same area as adult birds.

Patient Cases