Marek's Disease

Marek’s Disease (MD) is a common, highly contagious tumor-causing viral infection which affects chickens and sometimes turkeys, worldwide. There are many different strains of the virus, which vary in pathogenicity from non-pathogenic to highly virulent. Once a chicken is infected, they’re infected for life. Although there is a vaccine available which is relatively effective against clinical disease from developing, it does not prevent the chicken from becoming infected, and spreading the virus to other birds throughout their life span.

Age Range

It most commonly occurs in chickens between 3 and 6 months of age, but can occur at any age.

Risk Factors

  • Mixing vaccinated birds with non-vaccinated birds will put those individuals who have not received the vaccine at risk, since the vaccine does not prevent infection or shedding of the virulent virus in birds, it only prevents clinical signs of disease.
  • Breed – Leghorns and light egg-type breeds are more vulnerable to disease than heavier breeds. Silkies, Sebrights, and Polish breeds are highly susceptible to Marek’s disease.
  • Brooding chicks under 6 months of age in the same environment as adult flock members
  • Stress
  • Co-infection with immunodepressive pathogens such as chicken anemia virus
  • Chickens with poor immune systems

Case Studies

Red the Rooster with Marek’s

Also Consider

  • Gastrointestinal obstruction
  • Lead poisoning
  • Lymphoid leukosis
  • Newcastle disease
  • Pendulous crop
  • Renal tumors
  • Reticuloendotheliosis
  • Tumors

Clinical Signs

  • One leg stretched forward and the other back
  • Paralysis of the legs, wings, or neck
  • Torticollis
  • Incoordination
  • Gasping
  • Delayed crop emptying
  • Partial on complete blindness
  • Irregular-shaped or unequally sized pupils
  • Change in eye color (to gray)
  • Enlarged feather follicles
  • Reddened legs
  • Paleness
  • Loss in appetite
  • Depression/lethargy
  • Weakness
  • Weight loss


  • History
  • Clinical signs
  • Physical exam
  • Complete Blood Count (CBC) and Biochemistry – monitored for changes in health status.
  • Histopathological exam of affected organs, including peripheral nerves.


  • Reduce stress
  • Vaccinate chicks – Administer MDV vaccine to chicks shortly after they hatch. It is given subcutaneously (under the skin) at the back of the neck. Chicks require 7 to 14 days in order to become immunized against MDV.
  • Newly hatched chicks should not be put in the same environment as older flock members until they are at least 6 months old.
Marek’s disease is characterized by peripheral nerve enlargement and development of visceral lymphomas that may affect a wide range of organs. The different forms of MD include the following.
Neurological (nerve) form: Lymphoid infiltration into peripheral nerves.
Clinical signs depend on which nerves are involved. Cervical nerve involvement causes wry neck, flaccid neck paralysis, and head tilt. When the vagus nerve is affected, the bird may have crop emptying issues and/or breathing difficulty. Occurs commonly; mortality up to 20%.
Transient paralysis (TD): Vasogenic brain edema; causes temporary incoordination (ataxia), partial to complete paralysis of the neck or legs, lasting only 1 to 2 days. Occurs occasionally; mortality up to 30%.

Ocular (eye) form: Lymphoid infiltration into eyes; causes change in iris color, pupil shape or size, partial or total blindness. Rare, occurs usually in older birds; mortality up to 25%.

Cutaneous (skin) form: Lymphoid infiltration into skin; causes enlarged feather follicles, often scattered or clustered together, especially on the legs. Occurs commonly; 0% morality rate.

Visceral (internal) form: Tumors develop on internal organs (kidneys, spleen, liver, gonads, heart, and proventriculus). Clinical signs vary based on the location of the tumors. Occurs very commonly; 60-80% mortality rate.
Chickens become infected with the virus by inhaling infected dust and dander shed in the form of flakes of the skin of infected birds, which has contaminated the environment. Once inhaled, the virus invades the chicken’s lung air space and infects epithelial cells, and spreads to other cells throughout the body. Similar to other herpes viruses, MDV has a tendency to be transported towards cutaneous sites such as the skin, and feather follicles. From there, MDV is shed into the environment via scales and feather debris from infected birds. The virus can remain infectious in the environment for up to 8 months.
The presence of maternal antibody against MD can protect baby chicks, and with the development of a functional immune system, a degree of resistance to MD is developed. However, husbandry-related stress, or concurrent infection with other immunosuppressive pathogens will significantly enhance their susceptibility to developing clinical disease. The percentage of clinically sick birds in a flock depends on the strain of virus and the breed of bird. The percentage of birds in a flock who are infected and develop clinical signs of disease can be anywhere from 1% to up to 50%. The clinical disease is usually seen in unvaccinated chickens between 6 weeks to 30 weeks of age. But the disease occurs in older birds as well.
Marek’s disease can be confirmed in unvaccinated birds by sending a couple of the bird’s feathers to a veterinary diagnostic laboratory for confirmation. If the chicken has already died, sending their body to a diagnostic lab for a necropsy and biopsy examination of the tissues is the best option.
Chickens vaccinated against Marek’s disease rarely get sick. However, they must have received the vaccine before they are exposed to the virus. Then the birds need about 4-14 days for the vaccine to start to become effective. There are different types of the vaccine available, and some are more effective, and require more lead time to work than others.
Any birds who have not received the MD vaccine should never be combined with any chickens who have been vaccinated. This is because the vaccine prevents the formation of tumors, but it does not prevent the chicken from getting infected and shedding the virus into their environment.

Scientific References

  1. Yang, Fan et al.. Antiviral effect of baicalin on Marek’s disease virus in CEF cells BMC veterinary research vol. 16,1 (2020)
  2. Shi, Meng-Ya et al.. The Emergence of a vv + MDV Can Break through the Protections Provided by the Current Vaccines Viruses vol. 12,9 (2020)
  3. Bailey, Richard I., et al.. Pathogen transmission from vaccinated hosts can cause dose-dependent reduction in virulence PLoS biology 18.3 (2020) Bertzbach, Luca D., et al. Latest Insights into Marek’s Disease Virus Pathogenesis and Tumorigenesis Cancers 12.3 (2020)
  4. Yu, Zhenghao, et al.. Natural co-infection with two virulent wild strains of Marek’s disease virus in a commercial layer flock Veterinary Microbiology 240 (2020)
  5. R Hauck et al. Two Cases of Marek’s Disease in Backyard Turkeys Avian diseases (2020)