Marek's Disease

Marek's disease is one of the most common chicken diseases, so much so that the virus is assumed to be present in nearly all commercial flocks worldwide.   The virus comes in various forms which affect the nervous system (Neural), eyes (Ocular), skin (Cutaneous), and the organs (Visceral).   Skin tumors can also be found in the feather follicles.  Symptoms vary depending on the form of the illness.  Growing birds under the age of 6 months are most commonly affected by Marek's, however it can affect birds at any age.  



  • Chickens  (More common in large fowl than bantams)
  • Game birds
  • Pheasants
  • Quail 
  • Turkeys

Where:  Worldwide  (Extremely common - prevalent anywhere poultry is raised)

When:  Any age but typically at 3 to 6 months.

Why:  Herpes Virus (no cure) 


  • Can be transmitted by air (dust, dander, feces, saliva)  
  • Survivors are carriers for life.  
  • Virus can survive for years in bedding.  

What Symptoms:

Common Symptoms: (vary by the form of Marek's)

  • Gray eye/dilated pupil (Occular variant - see photo) 
  • Labored Breathing
  • Lameness  
  • Lethargy
  • One wing hanging down
  • Paleness
  • Paralysis (one leg forward, one leg backward) 
  • Poor coordination
  • Swollen feather follicles (tumors)
  • Unthriftiness - failure to grow and put on weight

Other Symptoms:

  •  Blindness (ocular form)
  • Greenish diarrhea (end stage)
  • Irregularly shaped pupils (ocular form) 
  • Tremors before death (older birds)
  • Weight loss due to inability to access food & water
  • Scaly discolored comb (end stage)
  • Sudden death


Treatment:  None.   Many recommend culling birds that show symptoms. 


Vaccine:  Yes (click here for more information) 

  • Typically vaccination of day-old chicks.     
  • Vaccine prevents symptoms, but not the virus. 


Also Known As: 

  • Acute leukosis
  • Gray eye (occular form)
  • Iritis (occular form)
  • MD
  • Neural leukosis
  • Neurolymphomatosis
  • Occular lymphomatosis
  • Range Paralysis
  • Skin leukosis
  • Uveitis 

Affects:  Muscles, neurological tissue ,organs, skin.  

Clinical Diagnosis:  Definite diagnosis us by necropsy through the presence of swollen organs and tumors in affected tissues.   Additional diagnosis by laboratory testing of blood/plasma.

Diseases with similar symptoms:  Lymphoid Leukosis has similar symptoms, however it generally affects birds that are 6-9 months old, whereas Marek's generally affects birds that are 3-6 months old.  Botulism can also cause paralysis. 

Duration of symptoms:  7 to 21 days, however it can be longer.

Incubation:  As little as 14 days. 

Morbidity:  Moderate (10-49%) occurrence in birds that have not been vaccinated.  Low (<10%) occurrence in vaccinated birds. 

Mortality:  Up to 100%. 

Transmission to people:  No. 


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A righteous man cares about his animal's health...

Proverbs 12:10a