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Marek's Disease

Marek’s Disease (MD) is a common, acute or chronic, very contagious disease of poultry, caused by an alpha herpes virus, called Marek’s disease virus (MDV) or Gallid herpesvirus 2 (GaHV-2). There are several different strains of the MDV, which vary in pathogenicity from non-pathogenic to very virulent. Chickens of any age are very susceptible to the virus, although the disease can also infect turkeys, quail, and pheasants. Marek's disease is found worldwide.

How Chickens are Infected with Marek's Disease Virus

Chickens become infected with MDV by inhaling infected dust and dander shed in the form of flakes of skin from MDV-infected birds, which has contaminated the environment. Once inhaled, it 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 several years. A 1:10 dilution of bleach inactivates the virus.
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 MDV.

What are the different forms of Marek's Disease?


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 include: Incoordination or stilted gait, progressive paralysis of legs, wings, or neck, hurdler's position (one leg stretched forward and the other leg backwards), torticollis, and/or muscle spasms. If vagus nerve is involved, crop dilation (presents often as crop impactions) and gasping. 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 tumor. Occurs very commonly; 60-80% mortality rate.

Chickens vaccinated against Marek’s disease rarely get sick. But the vaccine does not prevent them from spreading Marek’s to unvaccinated birds. 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. Studies have shown that a vaccinated bird sheds 10,000 times more virus than an unvaccinated bird.

Clinical Signs

One leg stretched forward and the other back
Paralysis of the legs, wings, or neck
Torticollis
Incoordination
Gasping
Delayed crop emptying
Partial or 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

Diagnosis

  • 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.

Treatment

NameSummary
Acyclovir (Zovirax®)10 mg/kg IM q24h x 5-14 daysB Speer, 2016, E Rush et al., 2005; T Norton et al., 1992
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
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
Sodium tanshinone IIA sulfonate (STS)A type of Chinese medicinal herb. Studies show evidence that it helps to promote apoptosis of cells.Wang J et al., 2016; Sun Y et al., 2014; Sun N et al., 2014; Zhang JQ et al., 2013
Thymulin 5cHDiluted into drinking waterSato C et al., 2012
MetyraponeIn one study, chickens fed a diet containing the cortisol-reducing drug Metyrapone showed regression or lack of MDV tumors compared to the control group.

Support

Prevention

  • 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.

Scientific References

Good Overviews

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, only prevents clinical signs of disease.
  • Silkies, Sebrights, and some strains of Polish chicken breeds are more susceptible to infection with the virus.
  • Inbreeding
  • Brooding chicks under 5 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 Stories