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Marek's Disease

Marek’s disease (MD) is a very common tumor-causing viral infection caused by a herpesvirus. It affects chickens (and sometimes turkeys) worldwide. The classic form of MD is characterized by the development of tumors in the peripheral nerves, particularly the sciatic nerve, and by lymphoma of various organs (liver, spleen, skin, heart, lungs, kidneys, and gonads). There is a vaccine available, however it does not protect the bird against infection, just the development of tumors. The vaccinated bird carries and excretes the virus in exactly the same way as unvaccinated birds.

The virus can survive for up to a year or more in the environment. It is shed in the feather follicles of infected birds, and is present in their feathers and dander. Both infected (this includes vaccinated birds) and recovered chickens are lifelong carriers of the MDV, and will continue to shed the virus for the remainder of their lives. 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, and did not receive the MD vaccine. MDV is not transmitted through the parents to the egg.

Often the first sign described is sudden lameness. 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 progress to developing the classic MD paralysis pose, where one leg is positioned straight out in front of their bodies, with the other straight out behind. Sometimes they may appear as if they are using their wings in an attempt to balance themselves, since they aren't able to use their legs.

There are several different forms of Marek's Disease.
FormGross pathologyClinical signs
Neurological form Enlarged 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, paralysis of one or more legs (often presents as 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.

Clinical Signs

One leg stretched forward and the other back
Paralysis
Torticollis
Incoordination
Nervous tics
Drooping wings
Inward curving of the toes
Gasping
Delayed crop emptying
Blindness
Irregular-shaped or unequally sized pupils
Change in eye color
Weight loss
Enlarged feather follicles, especially on legs
Reddened, bloody looking shanks
Paleness
Loss in appetite
Depression/lethargy
Muscle atrophy
Weakness

Diagnosis

  • History
  • Clinical signs
  • Physical exam
  • Complete Blood Count (CBC) and Biochemistry - monitored for changes in health status.
  • ELISA
  • PCR
  • Histopathology
  • Necropsy

Treatment

NameSummary
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;
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

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

Blogs

Age Range

It most commonly occurs in chickens between 2 and 4 months old.

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