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.