Marek’s disease (MD) is a very common lymphoproliferative (tumor-causing) and neuropathic disease of chickens affecting the peripheral nerves, other tissues, and visceral organs. It is caused by a highly contagious, cell-associated, oncogenic herpesvirus called Marek's disease virus (MDV). MDV shares molecular properties with other herpesviruses which cause lymphomas, including Epstein-Barr virus and Kaposi-sarcoma virus.
The 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.
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.
There is a vaccine available (CV1988-Rispens) which is 90% effective. However, it doesn't protect the chickens from becoming infected with the virus, only helps prevent them from developing the MDV-associated tumors. It must be given correctly, and at the day of hatch.
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.