Marek's disease is a highly contagious viral neoplastic disease in chickens. Occasionally misdiagnosed as an abtissue pathology it is caused by an alphaherpesvirus known as Marek's disease virus (MDV) or gallid herpesvirus 2 (GaHV-2).
Some chickens die without any clinical signs of Marek's disease. Most of the affected birds will have some degree of paralysis, although chickens with the acute form may not show this condition. Those with paralysis may die because they are unable to reach feed and water. The first indication of infection is a variation in the growth rate and degree of feathering.
Caused by a herpesvirus, the disease is often characterized by abnormal cell growth in the peripheral nerves and central nervous system. Hence, the common name for one form of Marek's: fowl paralysis. In addition to the nerves, however, the disease also may cause lesions on visceral organs and other tissues, including feather follicles of the skin. The most prominent lesions may be tumors on the liver, kidneys, testes, ova, spleen and lungs. In such cases, nerve swelling may not be involved.
Vaccination is the only known method to prevent the development of tumors when chickens are infected with the virus. However, administration of vaccine does not prevent transmission of the virus; i.e., the vaccine is non-sterilizing. However, it does reduce the amount of virus shed in the dander and hence reduce horizontal spread of the disease. Marek's Disease does not spread vertically. The vaccine was introduced in 1970. Before that, Marek's disease caused substantial revenue loss in the poultry industries of the United States and the United Kingdom. The vaccine can be administered to one day old chicks through sub-cutaneous inoculation or by in-ovo vaccination when the eggs are transferred from the incubator to the hatcher. In-ovo vaccination is the preferred method, as in does not require handling of the chicks and can be done rapidly by automated methods. Immunity develops within two weeks. The vaccine originally contained the antigenically similar turkey herpesvirus, which is serotype 3 of MDV. However, because vaccination does not prevent infection with the virus, the Marek's Disease virus has evolved increased virulence and resistance to this vaccine. As a result, current vaccines use a combination of vaccines consisting of HVT and gallid herpesvirus type 3 or an attenuated MDV strain, CVI988-Rispens.
The demonstration of nerve enlargement, especially with the ischiatic nerve along with suggestive clinical signs in a bird that is around three to four months old is highly suggestive of Marek's Disease. The presence of nodules on the internal organs may also suggest Marek's disease but further testing is required for confirmation. This is done through histological demonstration of lymphomatous infiltration into the affected tissue. A range of leukocytes can be involved, including lyphocytic cell lines such as large lymphocyte, lymphoblast, primitive reticular cells and occasional plasma cells as well as macrophage and plasma cells. The T-cells are involved in the malignancy, showing neoplastic changes with evidence of mitosis. The lymphomatous infiltrates need to be differentiated with another condition that affects poultry known as Lymphoid Leukosis as well as an inflammatory event associated with hyperplastic changes of the affected tissue.