Marek's disease

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). The disease is characterized by presence of T cell lymphoma as well as infiltration of nerves and organs by lymphocytes. Viruses related to MDV appear to be benign and can be used as vaccine strains to prevent Marek's disease. For example, the related Herpesvirus of Turkeys (HVT), causes no apparent disease in turkeys and continues to be used as a vaccine strain for prevention of Marek's diease (see below). Birds infected with GaHV-2 can be carriers and shedders of the virus for life. Newborn chicks are protected by maternal antibodies for a few weeks. After infection, microscopic lesions are present after one to two weeks, and gross lesions are present after three to four weeks. The virus is spread in dander from feather follicles and transmitted by inhalation.

There are four syndromes known to occur after infection with Marek's disease. These syndromes may overlap.
 * Classical Marek's disease or neurolymphomatosis causes asymmetric paralysis of one or more limbs. With vagus nerve involvement, difficulty breathing or dilation of the crop may occur.
 * Acute Marek's disease is an epidemic in a previously uninfected or unvaccinated flock, causing depression, paralysis, and death in a large number of birds (up to 80 percent).
 * Ocular lymphomatosis causes lymphocyte infiltration of the iris (making the iris turn grey), anisocoria, and blindness.
 * Cutaneous Marek's disease causes round, firm lesions at the feather follicles.

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. 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 is administered to one day old chicks, and immunity develops within two weeks. It usually contains the antigenically similar turkey herpesvirus, which is serotype 3 of MDV.

The disease is named after Dr. Josef Marek.