Marek’s Disease: A Mini-Review

Marek’s disease is a lymphoproliferative disease principally of the domestic chicken known to be caused by herpesvirus i-e oncogenic (serotype 1) strains of Marek’s disease herpesvirus (MDV) that replicates in the lymphoid and epithelial tissues. Pathogenesis is complex, involving cytolytic and latent infection of lymphoid cells and oncogenic transformation of CD4+ T cells in susceptible chickens. Infection of a young susceptible chick with the alpha herpesvirus is followed after 4 to 7 days by a short period of virus replication in lymphoid cells and reticulum cells in thymus, spleen and bursa of fabricius. Following an early cell associated cytolytic infection of lymphocytes a switch to latent infection occurs at about 7 days. Latently infected T lymphocytes harbor the MDV genome with limited viral antigen expression and they carry the virus through the bloodstream to the visceral organs, peripheral nerves, and feather follicle epithelium (FFE). MDV replication in feather tissues results in shedding of cell-free virus with skin and feather debris, and this is the source of infection for other chickens.This virus presents a complex and variable pathological picture in which both neoplastic and inflammatory changes are observed. The most commonly affected organs and tissues are peripheral nerves, iris gonads, spleen, heart, lungs, liver and muscle. Paralysis is evident with ataxia for period of several days. Diagnosis is based on enlarged nerves, lymphoid tumors in viscera and confirmation is by demonstration of tumor – associated surface antigen on some of the individual cells by immunofluorescence. A real-time PCR method is developed, optimised and validated, to enable quantitation of Marek’s disease virus genomes. Vaccination is the principal method of control. Genetic resistance of chickens to the disease has been exploited in the laboratory to develop resistant breeds. Chemoprophylaxis is of little success.