Marek's disease

Marek's disease is a lymphoproliferative disease of chicken, quails, bantams and other free-flying birds. It is caused by the Gallid Herpesvirus 2 of subfamily Alphaherpesvirinae in the family Herpesviridae. The presence of lymphomas in various visceral organs, nerves, gonands, skin and muscles constitute the clinical symptoms. The mortality rate varies from 10 to 15% in unvaccinated flocks. There are several strains of Marek's disease virus possessing varying degrees of oncogenicity. These are placed in two groups; Group 1 contains highly oncogenic strains and group 2 low or non - oncogenic strains. Among other factors influencing the pathogenesis of the disease are genetic resistance and age.

Clinical signs: Four overlapping syndromes are described. Neurolymphomatosis or classical Marek's disease is associated with an asymmetric paralysis of one or both legs or wings. Incoordination is a common early sign; one leg is held forward and the other backward. Wing dropping and lowering of head and neck are common.

Acute Marek's disease is common in explosive outbreaks with depression, ataxia and paralysis.Occular lymphomatosis leads to graying of iris of one or both eyes. Pupil is irregular and eccentric leading to  total or partial blindness. Cutaneous Marek's disease with round nodular lesions up to 1 cm in diameter are seen in feather follicles.

Diagnosis: The disease is diagnosed by the isolation of the virus in embryonated eggs, chick kidney or duck embryo fibroblast culture using the living cells from buffy coat, spleen, tumor tissue or other lymphoid cells, skin and feather tips. The detection of viral antigens in non-lymphoid cells and Marek's associated tumor specific surface antigen (MATSA) in lymphoid cells is done by immunofluorescence in frozen tissue sections. Virus neutralization test is also used.

Prevention and control: Three types of vaccines viz. Herpes virus of turkey (HVT),naturally occurring low pathogenic strains of Marek's disease virus (Serotype-2) and cell-culture attenuated MDV are available for immunization. Vaccine is available either a lyophilized cell free preparation or a cell associated preparation. Because of its economy in production, HVT vaccine is most widely used. One day old chicks are vaccinated parentrally.