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What is Marek’s Disease? A highly contagious infection in chickens caused by a specific herpes virus that results in a rapidly fatal polyclonal lymphoma.

Marek’s Disease Prof. Dr. Salah M. Hassan

The virus survives at ambient temperature for a long time (65 weeks) when cell associated and is resistant to some disinfectants (quaternary ammonium and phenol). It is inactivated rapidly when frozen and thawed.


How does Marek’s disease virus cause a lymphoma? An oncogenic variant of the virus enters through the respiratory route. An early productive infection occurs in non-lymphocytic cells. (At this stage, virus can spread rapidly through a chicken population.) The virus particles enter the lymphoid organs and causes short nonproductive latent infections, mainly in T cells. Nonproductive infection of these T lymphoid cells leads to lymphoma formation in each infected T cell, giving rise to a polyclonal lymphoma.

Signs

Paralysis of legs, wings and neck. ( Classical form , neurological form ) Loss of weight. Grey iris or irregular pupil. ( Ocular form , Gray eye ) Vision impairment. Skin around feather follicles raised and roughened. ( Skin form ). Visceral form - Tumours in heart, ovary, tests, muscles, lungs. Transient paralysis form,

MD affected layer bird with unilateral leg paralysis displaying “sportsman-like” posture

MD lesions in liver with numerous greyish-white coalescing tumour nodules

Post-mortem lesions
Grey-white foci of neoplastic tissue in liver, spleen, kidney, lung, gonads, heart, and skeletal muscle. Thickening of nerve trunks and loss of striation. Microscopically - lymphoid infiltration is polymorphic.

Peripheral Nerve. A-Type Lesion (Neoplastic Type), Marked Lymphoid Cell Proliferation, Absence Of Oedema. H/E, Bar = 25 Μm.

Symmetrical Neoplastic Foci Of Pleomorphic Cells, Liver, Hen. H/E, Bar = 50 Μm.

Acute (Visceral) Form, Liver, Hen. Lymphomatous Lesions, Mainly Consisting Of Lymphoblasts, And Small To Medium-Sized Lymphocytes. Among The Proliferate, Single Plasmatic And Reticular Cells Are Outlined. H/E, Bar = 10 Μm.

Muscle Tumour, Marek’s Disease. Focal Pleomorphic Cell Proliferation. H/E, Bar = 35 Μm.

Diffuse Pleomorphic Cell Proliferation In The Myocardium, Resulting In Atrophy Of Myofibrils. H/E, Bar = 35 Μm.

Pleomorphic Cell Proliferation, Ovary, Hen. H/E, Bar = 25 Μm

Intertubular, Pleomorphic Cell Proliferation, Testis, Cock. H/E, Bar = 30 Μm

Diffuse Pleomorphic Cell Proliferation In The Proventriculus Mucous Coat (Arrow), Hen. Compression And Atrophy Of Glandular Acini. H/E, Bar = 40 Μm.

Lymphoid Cell Proliferations In The Iris And Ciliary Muscles In The Ocular Form Of Marek’s Disease. H/E, Bar = 50 Μm.


Diagnosis

Prevention

It is common practice to use combinations of the different vaccine types in an effort to broaden the protection achieved. Genetics can help by increasing the frequency of the B21 gene that confers increased resistance to Marek's disease challenge.