Marek's disease is a Herpes virus infection of chickens, and in some cases - turkeys. It occurs in 2-5 month-old chickens although it can also occur after the onset of egg production. Marek’s disease may vary from asymptomatic infection to neurologic disease and ocular lesions, skin disease, internal organ, or visceral disease.

Neurological form is acute infiltration of the CNS and nerves resulting in 'floppy broiler syndrome' and transient paralysis, as well as more long-standing paralysis of legs or wings and eye lesions. Cutaneous form is displayed as tumours of feather follicles.  In visceral form we see tumours in heart, ovary, tests, muscles, lungs. Around 10-50% birds in the poultry house may get sick, and up to 100% of the ill animals die. Affected birds are more susceptible to other diseases, both parasitic and bacterial. The route of infection is usually airborne and the disease is highly contagious.  Infected birds spread the virus for the rest of their lives. The virus survives at ambient temperature for up to 65 weeks, and is resistant to some disinfectants, but is inactivated rapidly when frozen and thawed.

Signs and diagnosis

In affected animals we see loss of weight, inappetence, transient or long term paralysis of legs, wings and neck, eye lesions: grey iris or irregular pupil, vision impairment, blindness; skin around feather follicles raised and roughened. During post-mortem examination these lesions maybe found: thickening of nerve trunks and loss of striation, grey-white foci of neoplastic tissue in liver, spleen, kidney, lung, gonads, heart, and skeletal muscle. Histological findings: polymorphic lymphoid infiltration.

Marek’s disease is diagnosed using disease history, clinical signs, distribution of lesions, age ofaffected birds, pathologic and histopathologic findings. 
It is important to differentiate Marek’s from lymphoid leukosis, botulism, deficiency of thiamine, deficiency of Ca, Phosphorus, Vitamin D, especially at the onset of egg production.

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