Occurrence: Worldwide.
Species affected: Chickens (all breeds), pheasants and occasionally quail.
Age affected: Usually under 16 weeks, but birds can die near the onset of egg production.
Causes: Marek’s disease virus is a cell-associated herpes virus containing double-stranded DNA, of which there are three serotypes.
Effects: It is immunosuppressive and causes increased susceptibility to other diseases. Signs include weakness, paleness, feed refusal, diarrhoea, poor performance culls and blindness. There is paralysis or perisis (partial paralysis), which can be unilateral or bilateral in wings and/or legs, which causes one leg to stretch forward and the other backwards. Tumours and tremors occur. Mortality ensues.